Prevention of pertussis: An unresolved problem

ABSTRACT

Pertussis is a highly contagious respiratory disease caused by Bordetella pertussis. However, after the introduction of the whole-cell pertussis vaccine (wP), the annual incidence rates of the disease progressively declined. Despite this result, the inclusion of wP in the national immunization schedule of infants and young children was debated regarding its safety. Several efforts to produce vaccines based on B. pertussis components capable of evoking protective immunity with no or limited adverse events were made. Of these others, five pertussis antigens were considered possible components of acellular vaccines (aPs): pertussis toxin (PT), filamentous haemagglutinin (FHA), pertactin (PRN) and fimbria proteins 2 and 3. However, the introduction of aPs was followed by a slight but progressive increase in the incidence of pertussis. This paper discusses the potential reasons for reduced aPs efficacy. Moreover, it attempts to evaluate the real effectiveness of aPs and the potential differences between available preparations. Data analysis showed that several boosters are needed to maintain protection against pertussis and additional studies are needed to confirm the antigens that should be included in aPs to improve the prevention of pertussis.

Introduction

Pertussis, also known as whooping cough, is a highly contagious respiratory disease caused by Bordetella pertussis, a gram-negative bacterium.¹ Until the 1940s, pertussis) was a universal disease of early childhood. In 1934, more than 265,000 cases were reported in the USA. The mortality rate was approximately 10%, a value higher than that of poliomyelitis and measles combined.² However, after the introduction of the whole-cell pertussis vaccine (wP), the annual incidence rates of the disease progressively declined. The total number of reported annual cases dropped to under 100,000 in 1948 and remained between 1,200 and 4,000 during the 1980s.³ Significant reductions in the pertussis incidence were generally observed in all of the other countries where this vaccine was largely used. Despite this result, the inclusion of wP in the national immunization schedule of infants and young children was debated regarding its true or simply supposed efficacy and for safety concerns. The effectiveness of wP reported by some studies was considered as too low to justify its use.^4-6 The development of significant injection site reactions and fever in almost half of all vaccinated children was stressed.⁷ The occurrence in a non-marginal number of cases of acute and serious systemic reactions a few hours after wP injection (e.g., seizures, temporary shock-like state, and persistent crying) was considered a serious problem.⁸ Finally, the possible relationship between wP use and the development of severe and chronic encephalopathy, causing refractory seizures and long-term neurologic disabilities was underlined.⁹

All of these concerns had a significant global effect. In the 1970s and 1980s, pertussis vaccination programmes were suspended in Japan.¹⁰ A similar decision was made in Sweden in 1979.¹¹ During the same period, pertussis vaccination coverage was significantly reduced in several other European countries.¹² Due to the cost of defending lawsuits, numerous companies withdrew the vaccine from the market in the USA.¹³ Fortunately, over the following years, the careful evaluation of the available data led to a more realistic evaluation of these limitations. Evidence showed that the low effectiveness of wP was related only to a preparation marketed by a single company that could be excluded from official immunization programmes.^4,5 Regarding safety concerns, evidence showed that the acute systemic reactions, although undeniable, did not cause serious long-term sequelae in any of the infants and that the cases of encephalopathy could not be ascribed to the vaccine because they were dependent on an inherited genetic defect amongst a subset of vaccinated infants. The central nervous system disease was found to be due to de novo mutations in the SCN1A gene that codes for the voltage-gated neuronal sodium channel,^14,15 and wP vaccinations could be considered only a trigger of the clinical manifestations of the disease.

Despite these clarifications, apprehension surrounding wP safety remained high and led to the development of new vaccines. Several efforts to produce vaccines based on B. pertussis components capable of evoking protective immunity with no or limited adverse events were made. Of the pertussis antigens considered for use in acellular vaccines (aPs) only five have been used so far: pertussis toxin (PT), filamentous haemagglutinin (FHA), pertactin (PRN) and fimbria proteins 2 and 3. Numerous aPs containing up to 5 components were developed. Several aP infant efficacy trials clearly showed that these prophylactic measures had short-term efficacy not substantially different from or even higher than that of Wp.^16-21 Moreover, all of the studies showed that all of the aPs, regardless of the number of included antigens or the schedule used, had significantly higher tolerability and safety. In particular, significant acute or chronic neurologic adverse events were rare, and local reactions were significantly less common.^16-21 Consequently, aPs were included in the national immunization schedules of many countries, particularly those in the industrialized world, in place of wP.

