Immunization of children with pertussis toxoid decreases spread of pertussis within the family

Protecting the herd with vaccination

How much do COVID-19 vaccines reduce transmission? The answer is a moving target.

Estimating population-level effects of the acellular pertussis vaccine using routinely collected immunization data

BACKGROUND

Measuring and reporting the different population-level effects of the acellular pertussis vaccine on pertussis disease in addition to direct effects can increase the cost-effectiveness of a vaccine.

METHODS

We conducted a retrospective cohort study of children born between January 1, 2008, and December 31, 2017, in King County, Washington, who were enrolled in the Washington State Immunization Information System. Diphtheria-Tetanus-acellular-Pertussis (DTaP) vaccination data from WA-IIS was linked with pertussis case data from Public Health Seattle and King County. Census-level vaccination coverage was estimated as proportion of age-appropriately vaccinated children residing in it. Direct vaccine effectiveness was estimated by comparing pertussis risk in fully-vaccinated and under-vaccinated children. Population-level vaccine effects were estimated by comparing pertussis risk in census tracts in the highest vaccination coverage quartile to that in the lowest vaccination coverage quartile.

RESULTS

For direct protection, estimated vaccine effectiveness was 76% (95% CI: 63% - 84%) in low vaccination coverage clusters and it decreased to 47% (95% CI: 13% - 68%) in high vaccination coverage clusters, after adjusting for potential confounders. The estimated indirect effect was 45.0% (95% CI: 1%, 70%), total effect was 93.9% (95% CI: 91%, 96%), and overall effect was 42.2% (95% CI: 19%, 60%).

CONCLUSION

Our findings suggest that DTaP vaccination provided direct as well as indirect protection in the highly immunized King County, WA. Routine DTaP vaccination programs may have the potential to provide not only protection for vaccinated individuals but also for the under-vaccinated individuals living in the same area.

The relationship between mucosal immunity, nasopharyngeal carriage, asymptomatic transmission and the resurgence of Bordetella pertussis

The incidence of whooping cough in the US has been rising slowly since the 1970s, but the pace of this has accelerated sharply since acellular pertussis vaccines replaced the earlier whole cell vaccines in the late 1990s. A similar trend occurred in many other countries, including the UK, Canada, Australia, Ireland, and Spain, following the switch to acellular vaccines. The key question is why. Two leading theories (short duration of protective immunologic persistence and evolutionary shifts in the pathogen to evade the vaccine) explain some but not all of these shifts, suggesting that other factors may also be important.

In this synthesis, we argue that sterilizing mucosal immunity that blocks or abbreviates the duration of nasopharyngeal carriage of Bordetella pertussis and impedes person-to-person transmission (including between asymptomatically infected individuals) is a critical factor in this dynamic. Moreover, we argue that the ability to induce such mucosal immunity is fundamentally what distinguishes whole cell and acellular pertussis vaccines and may be pivotal to understanding much of the resurgence of this disease in many countries that adopted acellular vaccines. Additionally, we offer the hypothesis that observed herd effects generated by acellular vaccines may reflect a modification of disease presentation leading to reduced potential for transmission by those already infected, as opposed to inducing resistance to infection among those who have been exposed.

The pertussis enigma: reconciling epidemiology, immunology and evolution

Pertussis, a highly contagious respiratory infection, remains a public health priority despite the availability of vaccines for 70 years. Still a leading cause of mortality in developing countries, pertussis has re-emerged in several developed countries with high vaccination coverage. Resurgence of pertussis in these countries has routinely been attributed to increased awareness of the disease, imperfect vaccinal protection or high infection rates in adults. In this review, we first present 1980-2012 incidence data from 63 countries and show that pertussis resurgence is not universal. We further argue that the large geographical variation in trends probably precludes a simple explanation, such as the transition from whole-cell to acellular pertussis vaccines. Reviewing available evidence, we then propose that prevailing views on pertussis epidemiology are inconsistent with both historical and contemporary data. Indeed, we summarize epidemiological evidence showing that natural infection and vaccination both appear to provide long-term protection against transmission and disease, so that previously infected or vaccinated adults contribute little to overall transmission at a population level. Finally, we identify several promising avenues that may lead to a consistent explanation of global pertussis epidemiology and to more effective control strategies.

