Evaluation of cold-recombinant influenza A/Korea (CR-59) virus vaccine in infants

Dose-response assessment for influenza A virus based on data sets of infection with its live attenuated reassortants

Reported data sets on infection of volunteers challenged with wild-type influenza A virus at graded doses are few. Alternatively, we aimed at developing a dose-response assessment for this virus based on the data sets for its live attenuated reassortants. Eleven data sets for live attenuated reassortants that were fit to beta-Poisson and exponential dose-response models. Dose-response relationships for those reassortants were characterized by pooling analysis of the data sets with respect to virus subtype (H1N1 or H3N2), attenuation method (cold-adapted or avian-human gene reassortment), and human age (adults or children). Furthermore, by comparing the above data sets to a limited number of reported data sets for wild-type virus, we quantified the degree of attenuation of wild-type virus with gene reassortment and estimated its infectivity. As a result, dose-response relationships of all reassortants were best described by a beta-Poisson model. Virus subtype and human age were significant factors determining the dose-response relationship, whereas attenuation method affected only the relationship of H1N1 virus infection to adults. The data sets for H3N2 wild-type virus could be pooled with those for its reassortants on the assumption that the gene reassortment attenuates wild-type virus by at least 63 times and most likely 1,070 times. Considering this most likely degree of attenuation, 10% infectious dose of H3N2 wild-type virus for adults was estimated at 18 TCID50 (95% CI = 8.8-35 TCID50). The infectivity of wild-type H1N1 virus remains unknown as the data set pooling was unsuccessful.

Safety of the trivalent, cold-adapted influenza vaccine in preschool-aged children

OBJECTIVE

To provide additional information on the safety of trivalent, cold-adapted influenza vaccine (CAIV-T) in children.

METHODS

Children 15 to 71 months of age were enrolled in a multicenter, prospective, randomized, double-blind, and placebo-controlled trial to receive by nasal spray CAIV-T or placebo. In year 1 (1996-1997), 1314 were enrolled in the 2-dose cohort and 288 were enrolled in the 1-dose cohort. In year 2 (1997-1998), 1358 of the original participants received 1 dose of vaccine or placebo according to their original treatment group assignment. In year 3 (1998-1999) and year 4, the trial continued as an open-label safety trial of CAIV-T. A total of 642 and 549 children enrolled in years 3 and 4, respectively, received their third and fourth sequential annual doses of CAIV-T. Measured were 1) the occurrence of specific respiratory, gastrointestinal and systemic symptoms, unexpected symptoms (not specified in the diary card), and use of medications within the first 10 days after vaccination; 2) the occurrence of an acute illness and use of medication within 11 to 42 days after vaccination; and 3) the occurrence of serious adverse events within 42 days after vaccination.

RESULTS

The adjusted odd ratios of specific respiratory and gastrointestinal symptoms during the 10 days after vaccination were determined in years 1 and 2. Runny nose or nasal congestion, vomiting, muscle aches, and fever were significantly associated with the first dose of CAIV-T. With the second dose, runny nose was the only symptom that was associated with CAIV-T. In year 2, CAIV-T did not cause excess in any of the specific respiratory and gastrointestinal symptoms. In years 3 and 4, specific respiratory and gastrointestinal symptoms were comparable to that observed in year 2. A CAIV-T-associated symptom was most likely to occur on day 2 with the first dose of vaccine. The occurrence of unexpected symptoms was primarily of the gastrointestinal system. Approximately 6% of CAIV-T and 3.6% of placebo recipients had a gastrointestinal symptom. CAIV-T seemed to be associated with a mild excess in abdominal pain and vomiting only with the first vaccine dose. A statistically significant increase in the use of analgesics/antipyretics was detected only with the first dose in CAIV-T vaccinees compared with placebo recipients (23.5% vs 16.6%). Between days 11 and 42, CAIV-T use was not associated with an excess of illness, otitis media, or use of medication. None of the 6 serious adverse events in CAIV-T recipients in years 1 to 4 was attributed to the vaccine.

CONCLUSIONS

CAIV-T was safe in children. Mild respiratory, gastrointestinal, and systemic symptoms of short duration were observed in a minority of children and primarily with the first vaccine dose. Sequential annual doses of CAIV-T were well tolerated.