However, the introduction of aPs was followed some years later by evidence that the demonstrated rise in the incidence of pertussis already seen in the last years of wP use, was further significantly and progressively increased. In the USA, the greatest number of pertussis cases was documented in 2012, when 48,277 diagnoses were made. Recent reports have indicated that the US incidence of pertussis is declining, but the number of reported cases in 2016 (17,972) remained significantly higher than the number found when the use of wP predominated or immediately after the introduction of aP. In 2016, the incidence rates per 100,000 were 70.9 for children <6 months, 31.9 for 6- to 11-month-olds, and 13.7, 14.8, and 16.3 for those aged 1–6, 7–10, and 11–19 years, respectively.²² Similar trends were found in Europe. In 2014, 40,727 cases of pertussis were reported to the European Surveillance System by 29 EU/EEA countries.²³ The notification rate was 9.1 cases per 100,000 population, which was higher than that in 2013 but lower than that in the epidemic year of 2012. Age-specific rates were highest amongst children <1 year of age (51.6 cases per 100,000 population), followed by 10– to 14-year-olds (24.4/100,000) and 15– to 19-year-olds (19.7/100,000). This sustained increase spread doubts regarding the real efficacy of aPs compared with wPs. This paper discusses the potential reasons for this reduced vaccine efficacy. Moreover, it attempts to evaluate the real effectiveness of aPs and the potential differences between available preparations.

Reasons for pertussis resurgence

Detection bias

One, if not the most important, cause of pertussis resurgence might be an artefact of the improved reporting due to changes in awareness and diagnostic methodologies.^24-26 For many years, particularly amongst adolescents and adults who generally suffer from mild/atypical pertussis,^27-29 diagnosis was not common, and incidence rates were greatly underreported.^30,31 As Cherry highlighted,³² it is highly likely that the studies of pertussis vaccines, particularly those concerning aPs, and the evidence of the pertussis resurgence have brought more attention to this disease and its epidemiology, leading to more diagnosis and reports. Furthermore, the diagnosis of more pertussis cases was further facilitated by the availability of molecular methods that are significantly more sensitive and less time consuming than the traditional culture for B. pertussis detection.³³ Moreover, PT can be presently diagnosed by single-serum and oral fluid antibody evaluation in adolescents and adults.³⁴

Several studies have shown that to address potential pertussis-related problems, the use of laboratory tests by physicians has progressively increased with a parallel increase in pertussis diagnoses and reports. The proportion of pertussis-related problems resulting in a pertussis test request increased from 0.25% between April 2000 and March 2004 to 1.7% between April 2010 and March 2011 (odds ratio [OR] 7.0; 95% confidence intervals [CIs] 5.5–8.8] in Australia.³⁵ Moreover, pertussis notifications were highly correlated (r = 0.99). In Toronto, Canada, the incidence of pertussis was 2 per 100,000 from 1993 to 2004, and this rate increased to 10 per 100,000 from 2005–2007. This increase was associated with a concomitant 6-fold surge in test specimen submissions after the introduction of a new, more sensitive PCR assay.³⁶ However, a detection bias does not fully explain certain epidemiological variations. The World Health Organization (WHO) conducted a detailed evaluation of pertussis incidence, vaccination coverage and schedule, surveillance methods, case definitions, and type of vaccine used across 19 countries supposedly providing high-quality data on vaccine coverage and trends in pertussis burden over time.³⁷ Although the increased incidence could be attributed to cyclic patterns amplified by detection bias in most countries, 5 of 19 countries (Australia, Chile, Portugal, the USA and the UK) showed a true resurgence in pertussis-related morbidity. Moreover, the diagnosis of pertussis in adolescents was more common, as was the number of hospitalizations, particularly for unvaccinated or incompletely vaccinated infants.