Dissecting the indirect effects caused by vaccines into the basic elements

Vaccination directly protects vaccinated individuals, but it also has the potential for indirectly protecting the unvaccinated in a population (herd protection). Unintended negative consequences such as the re-manifestation of infection, mainly expressed as age shifts, result from vaccination programs as well. We discuss the necessary conditions for achieving optimal herd protection (i.e., high quality vaccine-induced immunity, substantial effect on the force of infection, and appropriate vaccine coverage and distribution), as well as the conditions under which age shifts are likely to occur. We show examples to illustrate these effects. Substantial ambiguity in observing and quantifying these indirect vaccine effects makes accurate evaluation troublesome even though the nature of these outcomes may be critical for accurate assessment of the economic value when decision makers are evaluating a novel vaccine for introduction into a particular region or population group. More investigation is needed to identify and develop successful assessment methodologies for precisely analyzing these outcomes.

Toward a new vaccine for pertussis

To overcome the limitations of the current pertussis vaccines, those of limited duration of action and failure to induce direct killing of Bordetella pertussis, a synthetic scheme was devised for preparing a conjugate vaccine composed of the Bordetella bronchiseptica core oligosaccharide with one terminal trisaccharide to aminooxylated BSA via their terminal ketodeoxyoctanate residues. Conjugate-induced antibodies, by a fraction of an estimated human dose injected into young outbred mice as a saline solution, were bactericidal against B. pertussis, and their titers correlated with their ELISA values. The carrier protein is planned to be genetically altered pertussis toxoid. Such conjugates are easy to prepare, stable, and should add both to the level and duration of immunity induced by current vaccine-induced pertussis antibodies and reduce the circulation of B. pertussis.

Herd protection by a bivalent killed whole-cell oral cholera vaccine in the slums of Kolkata, India

BACKGROUND

We evaluated the herd protection conferred by an oral cholera vaccine using 2 approaches: cluster design and geographic information system (GIS) design.

METHODS

Residents living in 3933 dwellings (clusters) in Kolkata, India, were cluster-randomized to receive either cholera vaccine or oral placebo. Nonpregnant residents aged≥1 year were invited to participate in the trial. Only the first episode of cholera detected for a subject between 14 and 1095 days after a second dose was considered. In the cluster design, indirect protection was assessed by comparing the incidence of cholera among nonparticipants in vaccine clusters vs those in placebo clusters. In the GIS analysis, herd protection was assessed by evaluating association between vaccine coverage among the population residing within 250 m of the household and the occurrence of cholera in that population.

RESULTS

Among 107 347 eligible residents, 66 990 received 2 doses of either cholera vaccine or placebo. In the cluster design, the 3-year data showed significant total protection (66% protection, 95% confidence interval [CI], 50%-78%, P<.01) but no evidence of indirect protection. With the GIS approach, the risk of cholera among placebo recipients was inversely related to neighborhood-level vaccine coverage, and the trend was highly significant (P<.01). This relationship held in multivariable models that also controlled for potentially confounding demographic variables (hazard ratio, 0.94 [95% CI, .90-.98]; P<.01).

CONCLUSIONS

Indirect protection was evident in analyses using the GIS approach but not the cluster design approach, likely owing to considerable transmission of cholera between clusters, which would vitiate herd protection in the cluster analyses.

CLINICAL TRIALS REGISTRATION

NCT00289224.

Components of the indirect effect in vaccine trials: identification of contagion and infectiousness effects

Vaccination of one person may prevent the infection of another either because the vaccine prevents the first from being infected and from infecting the second, or because, even if the first person is infected, the vaccine may render the infection less infectious. We might refer to the first of these mechanisms as a contagion effect and the second as an infectiousness effect. In the simple setting of a randomized vaccine trial with households of size two, we use counterfactual theory under interference to provide formal definitions of a contagion effect and an unconditional infectiousness effect. Using ideas analogous to mediation analysis, we show that the indirect effect (the effect of one person's vaccine on another's outcome) can be decomposed into a contagion effect and an unconditional infectiousness effect on the risk difference, risk ratio, odds ratio, and vaccine efficacy scales. We provide identification assumptions for such contagion and unconditional infectiousness effects and describe a simple statistical technique to estimate these effects when they are identified. We also give a sensitivity analysis technique to assess how inferences would change under violations of the identification assumptions. The concepts and results of this paper are illustrated with hypothetical vaccine trial data.

