Forecasting Epidemiological Consequences of Maternal Immunization

Maternal pertussis immunization and the blunting of routine vaccine effectiveness: a meta-analysis and modeling study

A key goal of pertussis control is to protect infants too young to be vaccinated, the age group most vulnerable to this highly contagious respiratory infection. In the last decade, maternal immunization has been deployed in many countries, successfully reducing pertussis in this age group. Because of immunological blunting, however, this strategy may erode the effectiveness of primary vaccination at later ages. Here, we systematically reviewed the literature on the relative risk (RR) of pertussis after primary immunization of infants born to vaccinated vs. unvaccinated mothers. The four studies identified had ≤6 years of follow-up and large statistical uncertainty (meta-analysis weighted mean RR: 0.71, 95% CI: 0.38-1.32). To interpret this evidence, we designed a new mathematical model with explicit blunting mechanisms and evaluated maternal immunization's short- and long-term impact on pertussis transmission dynamics. We show that transient dynamics can mask blunting for at least a decade after rolling out maternal immunization. Hence, the current epidemiological evidence may be insufficient to rule out modest reductions in the effectiveness of primary vaccination. Irrespective of this potential collateral cost, we predict that maternal immunization will remain effective at protecting unvaccinated newborns, supporting current public health recommendations.

Maternal vaccination: shaping the neonatal response to pertussis

Antepartum maternal vaccination can protect highly sensitive newborns before they are old enough to receive their own vaccines. Two vaccines are currently recommended during pregnancy: the flu vaccine and the Tdap vaccine against tetanus, diphtheria, and pertussis. Although there is strong evidence that maternal vaccination works to protect the offspring, limitations in the understanding of vaccines and of maternal transfer of immunity compound to obscure our understanding of how they work. Here we focus on the example of pertussis to explore the possible mechanisms involved in the transfer of protection to offspring and how these may impact the newborn's response to future exposure to pertussis. For example, Tdap vaccines induce pathogen specific antibodies, and those antibodies are known to be transferred from mother to the fetus in utero and to the newborn via milk. But antibodies alone have modest impact on pertussis disease, and even less effect on colonization/transmission. Maternal immune cells can also be transferred to offspring and may play a direct role in protection from disease and/or influence the developing neonatal immune system. However, some of the transferred immunity may also blunt the offspring's response to subsequent vaccination. In this review we will summarize the protection conferred to offspring by maternal vaccination against pertussis and the likely mechanisms by which protection is transferred, identifying the many knowledge gaps that limit our most effective application of this approach.

Evaluation of the effectiveness of maternal immunization against pertussis in Alberta using agent-based modeling: A Canadian immunization research network study

INTRODUCTION

The re-emergence of pertussis has occurred in the past two decades in developed countries. The highest morbidity and mortality is seen among infants. Vaccination in pregnancy is recommended to reduce the pertussis burden in infants.

METHODS

We developed and validated an agent-based model to characterize pertussis epidemiology in Alberta. We computed programmatic effectiveness of pertussis vaccination during pregnancy (PVE) in relation to maternal vaccine coverage and pertussis disease reporting thresholds. We estimated the population preventable fraction (PFP) of different levels of maternal vaccine coverage against counterfactual "no-vaccination" scenario. We modeled the effect of immunological blunting and measured protection through interruption of exposure pathways.

RESULTS

PVE was inversely related to duration of passive immunity from maternal immunization across most simulations. In the scenario of 50% maternal vaccine coverage, PVE was 87% (95% quantiles 82-91%), with PFP of 44% (95% quantiles 41-45%). For monthly age intervals of 0-2, 2-4, 4-6 and 6-12, PVE ranged between 82 and 99%, and PFP ranged between 41 and 49%. At 75% maternal vaccine coverage, PVE and PFP were 90% (95% quantiles 86-92%) and 68% (95% quantiles 65-69%), respectively. At 50% maternal vaccine coverage and 10% blunting, PVE and PFP were 86% (95% quantiles 77-87%) and 43% (95% quantiles 39-44%), respectively, while at 50% blunting, the corresponding values of PVE and PFP were 76% (95% quantiles 70-81%) and 38% (95% quantiles 35-40%). PVE attributable to interruption of exposure pathways was 54-57%.

CONCLUSIONS

Our model predicts significant reduction in future pertussis cases in infants due to maternal vaccination, with immunological blunting slightly moderating its effectiveness. The model is most sensitive to maternal vaccination coverage. The interruption of exposure pathways plays a role in the reduction of pertussis burden in infants due to maternal immunization. The effect of maternal immunization on population other than infants remains to be elucidated.