Poor protection persistence

Studies conducted of unvaccinated participants have shown that the protection offered by natural B. pertussis infection is not permanent.³⁸ Consequently, it is unsurprising that children who have received pertussis vaccines can be infected by B. pertussis years after their last immunizing dose. However, the duration of protection offered by aP was significantly briefer than that due to wP.³⁹ Clark et al. reported that some children who were fully vaccinated with aP during the first year of life suffered from pertussis at 7–10 years of age, whereas those who received wP were at risk of developing pertussis only during adolescence.⁴⁰ In Canada, a study of the transitional period from wP to aP administration revealed that children who received aP during infancy were at the highest risk of suffering from pertussis during the first 4 years of life, whereas the highest incidence amongst those vaccinated with wP was between 5 and 9 years of age.⁴¹ Consistent with this finding are the numerous unexpected pertussis cases described amongst older children and adolescent who received aP, including those who were given a booster dose only two or three years before. Khan et al. conducted a 1:2 matched case-control study of schoolchildren during a pertussis outbreak in the USA and found that of the 127 pertussis cases reported, the majority were adolescents (10–19 years of age, 50%) and adults (20 years or older, 22%); only 10% were infants and children less than 5 years of age.⁴²

The risk of pertussis was strictly related to the last administration of aP. When this dose was given at 4 years, the risk was higher than when it was given at 5 years of age (adjusted OR, 2.45; 95% CIs, 1.16–5.16). Similar data were collected in California, where adolescents who were given 4 doses of aP during infancy were at a significant higher risk for infection with Bp than those who were vaccinated with 4 doses of wP.⁴³ On the other hand, pertussis cases were diagnosed in school-age children with chronic cough who received a booster dose of aP a few years before. Esposito et al. reported that 18 of 96 (18.7%; 95% CIs 11.5 – 28.0) children and adolescent with chronic cough had pertussis. In 83.3% of these cases, the disease occurred despite booster aP administration. In 2 cases, pertussis was diagnosed less than 2 years after a booster injection.⁴⁴

Immunological reasons can explain the short-term protective effect of aPs together with the evidence that these vaccines, contrary to wPs, prevent the development of the signs and symptoms of pertussis but are not effective against colonization and do not prevent the transmission of B. pertussis.⁴⁵ Several studies have reported that the serum levels of antibodies against B. pertussis components of aPs rapidly decline.^46-50 One of the earliest examples of this finding was provided by Esposito et al. who studied 38 children previously received the dose at the proper time during infancy. In most of these cases, a booster was provided before entering school of a combined vaccine (TdaP-HBV) containing, together with aP, tetanus, diphtheria, and hepatitis B antigens.⁴⁶ Serum antibody titres against aP antigens included in the vaccine (PT, FHA and PRN), peripheral blood mononuclear cell-specific proliferation in the presence of aP antigens and the secretion of the cytokines interferon γ (IFNy), interleukin (IL) 2, IL-4 and IL-5 by those cells were measured approximately 5 years after vaccination. Only a few of the studied children had significant serum concentrations of specific IgG antibodies against all three B. pertussis antigens. In addition, a minority of the children had persistent T-cell responses, clearly demonstrating that protection had rapidly declined.

