Correlates of immune protection induced by live, attenuated, cold-adapted, trivalent, intranasal influenza virus vaccine

Sang Froid in a time of trouble: is a vaccine against HIV possible?

Since the announcement of the STEP trial results in the past months, we have heard many sober pronouncements on the possibility of an HIV vaccine. On the other hand, optimistic quotations have been liberally used, from Shakespeare's Henry V's "Once more unto the breach, dear friends" to Winston Churchill's definition of success as "going from one failure to another with no loss of enthusiasm". I will forgo optimistic quotations for the phrase "Sang Froid", which translates literally from the French as "cold blood"; what it really means is to avoid panic when things look bad, to step back and coolly evaluate the situation. This is not to counsel easy optimism or to fly in face of the facts, but I believe that while the situation is serious, it is not desperate.I should stipulate at the outset that I am neither an immunologist nor an expert in HIV, but someone who has spent his life in vaccine development. What I will try to do is to provide a point of view from that experience.There is no doubt that the results of STEP were disappointing: not only did the vaccine fail to control viral load, but may have adversely affected susceptibility to infection. But HIV is not the only vaccine to experience difficulties; what lessons can we glean from prior vaccine development?

Live attenuated vaccines for pandemic influenza

In this chapter, we will review the development of and clinical experience with the currently licensed seasonal live attenuated influenza vaccines (LAIV) and preclinical studies of H5, H7, and H9 live attenuated pandemic influenza vaccine candidates. Vectored vaccine approaches will not be reviewed in this chapter. Experience with seasonal influenza vaccination has demonstrated the safety and efficacy of LAIV in both children and adults; moreover, cross-protection among antigenically distinct viruses within the same subtype may be induced by LAIV. While clinical studies and further characterization of the immunologic response to avian influenza viruses are still needed, the experience with seasonal LAIV underscores the potential of live attenuated vaccines to play an important role in the event of a pandemic.

Estimating influenza vaccine efficacy from challenge and community-based study data

In this paper, the authors provide estimates of 4 measures of vaccine efficacy for live, attenuated and inactivated influenza vaccine based on secondary analysis of 5 experimental influenza challenge studies in seronegative adults and community-based vaccine trials. The 4 vaccine efficacy measures are for susceptibility (VE_S), symptomatic illness given infection (VE_P), infection and illness (VE_SP), and infectiousness (VE_I). The authors also propose a combined (VE_C) measure of the reduction in transmission in the entire population based on all of the above efficacy measures. Live influenza vaccine and inactivated vaccine provided similar protection against laboratory-confirmed infection (for live vaccine: VE_S  = 41%, 95% confidence interval (CI): 15, 66; for inactivated vaccine: VE_S  = 43%, 95% CI: 8, 79). Live vaccine had a higher efficacy for illness given infection (VE_P  = 67%, 95% CI: 24, 100) than inactivated vaccine (VE_P  = 29%, 95% CI: −19, 76), although the difference was not statistically significant. VE_SP for the live vaccine was higher than for the inactivated vaccine. VE_I estimates were particularly low for these influenza vaccines. VE_SP and VE_C can remain high for both vaccines, even when VE_I is relatively low, as long as the other 2 measures of vaccine efficacy are relatively high.

Influence of prior influenza vaccination on antibody and B-cell responses

Currently two vaccines, trivalent inactivated influenza vaccine (TIV) and live attenuated influenza vaccine (LAIV), are licensed in the USA. Despite previous studies on immune responses induced by these two vaccines, a comparative study of the influence of prior influenza vaccination on serum antibody and B-cell responses to new LAIV or TIV vaccination has not been reported. During the 2005/6 influenza season, we quantified the serum antibody and B-cell responses to LAIV or TIV in adults with differing influenza vaccination histories in the prior year: LAIV, TIV, or neither. Blood samples were collected on days 0, 7–9 and 21–35 after immunization and used for serum HAI assay and B-cell assays. Total and influenza-specific circulating IgG and IgA antibody secreting cells (ASC) in PBMC were detected by direct ELISPOT assay. Memory B cells were also tested by ELISPOT after polyclonal stimulation of PBMC in vitro. Serum antibody, effector, and memory B-cell responses were greater in TIV recipients than LAIV recipients. Prior year TIV recipients had significantly higher baseline HAI titers, but lower HAI response after vaccination with either TIV or LAIV, and lower IgA ASC response after vaccination with TIV than prior year LAIV or no vaccination recipients. Lower levels of baseline HAI titer were associated with a greater fold-increase of HAI titer and ASC number after vaccination, which also differed by type of vaccine. Our findings suggest that the type of vaccine received in the prior year affects the serum antibody and the B-cell responses to subsequent vaccination. In particular, prior year TIV vaccination is associated with sustained higher HAI titer one year later but lower antibody response to new LAIV or TIV vaccination, and a lower effector B-cell response to new TIV but not LAIV vaccination.

Live, attenuated influenza virus (LAIV) vehicles are strong inducers of immunity toward influenza B virus

Historically, vaccines developed toward influenza viruses of the B type using methodologies developed for influenza A viruses as a blueprint have not been equally efficacious or effective. Because most influenza research and public attention concerns influenza A viruses, these shortcomings have not been adequately addressed. In this manuscript, we utilized different influenza vaccine vehicles to compare immunogenicity and protection in mice and ferrets after vaccination against an influenza B virus. We report that plasmid DNA vaccines demonstrate low immunogenicity profiles and poor protection compared to either whole, inactivated influenza virus (IIV) or, live, attenuated influenza virus (LAIV) vaccines. When mixed prime:boost regimens using LAIV and IIV were studied, we observed a boosting effect in mice after priming with LAIV that was not seen when IIV was used as the prime. In ferrets LAIV induced high antibody titers after a single dose and provided a boost in IIV-primed animals. Regimens including LAIV as a prime demonstrated enhanced protection, and adjuvantation was required for efficacy using the IIV preparation. Our results differ from generally accepted influenza A virus vaccine models, and argue that strategies for control of influenza B virus should be considered separately from those for influenza A virus.

Vaccines: correlates of vaccine-induced immunity

The immune system is redundant, and B and T cells collaborate. However, almost all current vaccines work through induction of antibodies in serum or on mucosa that block infection or interfere with microbial invasion of the bloodstream. To protect, antibodies must be functional in the sense of neutralization or opsonophagocytosis. Correlates of protection after vaccination are sometimes absolute quantities but often are relative, such that most infections are prevented at a particular level of response but some will occur above that level because of a large challenge dose or deficient host factors. There may be >1 correlate of protection for a disease, which we term "cocorrelates." Either effector or central memory may correlate with protection. Cell-mediated immunity also may operate as a correlate or cocorrelate of protection against disease, rather than against infection. In situations where the true correlate of protection is unknown or difficult to measure, surrogate tests (usually antibody measurements) must suffice as predictors of protection by vaccines. Examples of each circumstance are given.