Strategies to increase uptake of maternal pertussis vaccination

INTRODUCTION

Pertussis is a highly contagious respiratory disease that results in disproportionate morbidity and mortality in infants who have yet to receive the primary diphtheria-tetanus-pertussis vaccine series. In the preceding decades numerous countries began to pursue either prenatal vaccination of pregnant women or postpartum vaccination of caregivers to protect infants. Despite proven benefit, maternal uptake of pertussis vaccine continues to remain suboptimal.

AREAS COVERED

Many studies have been conducted to address the suboptimal uptake of maternal pertussis vaccination. This systematic review was undertaken to systematically identify those studies, highlight the most successful strategies and find the knowledge gaps that need to be filled over the coming years to improve vaccine uptake. Twenty-five studies were identified from six different databases.

EXPERT OPINION

Five different interventions were shown to be successful in promoting uptake of pertussis vaccination: (1) standing orders, (2) opt-in orders, (3) provider education, (4) on-site vaccination and (5) interactive patient education. Three major knowledge gaps were also identified that need to be filled over the coming years: (1) lack of studies in low- and middle-income countries, (2) lack of studies targeting midwives and/or home birth and (3) lack of studies on the process of vaccine communication.

Comparison of static and dynamic models of maternal immunization to prevent infant pertussis in Brazil

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Dynamic transmission models of infectious disease capture the herd immunity effects of vaccination.

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We compared dynamic and static models of maternal acellular pertussis (aP) immunization built with Brazilian data.

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At infant vaccine coverage < 90–95%, both models estimate that maternal immunization is cost-effective.

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Only the dynamic model shows that maternal immunization is not cost-effective at infant coverage > 90–95%.

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The background effect of routine infant vaccination is critical to the cost-effectiveness of maternal aP immunization.

Background

This paper compares cost-effectiveness results from two models of maternal immunization to prevent pertussis in infants in Brazil, one static, one dynamic, to explore when static models are adequate for public health decisions and when the extra effort required by dynamic models is worthwhile.

Methods

We defined two scenarios to explore key differences between static and dynamic models, herd immunity and time horizon. Scenario 1 evaluates the incremental cost/DALY of maternal acellular pertussis (aP) immunization as routine infant vaccination coverage ranges from low/moderate up to, and above, the threshold at which herd immunity begins to eliminate pertussis. Scenario 2 compares cost-effectiveness estimates over the models’ different time horizons. Maternal vaccine prices of $9.55/dose (base case) and $1/dose were evaluated.

Results

The dynamic model shows that maternal immunization could be cost-saving as well as life-saving at low levels of infant vaccination coverage. When infant coverage reaches the threshold range (90–95%), it is expensive: the dynamic model estimates that maternal immunization costs $2 million/DALY at infant coverage > 95% and maternal vaccine price of $9.55/dose; at $1/dose, cost/DALY is $200,000. By contrast, the static model estimates costs/DALY only modestly higher at high than at low infant coverage. When the models’ estimates over their different time horizons are compared at infant coverage < 90–95%, their projections fall in the same range.

Conclusions

Static models may serve to explore an intervention’s cost-effectiveness against infectious disease: the direction and principal drivers of change were the same in both models. When, however, an intervention too small to have significant herd immunity effects itself, such as maternal aP immunization, takes place against a background of vaccination in the rest of the population, a dynamic model is crucial to accurate estimates of cost-effectiveness. This finding is particularly important in the context of widely varying routine infant vaccination rates globally.

Clinical Trial registry

Clinical Trial registry name and registration number: Not applicable.

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.

Impact of maternal diphtheria-tetanus-acellular pertussis vaccination on pertussis booster immune responses in toddlers: Follow-up of a randomized trial

BACKGROUND

Transplacentally transferred antibodies induced by maternal pertussis vaccination interfere with infant immune responses to pertussis primary vaccination. We evaluated whether this interference remains in toddlers after booster vaccination.

METHODS

In a prior phase IV, observer-blind, placebo-controlled, randomized study (NCT02377349), pregnant women in Australia, Canada and Europe received intramuscular tetanus-reduced-antigen-content diphtheria-three-component acellular pertussis vaccine (Tdap group) or placebo (control group) at 27^0/7-36^6/7 weeks' gestation, with crossover immunization postpartum. Their infants were primed (study NCT02422264) and boosted (at 11-18 months; current study NCT02853929) with diphtheria-tetanus-three-component acellular pertussis-hepatitis B virus-inactivated poliovirus/Haemophilus influenzae type b vaccine (DTaP-HepB-IPV/Hib) and 13-valent pneumococcal conjugate vaccine. Immunogenicity before and after booster vaccination, and reactogenicity and safety of the booster were evaluated descriptively.