On the other hand, studies that have compared immune responses after natural infection and the administration of both vaccines have clearly shown that the immune response evoked by aPs significantly differs from that due to natural infection or wP.⁵¹ Natural infection and wP induce similar immune response (slightly lower for wP) with the production of antibodies of IgG1, IgG2 and IgG3 subclasses that reflects a strong induction of Th1 cells.⁵² Moreover, a robust Th17 immunity is induced. Practically, both natural infection and wP evoke a Th1/Th17 immune response that is likely particular important because of its enhanced innate immune response to B. pertussis. Conversely, aPs evoke primarily IgG1 antibodies and to lesser extent IgG4, and this finding is consistent with a Th2-type response. Th1 and Th17 response was weak.⁵³ These differences were confirmed by studies of CD4⁺ T-cell responses. Experimental animal models demonstrate that aP evokes CD4+ T cells producing high amounts of IL-4, IL-5 and limited concentrations of IFNγ, a condition consistent with a Th2 response.⁵⁴ Conversely, wP administration is associated with the production of IFNγ and IL17A, suggesting a mixed Th1/Th17 response.⁵⁵ Finally, the production of specific CD4+ T-cell cytokines was significantly higher in children vaccinated with aP than in those who received wP at 4 years of age. However, the administration of a booster did not lead to a significant increase, whereas this effect occurred in children vaccinated with wP.⁵⁶ Additional controls, two years after the booster, revealed that the cytokine production of IL-17 in aP-primed children was poor, whereas this production was high in children given wP.⁵⁷

Although serological or cell-mediated markers of protection are presently unavailable,⁵¹ all these findings seem to indicate that wP is able to confer higher protection than aP and this might be one of the reasons for pertussis resurgence.

The emergence of Bordetella pertussis strains with different genetic characteristics

During the prevaccine era and after the introduction of wP, circulation of B. pertussis strains with modified or absent vaccine antigens, particularly PRN, had already been reported. However, the use of aP vaccines was clearly associated with a greater circulation of these strains.^58-60 Although, there is no public health surveillance data showing that this has resulted in increased rate of disease, it is possible that these strains might have played a role in reducing vaccine effectiveness. Immune pressure from vaccinations with the emergence of vaccine escape mutants is likely the main cause of this phenomenon as suggested by the evidence that the evolution of the B. pertussis population varied across countries and was strictly associated with the country vaccination coverage.^60,61

Genetic variations of the genes encoding the proteins that are included in aP were observed with a higher frequency than those encoding other B. pertussis genes. Bart et al. reported that the major changes in the antigen gene alleles were from PTA2 to PTA1, fim2-1 to fim2-2, PRN1 to PRN2, and fim3-1 to fim3-2.⁵⁹ Moreover, the emergence of more virulent B. pertussis strains with significant polymorphisms in genes encoding for PT was demonstrated.^59,63-65 Strains with the PT promoter allele PTP3 were found to be able to produce a greater amount of PT and have greater virulence than those containing the aP allele PTP2,⁶⁶ thereby causing more severe disease in younger children.⁶⁷

Finally, the deletion of antigens covered by aPs was demonstrated. Strains that did not express in isolation or combination with FHA, PT and PRN were observed.⁶⁸ Of these strains, the loss of PRN expression was the most largely studied primarily because it was reported in several geographic areas, although with significant differences across studies. A low frequency of B. pertussis strains lacking pertactin has been found in Finland,⁶⁹ France,⁷⁰ Italy,⁷¹ and Japan.⁷² Significantly higher and increased detection frequencies of PRN-deficient strains were found in Australia,⁷³ Israel,⁷⁴ and the USA.⁷⁵ From 2008–2012, when a large outbreak of PT occurred in Australia, 30% (96/320) of B. pertussis isolates did not express PRN. The pertussis resurgence in Israel was associated with the detection of the increasing prevalence of PRN-negative strains.⁷⁴ In the USA, 640 (85%) of 753 isolates of B. pertussis detected across 8 states from May 2011 to February 2013 (a period with a high incidence of PT) were PRN negative.⁷⁵ The association between the outbreaks of pertussis and the detection of PRN negative strains have led to the supposition that loss of PRN provides a selective advantage to the B. pertussis strains, causing a reduced response to the aPs containing this adhesin. This hypothesis was further supported by the evidence showing that isolates not producing PRN are capable of sustaining longer infection as compared to PRN producing isolates in an in vivo model of aP immunization.⁷⁶ However, the real clinical relevance of this genetic modification is debated. Martin et al. retrospectively calculated that the risk of developing pertussis due to a PRN-negative strain was significantly higher amongst those who had received at least one dose of aP (unadjusted OR 3.2; 95% CIs 1.9–5.3).⁷⁵ Conversely, Breakwell et al. reported different results; these authors studied the effectiveness of TDaP and TdaP in participants who were infected 90% of the time by PRN-negative B. pertussis.⁷⁷ In both children and adolescents, vaccine effectiveness estimates were similar to those reported in different settings, where the prevalence of PRN was low. On the other hand, the clinical relevance of all of the other genetic modifications of B. pertussis, together with their role favouring pertussis resurgence, were not precisely defined.