Correlation of cellular immune responses with protection against culture-confirmed influenza virus in young children

The highly sensitive gamma interferon (IFN-gamma) enzyme-linked immunosorbent spot (ELISPOT) assay permits the investigation of the role of cell-mediated immunity (CMI) in the protection of young children against influenza. Preliminary studies of young children confirmed that the IFN-gamma ELISPOT assay was a more sensitive measure of influenza memory immune responses than serum antibody and that among seronegative children aged 6 to <36 months, an intranasal dose of 10(7) fluorescent focus units (FFU) of a live attenuated influenza virus vaccine (CAIV-T) elicited substantial CMI responses. A commercial inactivated influenza virus vaccine elicited CMI responses only in children with some previous exposure to related influenza viruses as determined by detectable antibody levels prevaccination. The role of CMI in actual protection against community-acquired, culture-confirmed clinical influenza by CAIV-T was investigated in a large randomized, double-blind, placebo-controlled dose-ranging efficacy trial with 2,172 children aged 6 to <36 months in the Philippines and Thailand. The estimated protection curve indicated that the majority of infants and young children with >or=100 spot-forming cells/10(6) peripheral blood mononuclear cells were protected against clinical influenza, establishing a possible target level of CMI for future influenza vaccine development. The ELISPOT assay for IFN-gamma is a sensitive and reproducible measure of CMI and memory immune responses and contributes to establishing requirements for the future development of vaccines against influenza, especially those used for children.

[Bird flu: what the intensivist must know]

In the last century, humankind has faced 3 major pandemics of influenza virus infections. The first one occurred in 1918 and caused a significant amount of deaths. It was also capable of crossing over species barrier and affecting mammals, and most worrisome, humans. Since then several outbreaks have been reported in the Southeast of Asia. Many patients with the flu-like illness have a severe course and the patient develops pneumonia and in some cases multiorgan failure involving liver, kidneys, brain and lungs. Since the virus lacks regulatory control of genetic division it undergoes constant mutations leading to new subtypes and, sometimes, new strains. The only drugs that have shown some protection are oseltamivir and zanamivir. It is crucial to develop effective and non-expensive vaccines to prevent the virus spread and infection not only in humans but in birds too.

Vaccines for preventing influenza in healthy children

BACKGROUND

The consequences of influenza in children and adults are mainly absenteeism from school and work. However, the risk of complications is greatest in children and people over 65 years old.

OBJECTIVES

To appraise all comparative studies evaluating the effects of influenza vaccines in healthy children; assess vaccine efficacy (prevention of confirmed influenza) and effectiveness (prevention of influenza-like illness) and document adverse events associated with influenza vaccines.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2007, issue 3); OLD MEDLINE (1950 to 1965); MEDLINE (1966 to September 2007); EMBASE (1974 to September 2007); Biological Abstracts (1969 to September 2007); and Science Citation Index (1974 to September 2007).

SELECTION CRITERIA

Randomised controlled trials (RCTs), cohort and case-control studies of any influenza vaccine in healthy children under 16 years of age.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and extracted data.

MAIN RESULTS

Fifty-one studies with 294,159 observations were included. Sixteen RCTs and 18 cohort studies were included in the analysis of vaccine efficacy and effectiveness. From RCTs, live vaccines showed an efficacy of 82% (95% confidence interval (CI) 71% to 89%) and an effectiveness of 33% (95% CI 28% to 38%) in children older than two compared with placebo or no intervention. Inactivated vaccines had a lower efficacy of 59% (95% CI 41% to 71%) than live vaccines but similar effectiveness: 36% (95% CI 24% to 46%). In children under two, the efficacy of inactivated vaccine was similar to placebo. Variability in study design and presentation of data was such that a meta-analysis of safety outcome data was not feasible. Extensive evidence of reporting bias of safety outcomes from trials of live attenuated vaccines impeded meaningful analysis.

AUTHORS' CONCLUSIONS

Influenza vaccines are efficacious in children older than two but little evidence is available for children under two. There was a marked difference between vaccine efficacy and effectiveness. No safety comparisons could be carried out, emphasizing the need for standardisation of methods and presentation of vaccine safety data in future studies. It was surprising to find only one study of inactivated vaccine in children under two years, given current recommendations to vaccinate healthy children from six months old in the USA and Canada. If immunisation in children is to be recommended as a public health policy, large-scale studies assessing important outcomes and directly comparing vaccine types are urgently required.

Influenza vaccines and vaccination strategies in birds

Although it is well accepted that the present Asian H5N1 panzootic is predominantly an animal health problem, the human health implications and the risk of human pandemic have highlighted the need for more information and collaboration in the field of veterinary and human health. H5 and H7 avian influenza (AI) viruses have the unique property of becoming highly pathogenic (HPAI) during circulation in poultry. Therefore, the final objective of poultry vaccination against AI must be eradication of the virus and the disease. Actually, important differences exist in the control of avian and human influenza viruses. Firstly, unlike human vaccines that must be adapted to the circulating strain to provide adequate protection, avian influenza vaccination provides broader protection against HPAI viruses. Secondly, although clinical protection is the primary goal of human vaccines, poultry vaccination must also stop transmission to achieve efficient control of the disease. This paper addresses these differences by reviewing the current and future influenza vaccines and vaccination strategies in birds.

Flu myths: dispelling the myths associated with live attenuated influenza vaccine

Live attenuated influenza vaccine (LAIV), commercially available since 2003, has not gained widespread acceptance among prescribers. This underuse can be traced to several misperceptions and fears regarding LAIV. This review examines both the facts (safety, immunogenicity, and effectiveness) and the most pervasive myths about LAIV. Live attenuated influenza vaccine is a safe, highly immunogenic, and effective vaccine. It is well tolerated; only mild and transient upper respiratory infection symptoms occur with LAIV vs placebo, even in higher-risk patients with asthma or the early stages of human immunodeficiency virus. It is immunogenic, especially in induction of mucosal immunity. In certain populations, LAIV is as effective as, and in some cases more effective than, inactivated influenza in preventing influenza infection. It appears to be more effective in preventing influenza infection than trivalent inactivated influenza vaccine when the vaccine virus strain does not closely match that of the circulating wild-type virus. Many myths and misperceptions about the vaccine exist, foremost among them the myth of genetic reversion. Independent mutation in 4 gene segments would be required for reversion of the vaccine strain of influenza virus to a wild type, an unlikely and as yet unobserved event. Although shedding of vaccine virus is common, transmission of vaccine virus has been documented only in a single person, who remained asymptomatic. In the age groups for which it is indicated, LAIV is a safe and effective vaccine to prevent influenza infection.

