Can vaccine legacy explain the British pertussis resurgence?

Pertussis vaccines, epidemiology and evolution

Pertussis, which is caused by Bordetella pertussis, has plagued humans for at least 800 years, is highly infectious and can be fatal in the unvaccinated, especially very young infants. Although the rollout of whole-cell pertussis (wP) vaccines in the 1940s and 1950s was associated with a drastic drop in incidence, concerns regarding the reactogenicity of wP vaccines led to the development of a new generation of safer, acellular (aP) vaccines that have been adopted mainly in high-income countries. Over the past 20 years, some countries that boast high aP coverage have experienced a resurgence in pertussis, which has led to substantial debate over the basic immunology, epidemiology and evolutionary biology of the bacterium. Controversy surrounds the duration of natural immunity and vaccine-derived immunity, the ability of vaccines to prevent transmission and severe disease, and the impact of evolution on evading vaccine immunity. Resolving these issues is made challenging by incomplete detection of pertussis cases, the absence of a serological marker of immunity, modest sequencing of the bacterial genome and heterogeneity in diagnostic methods of surveillance. In this Review, we lay out the complexities of contemporary pertussis and, where possible, propose a parsimonious explanation for apparently incongruous observations.

Pertussis in Mexico from 2000 to 2019: A real-world study of incidence, vaccination coverage, and vaccine effectiveness

BACKGROUND

The national immunization program in Mexico includes a 3-dose primary series of pertussis vaccine and a toddler booster dose. In Mexico, whole-cell pertussis vaccines (wP) were switched in 2007 to acellular pertussis vaccines (aP).

METHODS

This retrospective study using Mexican National Databases of Health and population surveillance (2000-2019) assessed the incidence of pertussis, infant pertussis vaccination coverage, and vaccine effectiveness (VE) against clinically-diagnosed and/or laboratory-confirmed pertussis in children aged 6.5-18.5 or 24.5 months for the primary series, and children aged 18.5 or 24.5-48.5 months for the toddler booster.

RESULTS

The incidence of pertussis sharply increased in 2012 and was highest in 2012, 2015, and 2016 (0.84-0.94/100,000 person-years). Coverage was highest for the first dose in the primary series, decreasing for each subsequent dose. The VE against notified pertussis was 96.4% (95% CI: 94.7, 97.6) for the first three doses of wP vaccine (2000-2007) and 95.7% (95% CI: 95.1, 96.2) for the first three doses of aP vaccine (2008-2019).

CONCLUSIONS

Our findings suggested high levels of vaccine effectiveness overall were achieved for the aP and wP vaccines in Mexico between 2000 and 2019.

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.

Disentangling the causes of mumps reemergence in the United States

Over the past two decades, multiple countries with high vaccine coverage have experienced resurgent outbreaks of mumps. Worryingly, in these countries, a high proportion of cases have been among those who have completed the recommended vaccination schedule, raising alarm about the effectiveness of existing vaccines. Two putative mechanisms of vaccine failure have been proposed as driving observed trends: 1) gradual waning of vaccine-derived immunity (necessitating additional booster doses) and 2) the introduction of novel viral genotypes capable of evading vaccinal immunity. Focusing on the United States, we conduct statistical likelihood-based hypothesis testing using a mechanistic transmission model on age-structured epidemiological, demographic, and vaccine uptake time series data. We find that the data are most consistent with the waning hypothesis and estimate that 32.8% (32%, 33.5%) of individuals lose vaccine-derived immunity by age 18 y. Furthermore, we show using our transmission model how waning vaccine immunity reproduces qualitative and quantitatively consistent features of epidemiological data, namely 1) the shift in mumps incidence toward older individuals, 2) the recent recurrence of mumps outbreaks, and 3) the high proportion of mumps cases among previously vaccinated individuals.

Collateral Impact of COVID-19 Prevention Measures on Re-Emergence of Scarlet Fever and Pertussis in Mainland China and Hong Kong China

The incidence of scarlet fever and pertussis has increased significantly in China in recent years. During the COVID-19 pandemic, stringent non-pharmaceutical intervention measures were widely adopted to contain the spread of the virus, which may also have essential collateral impacts on other infectious diseases, such as scarlet fever and pertussis. We compared the incidence data of scarlet fever and pertussis in Mainland China and Hong Kong from 2004 to 2021 before and after the COVID-19 pandemic. The results show that the incidence of both diseases decreased significantly in 2020-2021 compared to the after-re-emergence stage in these two locations. Specifically, in 2020, scarlet fever decreased by 73.13% and pertussis by 76.63% in Mainland China, and 83.70% and 76.10%, respectively, in Hong Kong. In the absence of COVID-19, the predicted incidence of both diseases was much higher than the actual incidence in Mainland China and Hong Kong in 2020-2021. This study demonstrates that non-pharmaceutical measures implemented during the COVID-19 pandemic can partially reduce scarlet fever and pertussis re-emergence in Mainland China and Hong Kong.