Effectiveness of presently available acellular pertussis vaccines

Presently, only aPs containing 3 or 5 B. pertussis antigens are marketed. GSK vaccines contain 25 µg PT, 25 µg FHA, and 8 µg PRN, regardless of the number and type of other antigens included in commercial preparations.⁷⁸ Merck's currently licenses vaccines contain 5 B. pertussis antigens, but the amount of some varies based on the number of additional antigens. In diphtheria-tetanus-acellular pertussis vaccine (TDaP) 10 µg PT, 5 µg FHA, 3 µg PRN, and 5 µg FIM types 2 and 3 are included. Conversely, when polio, Haemophilus influenzae type b and hepatitis B vaccines are added, the amount of both PT and FHA are increased to 20 µg.⁷⁹ In most countries, vaccines that combine TDaP with other antigens (i.e. HB, HiB, and/or IPV) are used for the first 2–5 years of life while TdaP vaccines (sometimes combined with IPV or other antigens) are used beginning at ages as young as 4 years.

Several studies have measured the protective effect of contemporary aPs. However, estimates of efficacy and effectiveness of these vaccines are limited by the brief follow-up period after infant administration and by the method used to diagnose pertussis. In most of these cases, the WHO's definition of pertussis was used.⁸⁰ This method most likely underestimated the true pertussis incidence because it identifies severe but fails to track mild cases without a prolonged cough.⁸¹ Although no comparative study of the presently marketed aP formulations is available, the efficacy and effectiveness of these vaccines seem similar. The efficacy of another three-component aP (3aP) and the 5-component aP (5aP) was evaluated in Italy by Greco et al.¹⁸ and in Sweden by Gustafsson et al.¹⁹ In these cases, the results are comparable because these authors used similar vaccine dose schedules, methods, follow-up durations (approximately 2 years), and laboratory assays for pertussis infection. The efficacy was 83.9% (95% CIs 75.8–89.4) for the 3aP and 85.2% (95% CIs 80.6–88.8) for the 5aP.

Comparisons between studies that have measured effectiveness are significantly more difficult because the study designs, administration schedules, follow-up durations and case-definition criteria were different. Moreover, as in the case of Bisgard et al. [82 and Misegades et al.,⁸³ the combined estimate of all the aP used over a given period was made, which makes it impossible to differentiate the weight of single preparations on the reduction of the pertussis incidence rate. However, the few data presently available seem to indicate that, although different in composition, 3aP and 5aP show a similar effectiveness.⁸⁴ On the other hand, both vaccines contain PT, a major contributor to the clinical picture of pertussis and most likely the essential antigen for the production of protective immunity.⁸⁵ A vaccine containing only this antigen has been effective in Sweden.⁸⁶ Moreover, a combined vaccine containing only PT for protection against pertussis has successfully controlled pertussis in Denmark since 1997.⁸⁷ In both cases, the effectiveness was similar to that obtained with multivalent aPs concomitantly used in other countries. To be used as an antigen, however, PT requires detoxification. In all currently available aPs, detoxification is achieved via treatment with many chemical agents. Although effective, this chemical treatment dramatically alters the immunological properties of PT. Detoxification with formaldehyde, glutaraldehyde, or both induces the greatest modifications and causes the weakest immune response, whereas that due to hydrogen peroxide is less destructive.^88,89 The positive results obtained in Denmark with a monovalent aP vaccine containing only PT are most likely because of the use of a PT detoxified using this more conservative method.⁸⁷ In contrast, PT genetically detoxified through the substitution of two residues necessary for its enzymatic activity maintains all functional and immunological properties.^90-91 Substitution of the presently used PT with this genetically modified option might significantly improve the global effictiveness of aPs.