Universal vaccination of healthy children against influenza: a role for the cold-adapted intranasal influenza vaccine

The incidence of influenza in children well exceeds that of the elderly and has been identified as the basis for 20% of doctor visits for children during the winter. The disease results in over 100 hospitalizations per 100000 person-months in children <2 years of age. Furthermore, children serve as the major vector in the community; thus, influenza in children results in significant costs to society. Although efficacious, the current intramuscular, inactivated influenza vaccine is infrequently used in children, and is currently targeted only at children at high risk and those who are household members of such individuals. Experts believe that vaccinating only high risk individuals has little impact on the cycle of annual epidemics, but that universal vaccination of children may very well have a substantial impact. Experimental data support this. A recently published cost-benefit analysis indicated that routine, school-aged vaccination through individual visits to a clinician would save 4 US dollars per child vaccinated. A group program such as a school-based one would save 35 US dollars. One obstacle to universal vaccination includes the real and perceived resistance to the addition of yet another annual injection to the already crowded schedule of routine childhood immunizations. Nearing licensure is an intranasal, live attenuated, cold-adapted intranasal influenza vaccine. Cold-adaptation prevents replication in the lower respiratory tract. Trials have demonstrated immunogenicity, safety, and tolerability in adults as well as children. Placebo-controlled trials have shown efficacy rates of 83 to 94%. This novel vaccine addresses obstacles to universal childhood immunization and would permit a program of routine use that would dramatically reduce transmission and stem epidemics of influenza.

Safety of the trivalent, cold-adapted influenza vaccine (CAIV-T) in children

The trivalent, cold-adapted influenza vaccine (CAIV-T, FluMist, Aviron, Mountain View, CA) is a live attenuated influenza virus vaccine that is administered by nasal spray. CAIV-T is efficacious in preventing influenza virus infection. The vaccine was submitted to the Food and Drug Administration for licensure in healthy children and adults. Universal immunization is being considered in children, and an effective vaccine with minimal adverse reactions is thus required. The published studies on the safety of CAIV-T in children reviewed in this article were clinical trials sponsored by the National Institutes of Health (NIH) conducted in children from 1975 to 1991, clinical trials from 1991 to 1993 sponsored by a cooperative agreement between NIH and Wyeth-Ayerst Research, and clinical trials from 1995 to the present sponsored by a cooperative agreement between NIH and Aviron. Safety assessments included the occurrence of: 1) specific influenza-like symptoms, unexpected symptoms, and use of medications within the first 10 days after vaccination; 2) acute illness and use of medication within 11 to 42 days postvaccination; 3) serious adverse events and rare events within 42 days after vaccination; 4) healthcare utilization within 14 days after vaccination; and 5) acute respiratory symptoms with annual sequential vaccine doses. CAIV-T was safe and well-tolerated. Transient, mild respiratory symptoms were observed in a minority (10%-15%) of children and primarily with the first CAIV-T dose. Vomiting and abdominal pain occurred in fewer than 2 percent of CAIV-T recipients. The gastrointestinal symptoms were mild and of short duration. An excess of illness or use of medication was not observed after the 10th day of vaccination. Sequential annual doses of CAIV-T were well-tolerated and not associated with increased reactogenicity. CAIV-T did not cause an increase in healthcare utilization. Thus CAIV-T is safe in healthy children and should complement the use of inactivated influenza vaccine, trivalent (IIV-T) in children with underlying chronic conditions.

Promises and challenges of live-attenuated intranasal influenza vaccines across the age spectrum: a review

Despite the availability of inactivated influenza vaccines, influenza continues to cause considerable mortality in the elderly, and morbidity in all age groups. Cold-adapted, live-attenuated, intranasally administered influenza vaccines, first developed in the 1960s, have been tested in more than 10,000 volunteers and have been shown to be safe, well-tolerated, and immunogenic. Recent trials suggest that efficacy in children may be superior to that of inactivated vaccines, and efficacy in healthy adults may be similar to that of inactivated vaccines, although there are limited data comparing the two vaccines directly. Advantages of the live-attenuated vaccines include acceptability, ease of administration, and the potential for mass immunization. The possibility of substantially higher vaccination rates across all age groups brings promise for the development of herd immunity and greatly improved control of influenza in the future.

Influenza vaccines: present and future

Immunization is the most feasible method for preventing influenza. Vaccination against influenza is recommended for everyone 65 years of age and older and for persons less than 65 years of age who are at risk for developing complications of influenza. Immune correlates of protection have been established, and a global network is in place to monitor the appearance and circulation of antigenic variants of influenza viruses, as well as the appearance of novel subtypes of influenza A. Antigenic and genetic analyses of circulating viruses and testing of serum from vaccine recipients guide vaccine composition updates. The efficacy of influenza vaccines depends in part on the closeness of the antigenic match between the vaccine strain and the epidemic strain. Currently licensed influenza vaccines are trivalent, formalin-inactivated, egg-derived vaccines; their efficacy ranges from 70 to 90% in young, healthy populations when there is a close antigenic match between vaccine strains and epidemic strains. Development of intranasally administered alternative vaccines and improvement of the existing vaccine are areas of active research. A trivalent, ca live vaccine is the most promising LAIV candidate. In a field trial, efficacy rates of LAIV in young children were 96% against influenza A (H3N2) and 91% against influenza B. However, few data are available to compare this formulation of the trivalent ca live vaccine with the trivalent, inactivated vaccine. Influenza vaccine recommendations will most likely be revised on licensure of LAIV; each vaccine may offer distinct advantages in specific populations.