Dissecting recurrent waves of pertussis across the boroughs of London

Pertussis has resurfaced in the UK, with incidence levels not seen since the 1980s. While the fundamental causes of this resurgence remain the subject of much conjecture, the study of historical patterns of pathogen diffusion can be illuminating. Here, we examined time series of pertussis incidence in the boroughs of Greater London from 1982 to 2013 to document the spatial epidemiology of this bacterial infection and to identify the potential drivers of its percolation. The incidence of pertussis over this period is characterized by 3 distinct stages: a period exhibiting declining trends with 4-year inter-epidemic cycles from 1982 to 1994, followed by a deep trough until 2006 and the subsequent resurgence. We observed systematic temporal trends in the age distribution of cases and the fade-out profile of pertussis coincident with increasing national vaccine coverage from 1982 to 1990. To quantify the hierarchy of epidemic phases across the boroughs of London, we used the Hilbert transform. We report a consistent pattern of spatial organization from 1982 to the early 1990s, with some boroughs consistently leading epidemic waves and others routinely lagging. To determine the potential drivers of these geographic patterns, a comprehensive parallel database of borough-specific features was compiled, comprising of demographic, movement and socio-economic factors that were used in statistical analyses to predict epidemic phase relationships among boroughs. Specifically, we used a combination of a feed-forward neural network (FFNN), and SHapley Additive exPlanations (SHAP) values to quantify the contribution of each covariate to model predictions. Our analyses identified a number of predictors of a borough's historical epidemic phase, specifically the age composition of households, the number of agricultural and skilled manual workers, latitude, the population of public transport commuters and high-occupancy households. Univariate regression analysis of the 2012 epidemic identified the ratio of cumulative unvaccinated children to the total population and population of Pakistan-born population to have moderate positive and negative association, respectively, with the timing of epidemic. In addition to providing a comprehensive overview of contemporary pertussis transmission in a large metropolitan population, this study has identified the characteristics that determine the spatial spread of this bacterium across the boroughs of London.

The impact of pertussis vaccine programme changes on pertussis disease burden in Manitoba, 1992-2017-an age-period-cohort analysis

BACKGROUND

Changes to pertussis vaccination programmes can have impacts on disease burden that should be estimated independently from factors such as age- and period-related trends. We used age-period-cohort (APC) models to explore pertussis incidence in Manitoba over a 25-year period (1992-2017).

METHODS

We identified all laboratory-confirmed cases of pertussis from Manitoba's Communicable Diseases Database and calculated age-standardized incidence rates. We used APC models to investigate trends in pertussis incidence.

RESULTS

During the study period, 2479 cases were reported. Age-standardized rates were highest during a large outbreak in 1994 (55 cases/100 000 person-years), with much lower peaks in 1998, 2012 and 2016. We saw strong age and cohort effects in the APC models, with a steady decrease in incidence with increasing age and increased risk in the cohort born between 1980 and 1995.

CONCLUSIONS

The highest risk for pertussis was consistently in young children, regardless of birth cohort or time period. The 1981 programme change to an adsorbed whole-cell pertussis vaccine with low effectiveness resulted in reduced protection in the 1981-95 birth cohort and contributed to the largest outbreak of disease during the 25-year study period.

Delay in booster schedule as a control parameter in vaccination dynamics

The use of multiple vaccine doses has proven to be essential in providing high levels of protection against a number of vaccine-preventable diseases at the individual level. However, the effectiveness of vaccination at the population level depends on several key factors, including the dose-dependent protection efficacy of vaccine, coverage of primary and booster doses, and in particular, the timing of a booster dose. For vaccines that provide transient protection, the optimal scheduling of a booster dose remains an important component of immunization programs and could significantly affect the long-term disease dynamics. In this study, we developed a vaccination model as a system of delay differential equations to investigate the effect of booster schedule using a control parameter represented by a fixed time-delay. By exploring the stability analysis of the model based on its reproduction number, we show the disease persistence in scenarios where the booster dose is sub-optimally scheduled. The findings indicate that, depending on the protection efficacy of primary vaccine series and the coverage of booster vaccination, the time-delay in a booster schedule can be a determining factor in disease persistence or elimination. We present model results with simulations for a vaccine-preventable bacterial disease, Heamophilus influenzae serotype b, using parameter estimates from the previous literature. Our study highlights the importance of timelines for multiple-dose vaccination in order to enhance the population-wide benefits of herd immunity.