The real importance of other proteins is debated, and certain authors think that they are not useful to increase the protection already given by PT.⁹² Pre-existing FHA antibodies do not prevent pertussis.⁹³ Moreover, a vaccine containing only PT was found as protective as a vaccine containing both PT and FHA. However, short-term results of this study remain debatable, as the PT-FHA vaccine was handicapped by containing little more than half the PT of the PT-only vaccine. Moreover, prolonged unblinded passive surveillance of the trial cohort enrolled in this study has indicated that the PT-FHA vaccine provided better long-term protection against pertussis than the monocomponent PT vaccine.⁹⁴ Pre-existing PRN antibodies do not prevent pertussis.⁹⁵ Moreover, as previously reported, the loss of PRN by B. pertussis strains has not systematically been associated with an increased failure rate of aPs.⁷⁷ Fimbriae are not included in 3aP. Fim2 and Fim3 are important antigens for wPs because clinical trials have shown an association of anti-fimbriae antibody-mediated agglutination and protection. In a study specifically designed to search for serologic correlates of immunity to B. pertussis cough illnesses, Cherry et al.⁹⁶ and Storsaeter et al.⁹⁷ reported that protection against household exposure was significantly correlated with the presence of antibodies to Fim2 and Fim3. Moreover, Olin et al.⁹⁸ collected interesting data regarding the role of fimbriae using a double-blind, randomized, controlled study. These authors compared a no-longer-available 3aP containing lower amounts of B. pertussis antigens (5 μg PT, 2.5 μg FHA, and 2.5 μg PRN) with the presently marketed 5aP, a two component aP and wP. The results indicated that during a mean follow-up period of 22 months after the third vaccine dose, all of the vaccines showed the same efficacy against typical pertussis. However, when only culture-positive infections, considered regardless of cough, wP and 5aP were more effective than 3aP, which in turn, was more effective than 2aP. The relative risk of developing pertussis for the 3aP compared with the 5aP was 1.82 (95% CIs 1.14–2.90). These authors concluded that the lower efficacy of the 3aP against mild disease suggests fimbriae plays a protective role against B. pertussis infection. However, it is not known if this conclusion can be transferred to the presently available 3aP. Although it has been reported that the correlation between antigen dosage and the magnitude of the immune response is poor (with the exception of PT, at least in certain studies),⁹⁹ the possibility of higher B. pertussis antigen concentrations cannot be excluded because those contained in the present 3aP might have led to different results. Additional studies are needed to clarify whether protection is really increased by the inclusion of fimbriae with regard to the other Bp antigens.

Conclusions

Although effective, aPs presently have less efficacy/effectiveness than desired. The long-term effect of infant vaccination is limited and it is extremely important to maintain adequate vaccination coverage with several aP boosters throughout life.¹⁰⁰ At this regard, it should be important to better clarify whether the booster aP dose should be anticipated in adolescents. Moreover, it is important to clarify whetherthe aPs with 5 components offer further advantages in terms of effectiveness in comparison with aPs characterized by a lower number of antigens.

Theoretically, an ideal pertussis vaccine must evoke a long-lasting protective response, prevent transmission and limit the clinical manifestations of B. pertussis infection. The recent evidence showing that a likely effective immune response is associated with a Th1/Th17 immune response suggests that attempts to induce this response with a safe and well-tolerated vaccine must be made. Unfortunately, the means through which this goal can be accomplished are unknown, and whether this kind of immune stimulation is associated with potential risks is not defined. The use of genetically detoxified PT can further increase the efficacy of aPs. The inclusion of antigens that evoke bactericidal, opsonophagocytic, or both types of antibodies is one possible further improvement, although present knowledge in this regard is poor. Additional studies are needed to confirm the antigens that should be included in aPs to improve the prevention of pertussis.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This review was supported by an unrestricted grant from the World Association for Infectious Diseases and Immunological Disorders (WAidid 2018_06).