RESULTS

263 (Tdap group) and 277 (control group) toddlers received a DTaP-HepB-IPV/Hib booster. Pre-booster vaccination, observed geometric mean concentrations (GMCs) for the three pertussis antigens and diphtheria were 1.4-1.5-fold higher in controls than in the Tdap group. No differences were observed for the other DTaP-HepB-IPV/Hib antigens. One month post-booster vaccination, booster response rates for pertussis antigens were ≥ 92.1% and seroprotection rates for the other DTaP-HepB-IPV/Hib antigens were ≥ 99.2% in both groups (primary objective). Higher post-booster GMCs were observed in controls versus the Tdap group for anti-filamentous hemagglutinin (1.2-fold), anti-pertussis toxoid (1.5-fold) and anti-diphtheria (1.4-fold). GMCs for the other DTaP-HepB-IPV/Hib antigens were similar between groups. Serious adverse events were reported for three toddlers (controls, not vaccination-related). One death occurred pre-booster (Tdap group, not vaccination-related).

CONCLUSIONS

As a consequence of interference of maternal pertussis antibodies with infant immune responses to pertussis primary vaccination, pertussis antibody concentrations were still lower in toddlers from Tdap-vaccinated mothers before DTaP-HepB-IPV/Hib booster vaccination. After the booster, antibody concentrations were lower for filamentous hemagglutinin and pertussis toxoid but not for pertactin. The clinical significance of this interference requires further evaluation.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov: NCT02853929.

Modeling the cost-effectiveness of maternal acellular pertussis immunization (aP) in different socioeconomic settings: A dynamic transmission model of pertussis in three Brazilian states

OBJECTIVES

Using dynamic transmission models we evaluated the health and cost outcomes of adding acellular pertussis (aP) vaccination of pregnant women to infant vaccination in three Brazilian states that represent different socioeconomic conditions. The primary objective was to determine whether the same model structure could be used to represent pertussis disease dynamics in differing socioeconomic conditions.

METHODS

We tested three model structures (SIR, SIRS, SIRSIs) to represent population-level transmission in three socio-demographically distinct Brazilian states: São Paulo, Paraná and Bahia. Two strategies were evaluated: infant wP vaccination alone versus maternal aP immunization plus infant wP vaccination. Model projections for 2014-2029 include outpatient and inpatient pertussis cases, pertussis deaths, years of life lost, disability-adjusted life-years (DALYs) lost, and costs (in 2014 USD) of maternal aP vaccination, infant vaccination, and pertussis medical treatment. Incremental cost per DALY averted is presented from the perspective of the Brazilian National Health System.

RESULTS

Based on goodness-of-fit statistics, the SIRSIs model fit best, although it had only a modest improvement in statistical quantitative assessments relative to the SIRS model. For all three Brazilian states, maternal aP immunization led to higher costs but also saved infant lives and averted DALYs. The 2014 USD cost/DALY averted was $3068 in Sao Paulo, $2962 in Parana, and $2022 in Bahia. These results were robust in sensitivity analyses with the incremental cost-effectiveness ratios exceeding per capita gross regional product only when the probability that a pertussis case is reported was assumed higher than base case implying more overt cases and deaths and therefore more medical costs.

CONCLUSIONS

The same model structure fit all three states best, supporting the idea that the disease behaves similarly across different socioeconomic conditions. We also found that immunization of pregnant women with aP is cost-effective in diverse Brazilian states.