The impact of past vaccination coverage and immunity on pertussis resurgence

The resurgence of pertussis over the past decades has resulted in incidence levels not witnessed in the United States since the 1950s. The underlying causes have been the subject of much speculation, with particular attention paid to the shortcomings of the latest generation of vaccines. We formulated transmission models comprising competing hypotheses regarding vaccine failure and challenged them to explain 16 years of highly resolved incidence data from Massachusetts, United States. Our results suggest that the resurgence of pertussis is a predictable consequence of incomplete historical coverage with an imperfect vaccine that confers slowly waning immunity. We found evidence that the vaccine itself is effective at reducing overall transmission, yet that routine vaccination alone would be insufficient for elimination of the disease. Our results indicated that the core transmission group is schoolchildren. Therefore, efforts aimed at curtailing transmission in the population at large, and especially in vulnerable infants, are more likely to succeed if targeted at schoolchildren, rather than adults.

A Network-Based Compartmental Model For The Spread Of Whooping Cough In Nebraska

Outbreaks of pertussis have increased over the past few years, drawing the attention of health care providers. Un- derstanding the transmission mechanisms of contagious disease is critically important, but depends on many intricate factors including pathogen and host environment, exposed population, and their activities. In this work, we try to improve upon the prediction model for the exposed population. The number of whooping cough reported cases in Nebraska between 2000-2017 was gathered. The standard Susceptible-Exposed-Infected-Recovered (SEIR) model is used to predict the infected numbers. The results show that the SEIR model prediction for the number of infected indi- viduals is much higher than the actual number. To overcome this problem, the Network Based-SEIR model is proposed, and is able to estimate the number of infected more accurately than the classic SEIR model.

Age-related differences in antibody avidities to pertussis toxin and filamentous hemagglutinin in a healthy Japanese population

To gain insights into the current Japanese pertussis immunization schedule, we examined the distributions of antibody titers and avidities to pertussis toxin (PT) and filamentous hemagglutinin (FHA) in 460 Japanese healthy subjects (aged 1-60 years) based on age category. Our avidity enzyme-linked immunosorbent assays revealed that young children aged 1-2 years, which corresponded to ages after receiving primary and/or booster pertussis vaccinations, had relatively high-avidity anti-PT IgG (mean avidity index [AI], 40.5%) compared with other age groups (AI, 26.5-31.9%); however, they had relatively low-avidity anti-FHA IgG (AI, 41.8%). In contrast, children aged 3-6 years had both low-avidity anti-PT IgG (AI, 26.5%) and low-avidity anti-FHA IgG (AI, 40.4%). A significant age-related difference in anti-PT IgG avidity was observed between children aged 1-2 years and 3-6 years (P < 0.05); however, the difference in anti-FHA IgG avidity was not significant. The anti-PT IgG avidity was positively correlated with the antibody titer, especially among children aged 1-15 years (r_s = 0.508-0.685; P < 0.01), indicating that the avidity of vaccine-induced anti-PT IgG decreases with decreasing IgG antibody titer to PT. Our findings strongly suggest that vaccine-induced anti-PT IgG avidity rapidly wanes after vaccination, but this is not observed for anti-FHA IgG avidity. Because children aged 3-6 years have both low-quantity and low-quality antibodies against PT, an additional booster vaccination with acellular pertussis vaccines is required for such children in Japan.

Acellular pertussis vaccines effectiveness over time: A systematic review, meta-analysis and modeling study

Background

Acellular pertussis vaccine studies postulate that waning protection, particularly after the adolescent booster, is a major contributor to the increasing US pertussis incidence. However, these studies reported relative (ie, vs a population given prior doses of pertussis vaccine), not absolute (ie, vs a pertussis vaccine naïve population) efficacy following the adolescent booster. We aim to estimate the absolute protection offered by acellular pertussis vaccines.

Methods

We conducted a systematic review of acellular pertussis vaccine effectiveness (VE) publications. Studies had to comply with the US schedule, evaluate clinical outcomes, and report VE over discrete time points. VE after the 5-dose childhood series and after the adolescent sixth-dose booster were extracted separately and pooled. All relative VE estimates were transformed to absolute estimates. VE waning was estimated using meta-regression modeling.

Findings

Three studies reported VE after the childhood series and four after the adolescent booster. All booster studies reported relative VE (vs acellular pertussis vaccine-primed population). We estimate initial childhood series absolute VE is 91% (95% CI: 87% to 95%) and declines at 9.6% annually. Initial relative VE after adolescent boosting is 70% (95% CI: 54% to 86%) and declines at 45.3% annually. Initial absolute VE after adolescent boosting is 85% (95% CI: 84% to 86%) and declines at 11.7% (95% CI: 11.1% to 12.3%) annually.

Interpretation

Acellular pertussis vaccine efficacy is initially high and wanes over time. Observational VE studies of boosting failed to recognize that they were measuring relative, not absolute, VE and the absolute VE in the boosted population is better than appreciated.