Causal inference for vaccine effects on infectiousness

If a vaccine does not protect individuals completely against infection, it could still reduce infectiousness of infected vaccinated individuals to others. Typically, vaccine efficacy for infectiousness is estimated based on contrasts between the transmission risk to susceptible individuals from infected vaccinated individuals compared with that from infected unvaccinated individuals. Such estimates are problematic, however, because they are subject to selection bias and do not have a causal interpretation. Here, we develop causal estimands for vaccine efficacy for infectiousness for four different scenarios of populations of transmission units of size two. These causal estimands incorporate both principal stratification, based on the joint potential infection outcomes under vaccine and control, and interference between individuals within transmission units. In the most general scenario, both individuals can be exposed to infection outside the transmission unit and both can be assigned either vaccine or control. The three other scenarios are special cases of the general scenario where only one individual is exposed outside the transmission unit or can be assigned vaccine. The causal estimands for vaccine efficacy for infectiousness are well defined only within certain principal strata and, in general, are identifiable only with strong unverifiable assumptions. Nonetheless, the observed data do provide some information, and we derive large sample bounds on the causal vaccine efficacy for infectiousness estimands. An example of the type of data observed in a study to estimate vaccine efficacy for infectiousness is analyzed in the causal inference framework we developed.

New approaches to the assessment of vaccine herd protection in clinical trials

Criteria for the introduction of new vaccines into routine public health practice are becoming increasingly stringent. For vaccines that are expensive and those that provide moderate protection, the ability to confer herd protection could be crucial to policy deliberations about vaccine introduction. Traditionally, herd protection has been assessed after a vaccine is introduced, delaying the availability of data on herd effects to inform decisions about vaccine introduction. New methodological developments now provide the possibility to assess herd protection before the introduction of a vaccine into public health programmes. One approach is a cluster-randomised trial, which allows assessment of herd protection in a way that minimises biases. Analysis of individually randomised trials by appropriately selected clusters created post hoc can also provide measurements of herd protection. Here we discuss the use of these designs, which can generate an improved evidence base at an early stage for making decisions about the introduction of new vaccines.

Seroprevalence of pertussis antitoxin (anti-PT) in Sweden before and 10 years after the introduction of a universal childhood pertussis vaccination program

The prevalence of IgG ELISA antibodies against pertussis toxin (anti-PT) was studied in two Swedish seroepidemiological studies. One was performed in 1997 when the new pertussis vaccination program was 1 year old (n = 3420). In 2007, when Pa vaccines had been used countrywide for 10 years in the universal child vaccination program, this study was repeated to analyze the effect of vaccination on anti-PT prevalence (n = 2379). Before the statistical analysis of seroprevalence, children vaccinated within the last 2 years before the serosurveys were excluded. The results indicate a reduced exposure to Bordetella pertussis in the population. The proportion of sera without measurable anti-PT antibodies increased significantly, aggregated over all comparable age groups, from 3.8% in people sampled in 1997 to 16.3% in people sampled in 2007. For cord blood, 1% was without measurable anti-PT antibodies in 1997 compared to a significantly higher level, 12%, in 2007. With anti-PT concentrations of ≥50 and ≥100 EU/ml as cutoff points for ‘recent infection’ the proportion above the cutoff points for younger children was significantly higher in 1997 than in 2007 at both cutoff points. For all adults, 20 years of age and older, the difference in proportions above the lower cutoff point was close to statistically significant, comparing 1997 with 2007. This was not the case at 100 EU/ml. In the 1997 samples of children, there was a significant downward trend of ‘recent infections’ at both cutoff points for three sampled age groups between 5 and 15 years of age from 21% at 5.0–5.5 years of age to 7% at 14.7–15.7 years for the lowest cutoff. In the 2007 samples of children, on the contrary, there was a significant continuous upward trend of ‘recent infections’, at both cutoff points, for four sampled age groups between 4 and 18 years of age – from 4% at 4–5 years of age to 16% at 17–18 years at the lowest cutoff. The continuous increase, with age of children with high anti-PT concentrations, supports the recent change in the general Swedish childhood vaccination program to include a pre-school booster at 5–6 years and a school-leaving booster at 14–16 years of age.