Towards an oral influenza vaccine: comparison between intragastric and intracolonic delivery of influenza subunit vaccine in a murine model

In this paper we investigated to which part of the gastro-intestinal (GI) tract, the upper or lower part, an oral influenza vaccine should be targeted to result in an effective immune response in mice. Our study demonstrates that without adjuvant substantial systemic but low respiratory mucosal immune responses were induced in mice after delivery of influenza subunit vaccine to the upper GI-tract (intragastric) as well as the lower GI-tract (intracolonically). When the vaccine was adjuvanted with Escherichia coli heat-labile enterotoxin (LT) these responses were significantly enhanced. Interestingly, intracolonic administration of vaccine with adjuvant also resulted in enhanced cellular immune responses and the desired Th1-skewing of these responses. Intragastric administration of the adjuvanted vaccine also increased T-helper responses. However, Th1-skewing was absent. In conclusion, the right combination of strong mucosal adjuvant (e.g. LT) and antigen delivery site (e.g. the lower part of the gastro-intestinal tract) might result in effective vaccination via the oral route.

Trivalent live attenuated intranasal influenza vaccine administered during the 2003-2004 influenza type A (H3N2) outbreak provided immediate, direct, and indirect protection in children

OBJECTIVE

Live attenuated influenza vaccine may protect against wild-type influenza illness shortly after vaccine administration by innate immunity. The 2003-2004 influenza A (H3N2) outbreak arrived early, and the circulating strain was antigenically distinct from the vaccine strain. The objective of this study was to determine the effectiveness of influenza vaccines for healthy school-aged children when administered during the influenza outbreak.

DESIGN/METHODS

An open-labeled, nonrandomized, community-based influenza vaccine trial was conducted in children 5 to 18 years old. Age-eligible healthy children received trivalent live attenuated influenza vaccine. Trivalent inactivated influenza vaccine was given to children with underlying health conditions. Influenza-positive illness was compared between vaccinated and nonvaccinated children. Medically attended acute respiratory illness and pneumonia and influenza rates for Scott and White Health Plan vaccinees were compared with age-eligible Scott and White Health Plan nonparticipants in the intervention communities. Herd protection was assessed by comparing age-specific medically attended acute respiratory illness rates in Scott and White Health Plan members in the intervention and comparison communities.

RESULTS

We administered 1 dose of trivalent live attenuated influenza vaccine or trivalent inactivated influenza vaccine to 6569 and 1040 children, respectively (31.5% vaccination coverage), from October 10 to December 30, 2003. The influenza outbreak occurred from October 12 to December 20, 2003. Significant protection against influenza-positive illness (37.3%) and pneumonia and influenza events (50%) was detected in children who received trivalent live attenuated influenza vaccine but not trivalent inactivated influenza vaccine. Trivalent live attenuated influenza vaccine recipients had similar protection against influenza-positive illness within 14 days compared with >14 days (10 of 25 vs 9 of 30) after vaccination. Indirect effectiveness against medically attended acute respiratory illness was detected in children 5 to 11 and adults 35 to 44 years of age.

CONCLUSION

One dose of trivalent live attenuated influenza vaccine was efficacious in children even when administered during an influenza outbreak and when the dominant circulating influenza virus was antigenically distinct from the vaccine strain. We hypothesize that trivalent live attenuated influenza vaccine provides protection against influenza by both innate and adaptive immune mechanisms.

The cotton rat as a model to study influenza pathogenesis and immunity

Influenza viruses continue to cause significant morbidity and mortality worldwide. With the threat of the emergence of a pandemic influenza strain, there is an urgency to develop new vaccine strategies that offer broad protection. The rational basis for the design of such vaccines comes from the use of animal models. Cotton rats are a helpful tool to study influenza disease pathogenesis and immunity because adaptation of human influenza strains is not required for virus replication in the lower respiratory tract and subsequent disease signs. This review describes innate and adaptive responses to influenza in infected cotton rats, and points out immune mechanisms that contribute to protection against disease.

A dose-response evaluation of inactivated influenza vaccine given intranasally and intramuscularly to healthy young adults

Epidemic influenza occurs annually throughout the world and is accompanied by excess morbidity and mortality. Increasing the antigen content and topical administration of vaccine are two strategies being explored to improve the immune responses to trivalent inactivated influenza vaccine (TIV). We conducted a randomized, double-blind, placebo-controlled trial to compare the immunogenicity and reactogenicity of intramuscular (IM), intranasal (IN), or combined IM and IN administration of a contemporary US vaccine formulation at escalating dosage levels in young healthy adults. Two hundred forty three healthy adults between the ages of 18 and 45 years received 15, 30, or 60mcg of trivalent inactivated influenza vaccine by either IN, IM or both routes, 120mcg of vaccine IM, or placebo IN and IM. All dosages and routes of vaccine administration were well-tolerated. A bad taste and mild nasal discomfort were more likely to be reported when influenza vaccine was administered IN, while arm tenderness was more common after IM administration. Significant increases in geometric mean serum antibody titers in both HAI and Nt assays were seen in all of the groups receiving influenza vaccine for all test antigens (P<or=.025, paired t-test), except for the B HAI antibody titer in the group that received 30mcg IN (P=.055, paired t-test). Postvaccination geometric mean serum antibody titers, the frequency of seroresponses, and the percentage achieving postvaccination serum HAI antibody titers of >or=32 were higher following delivery of the study vaccines by an IM route than by the IN route, but significant increases in serum antibody were seen after IN vaccination. Nasal IgA antibody responses were more common when vaccine was administered IN; and, when the IN dosage was increased, the primary benefit from IN vaccine over IM vaccine appeared to be greater induction of nasal secretory antibody.

[Immunity and antiviral vaccinations. Example: the respiratory mucosa]

OBJECTIVE

As the mucosal surfaces of the respiratory tract represent a major portal of entry for most human viruses and many bacteria, they seem to be a critical component of the mammalian immunologic repertoire. Thus, vaccines stimulating this local immunity could represent an interesting approach to prevent these infections. After detailing the different mechanisms implied in this mucosal immunity, the aim of this study is to analyze the basis of such a vaccination and the different vaccines available to mucosal respiratory tract use.