Controlling pertussis: how can we do it? A focus on immunization

INTRODUCTION

Pertussis is a highly contagious disease of the respiratory tract which is caused by the bacterium Bordetella pertussis and is most severe in those <1 year of age. A vaccine against pertussis, introduced in the 1950's, led to a significant decrease in incidence of the disease, but recent increases in outbreaks have been attributed to insufficient vaccine uptake, suboptimal protection conferred by vaccines, and waning immunity after immunization. Areas covered: In this review we discuss the major challenges for controlling pertussis, and what we believe the best strategies are to overcome these challenges, focusing on immunization against pertussis in Europe, but with recommendations that are relevant worldwide. Expert commentary: To provide maximum vaccine coverage we propose a schedule that incorporates immunization of infants, preschoolers, adolescents, adults, and pregnant women. Uptake of vaccines may also vary between populations due to a variety of causes, including hesitancy to vaccinate, so any national strategy to control pertussis should also include sustaining public and healthcare provider confidence in vaccination. Addressing and improving regional variations in surveillance will also help better monitor annual incidence and outbreaks. Looking towards the future, the development of new pertussis vaccines with longer duration of protection would be advantageous.

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.

Seroepidemiology of diphtheria and pertussis in Chongqing, China: serology-based evidence of Bordetella pertussis infection

OBJECTIVES

In order to gain further insight into the seroepidemiology of Bordetella pertussis infection and immunity against diphtheria in Chongqing, China, the concentrations of antibodies to pertussis toxin (PT) and diphtheria toxin (DT) were investigated in a healthy population.

STUDY DESIGN

Cross-sectional study.

METHODS

In total, 1080 healthy people were recruited into this study. Sera antibodies to DT and PT were measured quantitatively using commercial enzyme-linked immunosorbent assay kits. Age-specific incidence of infection with B. pertussis was estimated and compared with notified cases of pertussis.

RESULTS

The mean concentration of anti-DT IgG was 0.71 IU/ml (95% confidence interval [CI] = 0.60-0.82), with a positive rate (>0.01 IU/ml) of 97.41% (1052/1080). The mean concentration of anti-PT IgG was 7.65 IU/ml (95% CI = 6.65-8.65), with a positive rate (>100 IU/ml) of 1.17% (11/944). The estimated pertussis infection rate was 7290/100,000, which was far higher than the reported incidence of 1.29/100,000 in 2015. The peaks of estimated incidence of infection were found in subjects aged 7-14 years (9971/100,000) and ≥20 years (13,898/100,000).

CONCLUSIONS

B. pertussis infection occurs frequently in young infants and adolescents/adults; the latter are often responsible for the transmission of pertussis to young infants. The existing surveillance system may underestimate the true incidence of pertussis in older age groups in Chongqing, and the immunisation programme should be improved to provide protection against pertussis for adolescents and young adults.

Core pertussis transmission groups in England and Wales: A tale of two eras

The recent resurgence of pertussis in England and Wales has been marked by infant deaths and rising cases in teens and adults. To understand which age cohorts are most responsible for these trends, we employed three separate statistical methods to analyze high-resolution pertussis reports from 1982 to 2012. The fine-grained nature of the time-series allowed us to describe the changes in age-specific incidence and contrast the transmission dynamics in the 1980s and during the resurgence era. Our results identified infants and school children younger than 10 years of age as a core group, prior to 2002: pertussis incidence in these populations was predictive of incidence in other age groups. After 2002, no core groups were identifiable. This conclusion is independent of methodology used. Because it is unlikely that the underlying contact patterns substantially changed over the study period, changes in predictability likely result from the introduction of more stringent diagnostics tests that may have inadvertently played a role in masking the relative contributions of core transmission groups.

Anticipating the emergence of infectious diseases

In spite of medical breakthroughs, the emergence of pathogens continues to pose threats to both human and animal populations. We present candidate approaches for anticipating disease emergence prior to large-scale outbreaks. Through use of ideas from the theories of dynamical systems and stochastic processes we develop approaches which are not specific to a particular disease system or model, but instead have general applicability. The indicators of disease emergence detailed in this paper can be classified into two parallel approaches: a set of early-warning signals based around the theory of critical slowing down and a likelihood-based approach. To test the reliability of these two approaches we contrast theoretical predictions with simulated data. We find good support for our methods across a range of different model structures and parameter values.

Forecasting Epidemiological Consequences of Maternal Immunization

BACKGROUND

 The increase in the incidence of whooping cough (pertussis) in many countries with high vaccination coverage is alarming. Maternal pertussis immunization has been proposed as an effective means of protecting newborns during the interval between birth and the first routine dose. However, there are concerns regarding potential interference between maternal antibodies and the immune response elicited by the routine schedule, with possible long-term population-level effects.

METHODS

 We formulated a transmission model comprising both primary routine and maternal immunization. This model was examined to evaluate the long-term epidemiological effects of routine and maternal immunization, together with consequences of potential immune interference scenarios.