Pertussis sources of infection and routes of transmission in the vaccination era

Vaccination against pertussis has resulted in reduction of the infection pressure of Bordetella pertussis (partial herd immunity), but the circulation of B. pertussis has persisted as a consequence of waning of vaccine-induced and naturally acquired immunity. An increase in the reported incidence of B. pertussis infection in older children, adolescents and young adults has been noted, resulting in a perceived resurgence of the disease in these age groups. Regardless of whether this resurgence is real or not, older groups are increasingly recognized as playing an important role in transmitting B. pertussis infection to incompletely immunized infants, in whom pertussis disease continues to cause severe and fatal illness, albeit at much lower levels than in the prevaccine era. Several studies have suggested that mothers, in particular, are a significant source of infection for infants. Adolescents, grandparents and health care workers can also play a role. By contrast, most adolescents acquire the infection from schoolmates and friends, whereas for adults the main sources are children and work colleagues. Furthermore teachers, child care workers and health care workers could be at increased risk of being exposed to, and transmitting, B. pertussis infection. Current immunization strategies inadequately control the circulation of B. pertussis, in part because of suboptimal adherence to current pediatric immunization guidelines. In addition to efforts to improve pertussis immunization rates in children, the expansion of pertussis immunization to target specific groups should be considered. Besides reducing morbidity in the targeted groups, these strategies could decrease the residual burden of pertussis morbidity and mortality in infants.

Duration of immunity against pertussis after natural infection or vaccination

Despite decades of high vaccination coverage, pertussis has remained endemic and reemerged as a public health problem in many countries in the past 2 decades. Waning of vaccine-induced immunity has been cited as one of the reasons for the observed epidemiologic trend. A review of the published data on duration of immunity reveals estimates that infection-acquired immunity against pertussis disease wanes after 4-20 years and protective immunity after vaccination wanes after 4-12 years. Further research into the rate of waning of vaccine-acquired immunity will help determine the optimal timing and frequency of booster immunizations and their role in pertussis control.

The potential cost-effectiveness of acellular pertussis booster vaccination in England and Wales

A cost-effectiveness analysis of the introduction of acellular pertussis booster doses at either 4 or 15 years of age was performed. A transmission dynamic model was used to predict the level of indirect protection in those too young to be vaccinated. Multivariate sensitivity analyses were performed. In England and Wales there are an estimated 35,000 general practitioner (GP) consultations, 5500 inpatient days, and nine deaths annually attributable to pertussis, despite high levels of coverage for the primary course (approximately 95%). Around 80% of the bed-days and 90% of the deaths occur in those too young to be immunised (< 3 months of age). The introduction of acellular booster doses at 4 years is expected to reduce morbidity and mortality in the younger age groups by 40-100%, and at 15 years by 0-100%. From the perspective of the health care provider, roughly 50% of the simulations result in a cost per life-year gained of less than 10,000 pounds for vaccination at 4 years, the corresponding proportion for vaccination at 15 years being only 35%. Apart from the degree of indirect protection the model was most sensitive to the discount rate, the price of the vaccine, and the mortality rate. Significant uncertainty remains regarding the epidemiology of pertussis and the impact of booster doses. Nevertheless, the introduction of acellular boosters, particularly at 4 years, has the potential to be cost-effective in the UK.

Immunologic and epidemiologic experience of vaccination with a monocomponent pertussis toxoid vaccine