MUCOSAL IMMUNITY

The major antibody isotype in external secretions is secretory immunoglobin A (S-IgA); the role of IgM (S-IgM) and IgG (S-IgG) are actually questionned. It is, however, interesting that the major effector cells in the mucosal surfaces are not IgA B cells, but T lymphocytes that may represent up to 80% of the entire mucosal lymphoid cell population.

IMMUNOPROPHYLAXIS BY THE MUCOSAL ROUTE

Passive antibodies were shown to protect against mucosal viral infections, such as those caused by RSV, but very high quantities of passive antibodies are needed to restrict virus replication on mucosal surface.In general, factors which favor development of mucosal antibody and cell mediated immune responses include the oral or respiratory immunization and the replicating nature of the vaccine agents. However, to date only a few vaccines have become available to mucosal respiratory tract use, and cold-adapted influenza virus vaccines is the only one available using nasal route. Other parenteral licensed vaccines have not been recommended for mucosal administration. Some of them have been experimentally used with nasal administration of replicating agents (varicella and measles vaccines) or non replicating agents (influenza inactivated vaccine), but have been found to induce a very low mucosal response.

CONCLUSION

Based on the experience with existing vaccines, the development of mucosal immunity or administration of vaccines via the mucosal route is clearly not a prerequisite today for control or prevention of most viral infectious respiratory diseases or diseases with respiratory tract as a route of contamination. But the example of live attenuated intranasal influenza vaccine inducing both systemic and local immune response without immunopathology, is promising for the future of the mucosal immunization against respiratory viral infections.

Estimating the protective concentration of anti-pneumococcal capsular polysaccharide antibodies

Estimates of minimum protective antibody concentrations for vaccine preventable diseases are of critical importance in assessing whether new vaccines will be as effective as those for which clinical efficacy was shown directly. We describe a method for correlating pneumococcal anticapsular antibody responses of infants immunized with pneumococcal conjugate (PnC) vaccine (Prevenar) with clinical protection from invasive pneumococcal disease (IPD). Data from three double blind controlled trials in Northern Californian, American Indian and South African infants were pooled in a meta-analysis to derive a protective concentration of 0.35 microg/ml for anticapsular antibodies to the 7 serotypes in Prevenar. This concentration has been recommended by a WHO Working Group as applicable on a global basis for assessing the efficacy of future pneumococcal conjugate vaccines. The WHO Working Groups anticipated that modifications in antibody assays for pneumococcal anticapsular antibodies would occur. The principles for determining whether such assay modifications should change the protective concentration are outlined. These principles were applied to an improvement in the ELISA for anticapsular antibodies, i.e. absorption with 22F pneumococcal polysaccharide, which increases the specificity of the assay for vaccine serotype anticapsular antibodies by removing non-specific antibodies. Using sera from infants in the pivotal efficacy trial in Northern California Kaiser Permanente (NCKP), 22F absorption resulted in minimal declines in pneumococcal antibody in Prevenar immunized infants but significant declines in unimmunized controls. Recalculation of the protective concentration after 22F absorption resulted in only a small decline from 0.35 microg/ml to 0.32 microg/ml. These data support retaining the 0.35 microg/ml minimum protective concentration recommended by WHO for assessing the efficacy of pneumococcal conjugate vaccines in infants.

Nouvelles voies d’administration : vaccinations par voie épidermique, intradermique, muqueuse

A successful vaccine triggers the interaction of various cells of the immune system as does a regular immune response. It is thus necessary to introduce the vaccine antigens into an anatomic site where they will contact immune cells. The route of administration is thus critical for the outcome of vaccination. Intramuscular or subcutaneous injections are the most popular. Antigens injected intramuscularly can form persistent precipitates that are dissolved and re-absorbed relatively slowly. If injecting antigens is a quick, easy and reproducible way to vaccination, it requires trained personnel. Alternatives exist, through non-invasive formulations which allow administration by the patient or a third party with no particular expertise. The skin, especially its epidermal layer, is an accessible and competent immune environment and an attractive target for vaccine delivery, through transcutaneous delivery or immunostimulant patches. Mucosal immunization is another strategy: its major rationale is that organisms invade the body via mucosal surfaces. Therefore, local protection at mucosal surface as well as systemic defense is beneficial. Various formulations of mucosal vaccines have been developed, such as the Sabin oral polio vaccine (OPV), rotavirus vaccines, cold-adapted influenza vaccines or vaccine against typhoid fever. Thus we are entering in an era where mucosal and transcutaneous immunisation will play an important role in disease management. However, it has not been so easy to obtain regulatory approval for mucosal or transcutaneous formulations and needle-based vaccines continue to dominate the market.

Evaluation of the ability of canarypox-vectored equine influenza virus vaccines to induce humoral immune responses against canine influenza viruses in dogs

OBJECTIVE

To evaluate canarypox-vectored equine influenza virus (EIV) vaccines expressing hemagglutinins of A/equine/Kentucky/94 (vCP1529) and A2/equine/Ohio /03 (vCP2242) for induction of antibody responses against canine influenza virus (CIV) in dogs.

ANIMALS

35 dogs.

PROCEDURES

Dogs were randomly allocated into 4 groups; group 1 (n = 8) and group 2 (9) were inoculated SC on days 0 and 28 with 1.0 mL (approx 10(5.7) TCID(50)) of vCP1529 and vCP2242, respectively. Dogs in group 3 (n = 9) were inoculated twice with 0.25 mL (approx 10(5.7) TCID(50)) of vCP2242 via the transdermal route. The 9 dogs of group 4 were control animals. All dogs were examined for adverse reactions. Sera, collected on days -1, 7, 13, 21, 28, 35, and 42, were tested by hemagglutination inhibition (HI) and virus neutralization (VN) assays for antibodies against CIV antigens A/Canine/FL/43/04-PR and A/Canine/NY/115809/05, respectively.

RESULTS

Inoculations were tolerated well. The HI and VN antibodies were detected by 7 days after primary inoculation. Most dogs of groups 1 and 2 and all dogs of group 3 had detectable antibodies by 14 days after initial inoculation. The second inoculation induced an anamnestic response, yielding geometric mean HI titers of 139, 276, and 1,505 and VN titers of 335, 937, and 3,288 by day 42 (14 days after booster inoculation) in groups 1, 2, and 3, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Canarypox-vectored EIV vaccines induce biologically important antibodies and may substantially impact CIV transmission within a community and be of great value in protecting dogs against CIV-induced disease.