RESULTS

 Overall, our model demonstrates that maternal immunization is an effective strategy in reducing the incidence of pertussis in neonates prior to the onset of the primary schedule. However, if maternal antibodies lead to blunting, incidence increases among older age groups. For instance, our model predicts that with 60% routine and maternal immunization coverage and 30% blunting, the incidence among neonates (0-2 months) is reduced by 43%. Under the same scenario, we observe a 20% increase in incidence among children aged 5-10 years. However, the downstream increase in the older age groups occurs with a delay of approximately a decade or more.

CONCLUSIONS

 Maternal immunization has clear positive effects on infant burden of disease, lowering mean infant incidence. However, if maternally derived antibodies adversely affect the immunogenicity of the routine schedule, we predict eventual population-level repercussions that may lead to an overall increase in incidence in older age groups.

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.

Using age-stratified incidence data to examine the transmission consequences of pertussis vaccination

Pertussis is a highly infectious respiratory disease that has been on the rise in many countries worldwide over the past several years. The drivers of this increase in pertussis incidence remain hotly debated, with a central and long-standing hypothesis that questions the ability of vaccines to eliminate pertussis transmission rather than simply modulate the severity of disease. In this paper, we present age-structured case notification data from all provinces of Thailand between 1981 and 2014, a period during which vaccine uptake rose substantially, permitting an evaluation of the transmission impacts of vaccination. Our analyses demonstrate decreases in incidence across all ages with increased vaccine uptake - an observation that is at odds with pertussis case notification data in a number of other countries. To explore whether these observations are consistent with a rise in herd immunity and a reduction in bacterial transmission, we analyze an age-structured model that incorporates contrasting hypotheses concerning the immunological and transmission consequences of vaccines. Our results lead us to conclude that the most parsimonious explanation for the combined reduction in incidence and the shift to older age groups in the Thailand data is vaccine-induced herd immunity.

Pertussis immunity and epidemiology: mode and duration of vaccine-induced immunity

The resurgence of pertussis in some countries that maintain high vaccination coverage has drawn attention to gaps in our understanding of the epidemiological effects of pertussis vaccines. In particular, major questions surround the nature, degree and durability of vaccine protection. To address these questions, we used mechanistic transmission models to examine regional time series incidence data from Italy in the period immediately following the introduction of acellular pertussis (aP) vaccine. Our results concur with recent animal-challenge experiments wherein infections in aP-vaccinated individuals proved as transmissible as those in naive individuals but much less symptomatic. On the other hand, the data provide evidence for vaccine-driven reduction in susceptibility, which we quantify via a synthetic measure of vaccine impact. As to the precise nature of vaccine failure, the data do not allow us to distinguish between leakiness and waning of vaccine immunity, or some combination of these. Across the range of well-supported models, the nature and duration of vaccine protection, the age profile of incidence and the range of projected epidemiological futures differ substantially, underscoring the importance of the remaining unknowns. We identify key data gaps: sources of data that can supply the information needed to eliminate these remaining uncertainties.

Cost-effectiveness of next-generation vaccines: The case of pertussis

Despite steady vaccination coverage rates, pertussis incidence in the United States has continued to rise. This public health challenge has motivated calls for the development of a new vaccine with greater efficacy and duration of protection. Any next-generation vaccine would likely come at a higher cost, and must provide sufficient health benefits beyond those provided by the current vaccine in order to be deemed cost-effective. Using an age-structured transmission model of pertussis, we quantified the health and economic benefits of a next-generation vaccine that would enhance either the efficacy or duration of protection of the childhood series, the duration of the adult booster, or a combination. We developed a metric, the maximum cost-effective price increase (MCPI), to compare the potential value of such improvements. The MCPI estimates the per-dose price increase that would maintain the cost-effectiveness of pertussis vaccination. We evaluated the MCPI across a range of potential single and combined improvements to the pertussis vaccine. As an upper bound, we found that a next-generation vaccine which could achieve perfect efficacy for the childhood series would permit an MCPI of $18 per dose (95% CI: $12-$31). Pertussis vaccine improvements that extend the duration of protection to an average of 75 years would allow for an MCPI of $22 per dose for the childhood series (CI: $10-$33) or $12 for the adult booster (CI: $4-$18). Despite the short duration of the adult booster, improvements to the childhood series could be more valuable than improvements to the adult booster. Combining improvements in both efficacy and duration, a childhood series with perfect efficacy and average duration of 75 years would permit an MCPI of $39 per dose, the highest of any scenario evaluated. Our results highlight the utility of the MCPI metric in evaluating potential vaccines or other interventions when prices are unknown.

Pertussis models to inform vaccine policy

Pertussis remains a challenging public health problem with many aspects of infection, disease and immunity poorly understood. Initially controlled by mass vaccination, pertussis resurgence has occurred in some countries with well-established vaccination programs, particularly among adolescents and young adults. Several studies have used mathematical models to investigate drivers of pertussis epidemiology and predict the likely impact of different vaccination strategies. We reviewed a number of these models to evaluate their suitability to answer questions of public health importance regarding optimal vaccine scheduling. We critically discuss the approaches adopted and the impact of chosen model structures and assumptions on study conclusions. Common limitations were a lack of contemporary, population relevant data for parameterization and a limited understanding of the relationship between infection and disease. We make recommendations for future model development and suggest epidemiologic data collections that would facilitate efforts to reduce uncertainty and improve the robustness of model-derived conclusions.