The Protective Role of Maternal Immunization in Early Life

With birth, the newborn is transferred from a quasi-sterile environment to the outside world. At this time, the neonatal immune system is inexperienced and continuously subject to a process of development as it encounters different antigenic stimuli after birth. It is initially characterized by a bias toward T helper 2 phenotype, reduced T helper 1, and cytotoxic responses to microbial stimuli, low levels of memory, and effector T and B cells and a high production of suppressive T regulatory cells. The aim of this setting, during fetal life, is to maintain an anti-inflammatory state and immune-tolerance. Maternal antibodies are transferred during pregnancy through the placenta and, in the first weeks of life of the newborn, they represent a powerful tool for protection. Thus, optimization of vaccination in pregnancy represents an important strategy to reduce the burden of neonatal infections and sepsis. Beneficial effects of maternal immunization are universally recognized, although the optimal timing of vaccination in pregnancy remains to be defined. Interestingly, the dynamic exchange that takes place at the fetal-maternal interface allows the transfer not only of antibodies, but also of maternal antigen presenting cells, probably in order to stimulate the developing fetal immune system in a harmless way. There are still controversial effects related to maternal immunization including the so called "immunology blunting," i.e., a dampened antibody production following infant's vaccination in those infants who received placentally transferred maternal immunity. However, clinical relevance of this phenomenon is still not clear. This review will provide an overview of the evolution of the immune system in early life and discuss the benefits of maternal vaccination. Current maternal vaccination policies and their rationale will be summarized on the road to promising approaches to enhance immunity in the neonate.

Immune interference (blunting) in the context of maternal immunization with Tdap-containing vaccines: is it a class effect?

INTRODUCTION

Maternal immunization with reduced antigen content tetanus-diphtheria-acellular pertussis (Tdap)-containing vaccines has been recommended to prevent infant pertussis. However, maternal antibodies may interfere with infant responses to routine immunization with diphtheria-tetanus-acellular pertussis (DTaP)-containing vaccines, raising concerns of suboptimal protection after infant vaccination. We performed a narrative literature review to assess whether blunting occurs regardless of the manufacturer of maternal and infant vaccines. Because internationally agreed correlates of protection are lacking, the clinical significance of blunting is not yet fully understood. We have reviewed the evidence available to date.

AREAS COVERED

Thirteen studies that evaluated blunting after maternal immunization and infant primary/booster series were identified. Blunting was observed with various combinations of Tdap- and DTaP-containing vaccines for maternal and pediatric immunization. Studies assessing the effectiveness of maternal Tdap immunization beyond the primary infant immunization series in England and in the United States suggested no evidence of a clinically relevant blunting effect so far.

EXPERT COMMENTARY

This review indicates that the phenomenon of blunting does not depend on the manufacturer/brand of the pertussis-containing vaccines used for immunizing mothers or children. Currently, there is no epidemiological evidence that children whose mothers received Tdap are at increased risk of pertussis after pediatric vaccinations, although longer follow-up is required.

Real-time predictive seasonal influenza model in Catalonia, Spain

Influenza surveillance is critical to monitoring the situation during epidemic seasons and predictive mathematic models may aid the early detection of epidemic patterns. The objective of this study was to design a real-time spatial predictive model of ILI (Influenza Like Illness) incidence rate in Catalonia using one- and two-week forecasts. The available data sources used to select explanatory variables to include in the model were the statutory reporting disease system and the sentinel surveillance system in Catalonia for influenza incidence rates, the official climate service in Catalonia for meteorological data, laboratory data and Google Flu Trend. Time series for every explanatory variable with data from the last 4 seasons (from 2010–2011 to 2013–2014) was created. A pilot test was conducted during the 2014–2015 season to select the explanatory variables to be included in the model and the type of model to be applied. During the 2015–2016 season a real-time model was applied weekly, obtaining the intensity level and predicted incidence rates with 95% confidence levels one and two weeks away for each health region. At the end of the season, the confidence interval success rate (CISR) and intensity level success rate (ILSR) were analysed. For the 2015–2016 season a CISR of 85.3% at one week and 87.1% at two weeks and an ILSR of 82.9% and 82% were observed, respectively. The model described is a useful tool although it is hard to evaluate due to uncertainty. The accuracy of prediction at one and two weeks was above 80% globally, but was lower during the peak epidemic period. In order to improve the predictive power, new explanatory variables should be included.

Maternal pertussis immunisation: clinical gains and epidemiological legacy

The increase in whooping cough (pertussis) incidence in many countries with high routine vaccination coverage is alarming, with incidence in the US reaching almost 50,000 reported cases per year, reflecting incidence levels not seen since the 1950s. While the potential explanations for this resurgence remain debated, we face an urgent need to protect newborns, especially during the time window between birth and the first routine vaccination dose. Maternal immunisation has been proposed as an effective strategy for protecting neonates, who are at higher risk of severe pertussis disease and mortality. However, if maternally derived antibodies adversely affect the immunogenicity of the routine schedule, through blunting effects, we may observe a gradual degradation of herd immunity. 'Wasted' vaccines would result in an accumulation of susceptible children in the population, specifically leading to an overall increase in incidence in older age groups. In this Perspective, we discuss potential long-term epidemiological effects of maternal immunisation, as determined by possible immune interference outcomes.