Distinct cellular immune responses following primary and secondary influenza virus challenge in cotton rats

To evaluate cell-mediated immunity in influenza-infected cotton rats, we compared the cellular composition of spleen, mediastinal lymph nodes (MLN) and bronchoalveolar lavage (BAL) after primary and secondary infection. There was an increase in cellularity in the MLN after primary infection that was further expanded upon rechallenge. CD4(+) T cells expanded after primary infection, but there was preferential increase in the number of CD4-negative T cells following secondary challenge. After primary infection, a large proportion of the monocytes and NK cells were present in the BAL while a T cell population dominated after secondary infection. CD4(+) T cells were predominant in this population unless the animals had been challenged with heterosubtypic influenza A virus. These studies are the first to show evidence of a memory T cell response to influenza infection in cotton rats and show quantitative and qualitative differences between the recall response to homosubtypic and heterosubtypic viruses.

Humoral and cellular immune responses in children given annual immunization with trivalent inactivated influenza vaccine

BACKGROUND

There have been no prior reports of the frequency of circulating influenza-specific, interferon gamma-producing memory CD4+ and CD8+ T-cells in healthy children who have received multiple influenza immunizations.

METHODS

We evaluated 21 previously immunized children, ages 3 to 9 years, before and 1 month after administration of trivalent inactivated influenza vaccine. Frequencies of influenza-specific CD4+ and CD8+ T-cells stimulated with trivalent inactivated influenza vaccine or A/Panama (H3N2) virus were determined by flow cytometry, and antibody responses to vaccine strains and a drifted H3N2 strain were measured by hemagglutination inhibition assay and neutralizing antibody assays.

RESULTS

Mean change in CD4+ and in CD8+ T-cell frequencies after immunization was 0.01% (P > 0.39) with postimmunization CD4+ frequencies higher than CD8+ frequencies. Children with more previous vaccinations had a higher baseline frequency of CD4+ T-cells (P = 0.0002) but a smaller increase or even a decline from baseline after immunization (P = 0.003). An association between age and change in frequency was not detected. Baseline geometric mean titers (GMTs) and seroprotection rates were significantly higher in older children against A/Panama (neutralizing baseline GMT, P = 0.0488) and A/New Caledonia (hemagglutination inhibition baseline GMT and seroprotection, P < 0.0297). Baseline GMTs against B/Hong Kong were not associated with age or quantity of prior vaccinations.

CONCLUSIONS

These findings suggest that children may plateau in CD4+ T-cell responses to influenza antigens with repeated exposures and that the number of exposures may play a large role in building a memory CD4+ T-cell response to influenza A, perhaps independently from age.

Community-acquired pneumonia: paving the way towards new vaccination concepts

Despite the availability of antimicrobial agents and vaccines, community-acquired pneumonia remains a serious problem. Severe forms tend to occur in very young children and among the elderly, since their immune competence is eroded by immaturity and immune senescence, respectively. The main etiologic agents differ according to patient age and geographic area. Streptococcus pneumoniae, Haemophilus influenzae, respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV-3) are the most important pathogens in children, whereas influenza viruses are the leading cause of fatal pneumonia in the elderly. Effective vaccines are available against some of these organisms. However, there are still many agents against which vaccines are not available or the existent ones are suboptimal. To tackle this problem, empiric approaches are now being systematically replaced by rational vaccine design. This is facilitated by the growing knowledge in the fields of immunology, microbial pathogenesis and host response to infection, as well as by the availability of sophisticated strategies for antigen selection, potent immune modulators and efficient antigen delivery systems. Thus, a new generation of vaccines with improved safety and efficacy profiles compared to old and new agents is emerging. In this chapter, an overview is provided about currently available and new vaccination concepts.

Prevention of antigenically drifted influenza by inactivated and live attenuated vaccines

BACKGROUND

The efficacy of influenza vaccines may decline during years when the circulating viruses have antigenically drifted from those included in the vaccine.

METHODS

We carried out a randomized, double-blind, placebo-controlled trial of inactivated and live attenuated influenza vaccines in healthy adults during the 2004-2005 influenza season and estimated both absolute and relative efficacies.

RESULTS

A total of 1247 persons were vaccinated between October and December 2004. Influenza activity in Michigan began in January 2005 with the circulation of an antigenically drifted type A (H3N2) virus, the A/California/07/2004-like strain, and of type B viruses from two lineages. The absolute efficacy of the inactivated vaccine against both types of virus was 77% (95% confidence interval [CI], 37 to 92) as measured by isolating the virus in cell culture, 75% (95% CI, 42 to 90) as measured by either isolating the virus in cell culture or identifying it through real-time polymerase chain reaction, and 67% (95% CI, 16 to 87) as measured by either isolating the virus or observing a rise in the serum antibody titer. The absolute efficacies of the live attenuated vaccine were 57% (95% CI, -3 to 82), 48% (95% CI, -7 to 74), and 30% (95% CI, -57 to 67), respectively. The difference in efficacy between the two vaccines appeared to be related mainly to reduced protection of the live attenuated vaccine against type B viruses.

CONCLUSIONS

In the 2004-2005 season, in which most circulating viruses were dissimilar to those included in the vaccine, the inactivated vaccine was efficacious in preventing laboratory-confirmed symptomatic illnesses from influenza in healthy adults. The live attenuated vaccine also prevented influenza illnesses but was less efficacious. (ClinicalTrials.gov number, NCT00133523.)

Live attenuated influenza vaccine in children

Live attenuated influenza vaccine (LAIV) offers a novel approach to influenza vaccination and is approved for healthy individuals 5 to 49 years of age. In placebo-controlled studies in children, LAIV was 73 to 93 percent efficacious, and protection lasted more than 12 months. In head-to-head studies in children, LAIV demonstrated a 35 to 53 percent reduction in influenza attack rates compared with injectable influenza vaccine (TIV) for matched strains. Compared with TIV, LAIV has demonstrated broader serum antibody responses, particularly against mismatched influenza A. The most common adverse events are runny nose and nasal congestion. Increased rates of asthma events were observed in young children. Additional large-scale safety and efficacy studies in young children, including a formal risk-benefit assessment, are ongoing. The results of these analyses will guide potential future use in young children.