Protecting Newborns Against Pertussis: Treatment and Prevention Strategies

Pertussis is a potentially severe respiratory disease, which affects all age groups from young infants to older adults and is responsible for an estimated 195,000 deaths occurred globally in 2008. Active research is ongoing to better understand the pathogenesis, immunology, and diagnosis of pertussis. For diagnosis, molecular assays (e.g., polymerase chain reaction) for detection of Bordetella pertussis have become more widely available and support improved outbreak detection. In children, pertussis vaccines have been incorporated into routine immunization schedules and deployed for pertussis outbreak control. Lower levels of vaccine coverage are now being observed in communities where vaccine hesitancy is rising. Additionally, recognition that newborn babies are at risk of pertussis in the USA and UK has led to recommendations to immunize pregnant women. Among adolescents and older adults in the USA, Tetanus Toxoid, Reduced Diphtheria Toxoid and Acellular pertussis (Tdap) Vaccines are recommended, but substantial individual- and system-level barriers exist that will make achieving national Healthy People 2020 targets for immunization challenging. Current antimicrobial regimens for pertussis are focused on reducing the severity of disease, reducing rates of sequelae, and minimizing transmission of infection to susceptible individuals. Continued surveillance for pertussis will be important to identify opportunities for reducing young infants' exposure and reducing the impact of outbreaks among school-aged children. Laboratory-based surveillance for newly emerging strains of B. pertussis will be important to identify strains that may evade protection elicited by currently available vaccines. Efforts to develop new-generation pertussis vaccines should be considered now in anticipation of vaccine development programs, which may require ten or more years to deliver a licensed vaccine.

Defining long-term drivers of pertussis resurgence, and optimal vaccine control strategies

Pertussis resurgence has been reported from several developed countries with long-standing immunisation programs. Among these, Australia in 2003 discontinued an 18 months (fourth) booster dose in favour of an adolescent (fifth) dose. We developed a model to evaluate determinants of resurgence in Australia and alternative vaccine strategies for mitigation. Novel characteristics of our model included the use of seroepidemiologic data for calibration, and broad investigation of variables relevant to transmission of, and protection against, pertussis. We simulated multiple parameter combinations, retaining those consistent with observed data for subsequent use in predictive models comparing alternative vaccination schedules. Reproducing the early control of pertussis followed by late resurgence observed in Australia required natural immunity to last decades longer than vaccine-acquired immunity, with mean duration exceeding 50 years in almost 90% of simulations. Replacement of the dose at 18 months with an adolescent dose in 2003 resulted in a 40% increase in infections in the age group 18-47 months by 2013. A six dose strategy (2, 4, 6, 18 months, 4 and 15 years) yielded a reduction in infection incidence (pre-school 43%, infants 8%) greater than any alternative strategies considered for timing of five administered doses. Our finding that natural immunity drives long-term trends in pertussis cycles is relevant to a range of pertussis strategies and provides the necessary context in which to consider maternal vaccination. Comparatively short-lived vaccine-acquired immunity requires multiple boosters over the first two decades of life to maximise reduction in infections.

Roads to the development of improved pertussis vaccines paved by immunology

Current acellular pertussis vaccines have various shortcomings, which may contribute to their suboptimal efficacy and waning immunity in vaccinated populations. This calls for the development of new pertussis vaccines capable of inducing long-lived protective immunity. Immunization with whole cell pertussis vaccines and natural infection with Bordetella pertussis induce distinct and more protective immune responses when compared with immunization with acellular pertussis vaccines. Therefore, the immune responses induced with whole cell vaccine or after infection can be used as a benchmark for the development of third-generation vaccines against pertussis. Here, we review the literature on the immunology of B. pertussis infection and vaccination and discuss the lessons learned that will help in the design of improved pertussis vaccines.

To develop improved pertussis vaccines capable of inducing long-lived protective immunity, lessons have to be learned from immunology of Bordetella pertussis infection and current vaccination.

Understanding Trends in Pertussis Incidence: An Agent-Based Model Approach

OBJECTIVES

We examined the impact of undetected infections, adult immunity, and waning vaccine-acquired immunity on recent age-related trends in pertussis incidence.

METHODS

We developed an agent-based model of pertussis transmission in Dakota County, Minnesota using case data from the Minnesota Department of Health. For outbreaks in 2004, 2008, and 2012, we fit our model to incidence in 3 children's age groups relative to adult incidence. We estimated parameters through model calibration.