Immune responses of infants to infection with respiratory viruses and live attenuated respiratory virus candidate vaccines

Respiratory viruses such as respiratory syncytial virus (RSV), the parainfluenza viruses (PIV), and the influenza viruses cause severe lower respiratory tract diseases in infants and children throughout the world. Experimental live attenuated vaccines for each of these viruses are being developed for intranasal administration in the first weeks or months of life. A variety of promising RSV, PIV-3, and influenza virus vaccine strains have been developed by classical biological methods, evaluated extensively in preclinical and clinical studies, and shown to be attenuated and genetically stable. The ongoing clinical evaluation of these vaccine candidates, coupled with recent major advances in the ability to develop genetically engineered viruses with specified mutations, may allow the rapid development of respiratory virus strains that possess ideal levels of replicative capacity and genetic stability in vivo. A major remaining obstacle to successful immunization of infants against respiratory virus associated disease may be the relatively poor immune response of very young infants to primary virus infection. This paper reviews the immune correlates of protection against disease caused by these viruses, immune responses of infants to naturally-acquired infection, and immune responses of infants to experimental infection with candidate vaccine viruses.

Cytotoxic T lymphocyte responses of infants after natural infection or immunization with live cold-recombinant or inactivated influenza A virus vaccine

The cytotoxic T lymphocyte (CTL) response of infants after immunization with either inactivated trivalent subvirion vaccine (TIV) or bivalent attenuated cold-recombinant (CR) vaccine or occurrence of natural influenza virus infection were compared in a blinded, placebo-controlled study during the 1987-1988 and 1988-1989 influenza epidemic seasons. Healthy infants between 6 and 13 months of age were randomly assigned and administered a single dose of intranasal bivalent (A/H3N2/A/H1N1) CR vaccine, a two-dose regimen of TIV (A/H3N2/A/H1N1/B) influenza vaccine, or placebo. Peripheral blood lymphocytes were obtained prior to and 2-8 weeks after vaccination and at the end of the epidemic season and stimulated with virus in vitro for 6 or 7 days. Lysis of autologous virus-infected target cells was assessed in a 4 hr 51Cr release assay. MHC class I-restricted influenza A-specific CTL was stimulated following natural influenza A virus infection but not after immunization with CR influenza A virus vaccine or TIV. These results demonstrate for the first time induction of influenza virus-specific CTL activity in infants under 1 year of age.

Studies on reactogenicity and immunogenicity of attenuated bivalent cold recombinant influenza type A (CRA) and inactivated trivalent influenza virus (TI) vaccines in infants and young children

Fifty-two infants seronegative to or without prior infection with influenza type A viruses were enrolled in a study to evaluate reactogenicity and immunogenicity of three bivalent cold recombinant type A (CRA) and two trivalent inactivated influenza (TI) vaccines. Controls consisted of infants receiving normal saline by nose drops (Pli.n.) or intramuscularly (Pli.m.). CRA and TI vaccines were monitored for local and systemic reactions after vaccination. Serum specimens obtained prior to and 6 weeks postvaccination were analysed for neutralizing antibody to influenza H1N1 and H3N2 viruses. CRA vaccines and Pli.n. recipients had similar numbers of acute respiratory infections and comparable rates of illnesses during the trial. Significantly fewer CRA vaccinees without an intercurrent viral infection had fever (0/16 versus 4/10, p = 0.04) and cough (4/16 versus 9/10, p = 0.002) than CRA vaccinees with a confirmed intercurrent viral infection. Recipients of TI vaccine and Pli.m. did not develop reactions at the injection site. For each of the CRA vaccines tested, a dominant CRA virus was identified. The dominant CRA viruses were isolated from a greater number of infants or for a longer duration than the non-dominant CRA viruses. All 14 non-dominant CRA viruses were recovered from infants within the first week after vaccination; 24 of 77 dominant CRA viruses were recovered more than 7 days after vaccination. The immunogenicity of CRA vaccines was not affected by a confirmed intercurrent viral infection or low titres of influenza-specific antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of a cold-adapted influenza B/Texas/84 reassortant virus (CRB-87) vaccine in young children

A cold-adapted (ca) influenza B reassortant virus vaccine that contained the six internal RNA segments from influenza B/Ann Arbor/1/66 ca virus and the neuraminidase and hemagglutinin genes from wild-type influenza B/Texas/1/84 virus was evaluated in children ranging in age from 8 months to 14 years. The children were vaccinated intranasally with doses ranging from 10(3.2) to 10(6.2) 50% tissue culture infective doses (TCID50). Thirty children were seropositive, and 26 were seronegative. Thirty-three children participated as unvaccinated controls. The vaccine was well tolerated by both seronegative and seropositive children. The amount of virus required to infect 50% of seronegative children was approximately 10(4.5) TCID50. Vaccine viruses recovered from airway secretions retained temperature-sensitive and cold-adapted characteristics. The results of this study indicate that the vaccine virus, influenza B/Texas/84 ca reassortant virus, is attenuated, immunogenic, and phenotypically stable when given to young seronegative children.