Distinct contributions of vaccine-induced immunoglobulin G1 (IgG1) and IgG2a antibodies to protective immunity against influenza

Vaccination represents the most effective form of protection against influenza infection. While neutralizing antibodies are typically measured as a correlate of vaccine-induced protective immunity against influenza, nonneutralizing antibodies may contribute to protection or amelioration of disease. The goal of this study was to dissect the individual contributions of the immunoglobulin G1 (IgG1) and IgG2a antibody isotypes to vaccine-induced immunity against influenza virus. To accomplish this, we utilized an influenza vaccine regimen that selectively enhanced IgG1 or IgG2a antibodies by using either DNA or viral replicon particle (VRP) vectors expressing influenza virus hemagglutinin (HA) (HA-DNA or HA-VRP, respectively). After HA-DNA vaccination, neutralizing antibodies were detected by both in vitro (microneutralization) and in vivo (lung viral titer) methods and were associated with increased IgG1 expression by enzyme-linked immunosorbent assay (ELISA). Vaccination with HA-VRP did not strongly stimulate either neutralizing or IgG1 antibodies but did induce IgG2a antibodies. Expression of IgG2a antibodies in this context correlated with clearance of virus and increased protection against lethal influenza challenge. Increased induction of both antibody isotypes as measured by ELISA was a better correlate for vaccine efficacy than neutralization alone. This study details separate but important roles for both IgG1 and IgG2a expression in vaccination against influenza and argues for the development of vaccine regimens that stimulate and measure expression of both antibody isotypes.

Cross-protective immunity in mice induced by live-attenuated or inactivated vaccines against highly pathogenic influenza A (H5N1) viruses

Because of the time required to identify and produce an antigenically well-matched pandemic vaccine, vaccines that offer broader cross-reactive immunity and protection are desirable. We have compared a live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV) based on a related H5 hemagglutinin (HA) from a nonpathogenic avian influenza virus, A/Duck/Pottsdam/1042-6/86 (H5N2), for the ability to induce cross-reactive immunity and/or cross-protective efficacy against a contemporary highly pathogenic H5N1 viruses. Both LAIV and IIV provided cross-protection from systemic infection, severe disease, and death following lethal challenges with antigenically distinct A/Vietnam/1203/2004 (VN/1203) virus. Substantial levels of serum anti-VN/1203 HA IgG were detected in mice that received either IIV or LAIV, while nasal wash anti-VN/1203 HA IgA was detected in mice that received LAIV. Formulation of IIV with alum adjuvant augmented neutralizing antibody responses and protective efficacy. These results demonstrated that vaccination of mice with H5 IIV or LAIV induced a high degree of cross-protection from illness and death following lethal challenges with a heterologous H5N1 virus.

Comparison of the influenza virus-specific effector and memory B-cell responses to immunization of children and adults with live attenuated or inactivated influenza virus vaccines

Cellular immune responses to influenza virus infection and influenza virus vaccination have not been rigorously characterized. We quantified the effector and memory B-cell responses in children and adults after administration of either live attenuated (LAIV) or inactivated (TIV) influenza virus vaccines and compared these to antibody responses. Peripheral blood mononuclear cells were collected at days 0, 7 to 12, and 27 to 42 after immunization of younger children (6 months to 4 years old), older children (5 to 9 years old), and adults. Influenza virus-specific effector immunoglobulin A (IgA) and IgG circulating antibody-secreting cells (ASC) and stimulated memory B cells were detected using an enzyme-linked immunospot assay. Circulating influenza virus-specific IgG and IgA ASC were detected 7 to 12 days after TIV and after LAIV immunization. Seventy-nine percent or more of adults and older children had demonstrable IgG ASC responses, while IgA ASC responses were detected in 29 to 53% of the subjects. The IgG ASC response rate to LAIV immunization in adults was significantly higher than the response rate measured by standard serum antibody assays (26.3% and 15.8% by neutralization and hemagglutination inhibition assays, respectively). IgG ASC and serum antibody responses were relatively low in the younger children compared to older children and adults. TIV, but not LAIV, significantly increased the percentage of circulating influenza virus-specific memory B cells detected at 27 to 42 days after immunization in children and adults. In conclusion, although both influenza vaccines are effective, we found significant differences in the B-cell and antibody responses elicited after LAIV or TIV immunization in adults and older children and between young children and older age groups.

Alternative routes of influenza vaccine delivery

The global emergence of virulent avian influenza and the concomitant raised threat of an influenza pandemic has increased interest in the development of improved influenza vaccines. Whereas conventional influenza vaccines are delivered by parenteral injection, an intranasal influenza vaccine has been marketed since 2003. Many other technologies are in development for intranasal, oral, epidermal and topical influenza vaccines. This editorial summarises the advances in clinical development of technologies for needle-free influenza vaccine delivery.

Live attenuated influenza virus expressing human interleukin-2 reveals increased immunogenic potential in young and aged hosts

Despite the reported efficacy of commercially available influenza virus vaccines, a considerable proportion of the human population does not respond well to vaccination. In an attempt to improve the immunogenicity of live influenza vaccines, an attenuated, cold-adapted (ca) influenza A virus expressing human interleukin-2 (IL-2) from the NS gene was generated. Intranasal immunization of young adult and aged mice with the IL-2-expressing virus resulted in markedly enhanced mucosal and cellular immune responses compared to those of mice immunized with the nonrecombinant ca parent strain. Interestingly, the mucosal immunoglobulin A (IgA) and CD8(+) T-cell responses in the respiratory compartment could be restored in aged mice primed with the IL-2-expressing virus to magnitudes similar to those in young adult mice. The immunomodulating effect of locally expressed IL-2 also gave rise to a systemic CD8(+) T-cell and distant urogenital IgA response in young adult mice, but this effect was less distinct in aged mice. Importantly, only mice immunized with the recombinant IL-2 virus were completely protected from a pathogenic wild-type virus challenge and revealed a stronger onset of virus-specific CD8(+) T-cell recall response. Our findings emphasize the potential of reverse genetics to improve the efficacy of live influenza vaccines, thus rendering them more suitable for high-risk age groups.

A model for immunological correlates of protection

Immunological assays measure characteristics of the immune system, such as antibody levels, specific to certain diseases. High assay values are often associated with protection from disease. A question of interest is how the relationship between assay values and subsequent development of disease should be quantitatively modelled. Existing approaches successfully model the relationship for high assay values, where the probability of developing disease is low. However at low assay values, the probability of developing disease is more closely associated with factors such as disease prevalence rates and an individual's chance of exposure to infection; these are less well captured by existing models. This paper presents a model that accommodates both assay values and factors independent of assay values, enabling protection from disease to be modelled over the whole range of assay values and proposing a method for predicting the efficacy of a vaccine from the assays of vaccinees and non-vaccinees.