RESULTS

The duration of vaccine-acquired immunity after completion of the 5-dose vaccination series decreased from 6.6 years in the 2004 model to approximately 3.0 years in the 2008 and 2012 models. Tdap waned after 2.1 years in the 2012 model. A greater percentage of adults were immune in the 2008 model than in the 2004 and 2012 models. On average, only 1 in 10 adult infections was detected, whereas 8 in 10 child infections were detected.

CONCLUSIONS

The observed trends in relative pertussis incidence in Dakota County can be attributed in part to fluctuations in adult immunity and waning vaccine-acquired immunity. No single factor accounts for current pertussis trends.

Asymptomatic transmission and the resurgence of Bordetella pertussis

BACKGROUND

The recent increase in whooping cough incidence (primarily caused by Bordetella pertussis) presents a challenge to both public health practitioners and scientists trying to understand the mechanisms behind its resurgence. Three main hypotheses have been proposed to explain the resurgence: 1) waning of protective immunity from vaccination or natural infection over time, 2) evolution of B. pertussis to escape protective immunity, and 3) low vaccine coverage. Recent studies have suggested a fourth mechanism: asymptomatic transmission from individuals vaccinated with the currently used acellular B. pertussis vaccines.

METHODS

Using wavelet analyses of B. pertussis incidence in the United States (US) and United Kingdom (UK) and a phylodynamic analysis of 36 clinical B. pertussis isolates from the US, we find evidence in support of asymptomatic transmission of B. pertussis. Next, we examine the clinical, public health, and epidemiological consequences of asymptomatic B. pertussis transmission using a mathematical model.

RESULTS

We find that: 1) the timing of changes in age-specific attack rates observed in the US and UK are consistent with asymptomatic transmission; 2) the phylodynamic analysis of the US sequences indicates more genetic diversity in the overall bacterial population than would be suggested by the observed number of infections, a pattern expected with asymptomatic transmission; 3) asymptomatic infections can bias assessments of vaccine efficacy based on observations of B. pertussis-free weeks; 4) asymptomatic transmission can account for the observed increase in B. pertussis incidence; and 5) vaccinating individuals in close contact with infants too young to receive the vaccine ("cocooning" unvaccinated children) may be ineffective.

CONCLUSIONS

Although a clear role for the previously suggested mechanisms still exists, asymptomatic transmission is the most parsimonious explanation for many of the observations surrounding the resurgence of B. pertussis in the US and UK. These results have important implications for B. pertussis vaccination policy and present a complicated scenario for achieving herd immunity and B. pertussis eradication.

Dynamics of Pertussis Transmission in the United States

Past patterns of infectious disease transmission set the stage on which modern epidemiologic dynamics are played out. Here, we present a comprehensive account of pertussis (whooping cough) transmission in the United States during the early vaccine era. We analyzed recently digitized weekly incidence records from Morbidity and Mortality Weekly Reports from 1938 to 1955, when the whole-cell pertussis vaccine was rolled out, and related them to contemporary patterns of transmission and resurgence documented in monthly incidence data from the National Notifiable Diseases Surveillance System. We found that, during the early vaccine era, pertussis epidemics in US states could be categorized as 1) annual, 2) initially annual and later multiennial, or 3) multiennial. States with predominantly annual cycles tended to have higher per capita birth rates, more household crowding, more children per family, and lower rates of school attendance than the states with multiennial cycles. Additionally, states that exhibited annual epidemics during 1938-1955 have had the highest recent (2001-2010) incidence, while those states that transitioned from annual cycles to multiennial cycles have had relatively low recent incidence. Our study provides an extensive picture of pertussis epidemiology in the United States dating back to the onset of vaccination, a back-story that could aid epidemiologists in understanding contemporary transmission patterns.

A change in vaccine efficacy and duration of protection explains recent rises in pertussis incidence in the United States

Over the past ten years the incidence of pertussis in the United States (U.S.) has risen steadily, with 2012 seeing the highest case number since 1955. There has also been a shift over the same time period in the age group reporting the largest number of cases (aside from infants), from adolescents to 7–11 year olds. We use epidemiological modelling and a large case incidence dataset to explain the upsurge. We investigate several hypotheses for the upsurge in pertussis cases by fitting a suite of dynamic epidemiological models to incidence data from the National Notifiable Disease Surveillance System (NNDSS) between 1990–2009, as well as incidence data from a variety of sources from 1950–1989. We find that: the best-fitting model is one in which vaccine efficacy and duration of protection of the acellular pertussis (aP) vaccine is lower than that of the whole-cell (wP) vaccine, (efficacy of the first three doses 80% [95% CI: 78%, 82%] versus 90% [95% CI: 87%, 94%]), increasing the rate at which disease is reported to NNDSS is not sufficient to explain the upsurge and 3) 2010–2012 disease incidence is predicted well. In this study, we use all available U.S. surveillance data to: 1) fit a set of mathematical models and determine which best explains these data and 2) determine the epidemiological and vaccine-related parameter values of this model. We find evidence of a difference in efficacy and duration of protection between the two vaccine types, wP and aP (aP efficacy and duration lower than wP). Future refinement of the model presented here will allow for an exploration of alternative vaccination strategies such as different age-spacings, further booster doses, and cocooning.