Pertussis re-emerged in Sweden with a cumulative incidence of about 60% during the first 10 years of life, when the locally produced cellular vaccine lost its efficacy around 1970 and general vaccination was discontinued in 1979. The epidemiology, clinical features, and immunology of pertussis and a monocomponent pertussis toxoid vaccine were studied in Göteborg, Sweden. After phase 1 and 2 studies, a randomized, double-blind, placebo-controlled trial of pertussis toxoid (PTox), compounded with diphtheria and tetanus toxoids, was administered to 3450 children according to the Swedish schedule at 3, 5, and 12 months of age. After a mean follow-up of 18 months, the efficacy was 71% overall and 75% in household contacts, respectively. A statistically significant correlation was found between the level of PTox-induced antibodies and protection against pertussis. As observed with cellular and with multicomponent acellular vaccines, PTox reduced the severity of disease and the percent of children with positive cultures. Furthermore, vaccination reduced the transmission of Bordetella pertussis to household contacts in the vaccinees compared with the controls who received only diphtheria and tetanus toxoids. Patients with culture-verified Bordetella parapertussis infection reacted with antibodies to pertactin and to filamentous hemagglutinin but not to pertussis toxin, and some subsequently developed pertussis. The antibody responses of patients with pertussis to the surface polysaccharides of B pertussis and to B parapertussis were cross-reactive serologically. Serosurveys showed that only antibodies to pertussis toxin were related to the occurrence of pertussis in the general population: antibodies to filamentous hemagglutinin and pertactin were probably stimulated by antigens of other bacteria as well as Bordetellae. Mass vaccination of Göteborg children born in the 1990s was started in 1995. In February 1999, about 55% had been vaccinated and both B pertussis and pertussis decreased significantly in individuals of all ages (herd immunity). Similar to diphtheria, PTox-induced immunity to pertussis occurs both on an individual and community basis. The apparent greater efficacy of multicomponent acellular pertussis vaccines compared with monocomponent PTox was proposed to be an artifact created when the diagnosis of pertussis was made by the serologic criteria of the World Health Organization only. Our conclusion is that PTox is both an essential and alone sufficient antigen in acellular pertussis vaccines.

Mass vaccination of children with pertussis toxoid--decreased incidence in both vaccinated and nonvaccinated persons

During 1979-1995, there was no vaccination against pertussis in Sweden. With the aim of studying the epidemiology and transmission of pertussis, mass vaccination with pertussis toxoid of children born during the 1990s was instituted in the Göteborg area (population, 778,597) in 1995. Infants were offered 3 doses of pertussis toxoid combined with diphtheria and tetanus toxoids. Children aged > or =1 year were offered 3 doses of pertussis toxoid alone. From June 1995 through February 1999, 167,810 doses of pertussis toxoid were given to 61,219 children born during the 1990s (56% received 3 doses). The number of Bordetella pertussis isolates per year declined from 1214 (1993-1995) to 64 (January 1997 through June 1999; P<.0001), and hospitalizations due to pertussis declined from 62 to 5 (P<.0001). Significant decreases in B. pertussis isolates and hospitalizations occurred in all age groups, including adults and nonvaccinated infants. Thus, mass vaccination of children with pertussis toxoid decreases spread of B. pertussis in the population.

Herd immunity and herd effect: new insights and definitions

The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed. It is defined as: "the reduction of infection or disease in the unimmunised segment as a result of immunising a proportion of the population". Herd immunity can be measured by testing a sample of the population for the presence of the chosen immune parameter. Herd effect can be measured by quantifying the decline in incidence in the unimmunised segment of a population in which an immunisation programme is instituted. Herd immunity applies to immunisation or infection, human to human transmitted or otherwise. On the other hand, herd effect applies to immunisation or other health interventions which reduce the probability of transmission, confined to infections transmitted human to human, directly or via vector. The induced herd immunity of a given vaccine exhibits geographic variation as it depends upon coverage and efficacy of the vaccine, both of which can vary geographically. Herd effect is determined by herd immunity as well as the force of transmission of the corresponding infection. Clear understanding of these phenomena and their relationships will help improve the design of effective and efficient immunisation programmes aimed at control, elimination or eradication of vaccine preventable infectious diseases.

Vaccines to prevent respiratory infection: opportunities on the near and far horizon

Illnesses caused by respiratory pathogens result in great loss of life, suffering and commitment of resources for treatment. That the suffering and loss of life can be prevented through immunization has already been clearly shown with existing vaccines, such as those for Haemophilus influenzae type b, Streptococcus pneumoniae, and influenza. The emergence of drug-resistant pathogens is making reliance on therapy more expensive and perhaps less successful, accentuating the need to focus on prevention. Although several effective vaccines to prevent respiratory infections currently exist, they are underutilized globally. Improvements in immunogenicity, efficacy, and ease of administration, and lowering the costs of some of the existing vaccines would augment the potential for prevention worldwide. The greatest opportunities for the prevention of respiratory infections will rest with vaccines that will become available in the future.