Human leukocyte antigen polymorphisms: variable humoral immune responses to viral vaccines

Antibody formation in response to antigen stimulation remains the basis for measuring an individual's response and protection for most viral vaccines. A significant proportion of the variation in individual humoral immune response to vaccination appears to be genetic. The collection of genes found on chromosome 6 forming the human leukocyte antigen system provides one of the greatest sources of genetic variation in individuals with respect to their immunological responses. Recent research has demonstrated significant associations between vaccine response and human leukocyte antigen alleles. These associations not only explain why vaccine-induced humoral immune responses vary among individuals and between populations, but these variations may also hold the key to the development of future generations of vaccines.

Characterization of an influenza A H5N2 reassortant as a candidate for live-attenuated and inactivated vaccines against highly pathogenic H5N1 viruses with pandemic potential

We generated a high-growth 7:1 reassortant (Len17/H5) that contained the hemagglutinin (HA) gene from non-pathogenic A/Duck/Potsdam/1402-6/86 (H5N2) virus and other genes from the cold-adapted (ca) attenuated A/Leningrad/134/17/57 (H2H2) strain. Len17/H5 demonstrated an attenuated phenotype in mice and did not infect chickens. Mice administered Len17/H5 either as a live-attenuated intranasal vaccine or as an inactivated intramuscular vaccine were substantially protected from lethal challenge with highly pathogenic A/Hong Kong/483/97 (H5N1) virus and were protected from pulmonary infection with antigenically distinct A/Hong Kong/213/2003 (H5N1) virus. The cross-protective effect correlated with the levels of virus-specific mucosal IgA and/or serum IgG antibodies. Our results suggest a new strategy of using classical genetic reassortment between a high-growth ca H2N2 strain and antigenically related non-pathogenic avian viruses to prepare live-attenuated and inactivated vaccines for influenza pandemic.

T‐cell memory and recall responses to respiratory virus infections

The respiratory tract is characterized by its large surface area and the close association of an extensive vasculature with the external environment. As such, the respiratory tract is a major portal of entry for many pathogens. The immune system is able to effectively control most pulmonary pathogens and establish immunological memory that is capable of mediating an accelerated and enhanced recall response to secondary pathogen challenge. A key component of the recall response in the lung involves the rapid response of antigen-specific memory CD8+ T cells. Recent studies have shown that memory CD8+ T cells are extremely heterogeneous in terms of phenotype, function, anatomical distribution, and longevity. However, we have little understanding of how the different subsets of memory cells actually contribute to the recall response, especially with respect to peripheral or mucosal sites, such as the lung. Since immunological memory is the cornerstone of vaccination, it is essential that we understand how different memory CD8+ T-cell subsets are initially generated, maintained over time, and contribute to recall responses. This review focuses on memory T cells that mediate recall responses to influenza and parainfluenza virus infections in the lung.

New viral vaccines

Vaccination is the most effective medical intervention against diseases caused by human viral pathogens. Viral vaccines prevent or modify the severity of illness in the individual and interrupt or reduce the transmission of the pathogens to other susceptible people. Through these mechanisms, vaccines against smallpox, polio, measles and hepatitis B have had an enormous impact on world health over the last 50 years. Advances in basic virology and understanding of human immunity promise more progress in the control of human viral diseases as the 21st century begins. Some important targets, including human immunodeficiency virus, respiratory syncytial virus and hepatitis C virus present challenges that require more basic research. The purpose of this review is to highlight four new viral vaccines that have recently, or will soon demonstrate the effective translation of basic investigations into clinical benefits for disease control in healthy and high-risk populations. These vaccines include the live attenuated vaccines against the RNA viruses, rotavirus and influenza A and B, and vaccines against human papilloma virus and varicella-zoster virus, which are DNA viruses that cause morbidity and mortality through their capacity to establish persistent infection. Although only the influenza vaccine has been licensed in the United States, these other new tools for disease prevention are likely to be introduced within the next few years, with profound effects on the diseases that they cause. Hence, as Virology celebrates its 50th anniversary, it is appropriate to examine these examples of recent advances in viral vaccines.

Zur Schätzung der Schutzwirkung der Influenzaimpfung aus Surveillancedaten

The continuous antigenic drift of influenza viruses requires annual adaptation of the vaccine. Protection depends largely on the match of the variants represented in the vaccine with the viruses actually known to be in circulation and may differ considerably from season to season. Therefore studies to assess the efficacy and effectiveness of the vaccine are conducted rather sporadically on an annual basis and it would be desirable to make use of routinely available data from surveillance programs. We compared two different approaches: (1) the "screening method" where cases are identified from laboratory data and controls are taken from data on vaccination rates and (2) a second method that uses the same cases, but controls were influenza-negative individuals with influenza-like illness (also identified from laboratory data). The sensitivity of the methods to confounders that were considered as relevant was tested with a simulation. Both methods were applied to the data of the German influenza surveillance data of the season 2004/2005. The estimated effectiveness over all age groups was rather low with both methods, but comparable with other estimations from the literature. We observed differences in certain age groups between the methods as well as large differences between particular age groups within one method. Possible explanations are random variations due to low numbers in age strata and other influences not yet considered. Therefore the estimations should be interpreted with care; however, relative comparisons among seasons may still be meaningful.

Repeated influenza vaccination of healthy children and adults: borrow now, pay later?

A growing number of publications are recommending annual influenza vaccination of healthy children and adults. However, the long-term consequences of repeated influenza vaccination are unknown. We used a simple model of recurrent influenza infection to assess the likely impact of various repeated influenza vaccination scenarios. The model was based on a Markovian framework and was fitted on annual incidence rates of influenza infection by age. We found that natural influenza infection reduced the risk of being re-infected by 15.4% (95% confidence interval 7.1-23.0). Various scenarios of repeated influenza vaccination were then simulated and compared with a reference scenario where vaccination is given from age 65 years onwards. We show that repeated vaccination at a young age substantially increases the risk of influenza in older age, by a factor ranging between 1.2 (vaccination after 50 years) to 2.4 (vaccination from birth). These findings have important implications for influenza vaccination policies.

Vaccines for preventing influenza in healthy children

BACKGROUND

In children and adults the consequences of influenza are mainly absences from school and work, however the risk of complications is greatest in children and people over 65 years old.

OBJECTIVES

To appraise all comparative studies evaluating the effects of influenza vaccines in healthy children; assess vaccine efficacy (prevention of confirmed influenza) and effectiveness (prevention of influenza-like illness) and document adverse events associated with receiving influenza vaccines.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 1, 2005); OLD MEDLINE (1966 to 1969); MEDLINE (1969 to December 2004); EMBASE (1974 to December 2004); Biological Abstracts (1969 to December 2004); and Science Citation Index (1974 to December 2004). We wrote to vaccine manufacturers and a number of corresponding authors of studies in the review.