Over the past ten years the incidence of pertussis in the United States (U.S.) has risen steadily, with 2012 seeing the highest case number since 1955. There has also been a shift over the same time period in the age group reporting the largest number of cases (aside from infants), from adolescents to 7–11 year olds. We investigate several hypotheses for the upsurge in pertussis cases by fitting a suite of epidemiological models to incidence data from the National Notifiable Disease Surveillance System (NNDSS) between 1990–2009. We find that: 1) the best-fitting model is one in which the vaccine efficacy and duration of protection of the acellular pertussis vaccine is lower than that of the whole-cell vaccine, 2) increasing the rate at which disease is reported to NNDSS is not sufficient to explain the upsurge and 3) 2010–2012 disease incidence is predicted well. These results demonstrate that the resurgence in pertussis in the U.S. can be explained by past changes in vaccination policy. However, our findings suggest that the efficacy of the currently-used acellular vaccine is not much lower than that of the whole-cell vaccine, and booster doses may be sufficient to curtail epidemics while vaccine research continues.

Immunogenicity and reactogenicity of a decennial booster dose of a combined reduced-antigen-content diphtheria-tetanus-acellular pertussis and inactivated poliovirus booster vaccine (dTpa-IPV) in healthy adults

BACKGROUND

Pertussis in adults and adolescents could be reduced by replacing traditional tetanus and diphtheria (Td) boosters with reduced-antigen-content diphtheria-tetanus-acellular pertussis (dTpa) vaccines. This study evaluated the administration of dTpa-IPV (dTpa-inactivated poliovirus) in adults ten years after they received a booster dose of either dTpa-IPV, dTpa+IPV or Td-IPV in trial NCT01277705.

METHODS

Open multicentre, phase IV study (www.clinicaltrials.govNCT01323959) in which healthy adults, who had received a previous dose of dTpa-IPV, dTpa+IPV or Td-IPV ten years earlier, received a single decennial booster dose of dTpa-IPV (Boostrix-polio, GlaxoSmithKline Vaccines). Blood samples were collected before and one month after booster vaccination. Antibody concentrations against all vaccine antigens were measured and reactogenicity and safety were assessed.

RESULTS

A total of 211 subjects (mean age 50.3 years) received vaccination of whom 201 were included in the according-to-protocol cohort for immunogenicity. Before the decennial dTpa-IPV booster, ≥71.0% subjects were seroprotected/seropositive against all vaccine antigens. One month after the booster dose, all subjects were seroprotected against tetanus and poliovirus types 2 and 3; ≥95.7% subjects were seroprotected against diphtheria and ≥98.3% against poliovirus type 1. Anti-pertussis booster responses for the various antigens were observed in ≥76.5% (pertussis toxoid; PT), ≥85.1% (filamentous haemagglutinin; FHA) and ≥63.2% (pertactin; PRN) of subjects. During the 4-day follow-up, the overall incidence of local AEs was 71.6%, 75.0% and 72.2% in dTpa-IPV, dTpa+IPV and Td-IPV groups, respectively. Pain was the most frequent solicited local adverse event (AE; ≥62.7% subjects) and fatigue the most frequent solicited general AE (≥18.5%). No serious AEs were reported during the study.

CONCLUSION

A booster dose of dTpa-IPV was immunogenic and well tolerated in adults who had received a booster dose of either dTpa-IPV, dTpa+IPV or Td-IPV, ten years previously and supports the repeated administration of dTpa-IPV.

Combating pertussis resurgence: One booster vaccination schedule does not fit all

Pertussis has reemerged as a major public health concern in many countries where it was once considered well controlled. Although the mechanisms responsible for continued pertussis circulation and resurgence remain elusive and contentious, many countries have nevertheless recommended booster vaccinations, the timing and number of which vary widely. Here, using a stochastic, age-stratified transmission model, we searched for cost-effective booster vaccination strategies using a genetic algorithm. We did so assuming four hypothesized mechanisms underpinning contemporary pertussis epidemiology: (I) insufficient coverage, (II) frequent primary vaccine failure, (III) waning of vaccine-derived protection, and (IV) vaccine "leakiness." For scenarios I-IV, successful booster strategies were identified and varied considerably by mechanism. Especially notable is the inability of booster schedules to alleviate resurgence when vaccines are leaky. Critically, our findings argue that the ultimate effectiveness of vaccine booster schedules will likely depend on correctly pinpointing the causes of resurgence, with misdiagnosis of the problem epidemiologically ineffective and economically costly.

Increase of pertussis incidence in 2010 to 2012 after 12 years of low circulation in Spain

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