SELECTION CRITERIA

Any randomised controlled trials (RCTs), cohort and case-control studies of any influenza vaccine in healthy children under 16 years old.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trial quality and extracted data.

MAIN RESULTS

Fifty-one studies involving 263,987 children were included. Seventeen papers were translated from Russian. Fourteen RCTs and 11 cohort studies were included in the analysis of vaccine efficacy and effectiveness. From RCTs, live vaccines showed an efficacy of 79% (95% confidence interval (CI) 48% to 92%) and an effectiveness of 33% (95% CI 28% to 38%) in children older than two years compared with placebo or no intervention. Inactivated vaccines had a lower efficacy of 59% (95% CI 41% to 71%) than live vaccines but similar effectiveness: 36% (95% CI 24% to 46%). In children under two, the efficacy of inactivated vaccine was similar to placebo. Thirty-four reports containing safety outcomes were included, 22 including live vaccines, 8 inactivated vaccines and 4 both types. The most commonly presented short-term outcomes were temperature and local reactions. The variability in design of studies and presentation of data was such that meta-analysis of safety outcome data was not feasible.

AUTHORS' CONCLUSIONS

Influenza vaccines are efficacious in children older than two years but little evidence is available for children under two. There was a marked difference between vaccine efficacy and effectiveness. That no safety comparisons could be carried out emphasizes the need for standardisation of methods and presentation of vaccine safety data in future studies. It was surprising to find only one study of inactivated vaccine in children under two years, given recent recommendations to vaccinate healthy children from six months old in the USA and Canada. If immunisation in children is to be recommended as public-health policy, large-scale studies assessing important outcomes and directly comparing vaccine types are urgently required.

A multiplex RT-PCR method for screening of reassortant live influenza vaccine virus strains

A system based on reverse transcription polymerase chain reaction (RT-PCR) of the RNA genome was established to identify genetic composition of influenza viruses generated by reassortment between an attenuated donor virus and virulent wild type virus. The primers were designed, by multiple sequence alignment of variable regions, specific for cold-adapted donor virus HTCA-A 101, as compared to other influenza A viruses. The specificity of each primer set was confirmed and the primers were combined to perform RT-PCR in multiplex manner. The multiplex PCR was adopted to distinguish the 6:2 reassortant viruses containing six internal genome segments of attenuated donor virus and two surface antigens of virulent strain from the wild type viruses. The method allowed us to optimize the reassorting process on a routine basis and to confirm the selection of reassortant clones efficiently. The method is suitable for analyzing the contribution of specific gene segments for growth and attenuating characteristics and for generation of live attenuated vaccine by annual reassortment.

Antimicrobial therapy in childhood asthma and wheezing

There is an increasing number of viral and bacterial pathogens suspected of contributing to asthma pathogenesis in childhood, making it more difficult for the practitioner to make specific therapy decisions. This review discusses the role of viruses, e.g. respiratory syncytial virus, human metapneumovirus, influenza viruses and rhinoviruses, as well as the role of the atypical bacteria Chlamydophila pneumoniae and Mycoplasma pneumoniae, as contributors to childhood asthma. Diagnosis, prevention, and therapy are discussed, including a summary of drugs, i.e. macrolide antibacterials, antivirals, and vaccine regimens already available, or at least in clinical trials. For the practitioner dealing with patients every day, drug regimens are assigned to the individual pathogens and an algorithm for the management of atypical infections in patients with asthma or recurrent wheezing is presented.

The use of a systemic prime/mucosal boost strategy with an equine influenza ISCOM vaccine to induce protective immunity in horses

In horses, natural infection confers long lasting protective immunity characterised by mucosal IgA and humoral IgGa and IgGb responses. In order to investigate the potential of locally administered vaccine to induce a protective IgA response, responses generated by vaccination with an immunostimulating complex (ISCOM)-based vaccine for equine influenza (EQUIP F) containing A/eq/Newmarket/77 (H7N7), A/eq/Borlänge/91 (H3N8) and A/eq/Kentucky/98 (H3N8) using a systemic prime/mucosal boost strategy were studied. Seven ponies in the vaccine group received EQUIP F vaccine intranasally 6 weeks after an initial intramuscular immunisation. Following intranasal boosting a transient increase in virus-specific IgA was detected in nasal wash secretions. Aerosol challenge with the A/eq/Newmarket/1/93 reference strain 4 weeks after the intranasal booster resulted in clinical signs of infection and viral shedding in seven of seven influenza-naive control animals whereas the seven vaccinated ponies had statistically significantly reduced clinical signs and duration of virus excretion. Furthermore, following this challenge, significantly enhanced levels of virus-specific IgA were detected in the nasal washes from vaccinated ponies compared with the unvaccinated control animals. These data indicate that the intranasal administration of EQUIP F vaccine primes the mucosal system for an enhanced IgA response following exposure to live influenza virus.

[Antiviral vaccination and respiratory mucosal immunity: still disappointing results from a seductive idea]

Mucosal surfaces of the respiratory tract represent a major portal of entry for most human viruses and a critical component of the mammalian immunologic repertoire. The major antibody isotype in external secretions is secretory immunoglobin A (S-IgA). The major effector cells in mucosal surfaces, however, are not IgA B cells, but T lymphocytes, which may account for up to 80% of the mucosal lymphoid cell population. Mucosal immunoprophylaxis is theoretically an important approach to control infections acquired through these portals. Passive antibodies can protect against mucosal viral infections, as shown for respiratory syncytial virus, but very high quantities of passive antibodies are needed to restrict virus replication on mucosal surface. Factors likely to induce mucosal antibody and cell-mediated immune responses include oral or respiratory routes of immunization and active (effectively replicating) vaccine agents. Very few antiviral vaccines have been developed to protect the mucosal surface of the respiratory tract, and only an attenuated influenza virus vaccine uses the nasal route. Other vaccines, approved for parenteral use, have been administered experimentally by the nasal route; these include active (replicating) and inactive (nonreplicating) vaccines. By this route they induce only a moderate local mucosal response. Neither the development of mucosal immunity nor the administration of vaccines via the mucosal route is essential for control or prevention of most respiratory viral infections and diseases acquired through the respiratory tract. Nonetheless, the example of the live attenuated intranasal influenza vaccine, which induces both systemic and local immune response, is promising for the future of mucosal immunization against respiratory viral infections.