The increasing number of whooping cough cases and fatalities globally has raised the spectre of new strains emerging that are increasingly resistant to the acellular vaccines introduced to tackle pertussis in the late 1990s.
The rise in pertussis cases over the past decade is linked to increased vigilance among paediatricians with the widespread use of PCR assays during the pandemic enabling increased screening and identification of cases within the community.
WA Health data showed 88 cases of pertussis have already been recorded this year in the South West, with 84 cases identified in the Perth metropolitan area.
China is currently in the grips of an outbreak, with decreased immunity after acellular vaccination being a key reason for the increase alongside the apparent changes in Bordetella pertussis.
Although China is among the few countries using copurified acellular pertussis vaccines in national immunisation programs, declining protection after inoculation is not a unique problem.
Lead author of the study Dr Kai-hu Yao, said that before COVID, numerous studies involving serological analyses and enhanced clinical surveillance suggested that reported pertussis rates were likely to be substantially underestimated, despite a ten-fold increase in the number of reported cases compared with the same period last year.
“The actual number of cases could be tens of thousands time greater than the reported cases, with one serological study revealing that the incidence of pertussis in individuals older than 15 years was 25,625 times higher than the reported incidence,” Dr Yao said.
“Doctors in general hospitals need to be more vigilant regarding pertussis because the disease can be challenging to identify in adolescents and adults owing to frequent atypical manifestations.”
“Reduced vaccine-induced immunity contributes to the resurgence of pertussis but remarkably, the coverage of the diphtheria, tetanus, and acellular pertussis vaccine in China has remained above 98% during 2009–22, despite the COVID-19 pandemic,” Dr Yao said.
“If waning immunity were solely responsible, logically more cases would be observed in adolescents and adults.”
Another crucial epidemiological factor was the evident resurgence in reported pertussis cases before the pandemic. By 2019, national reports registered 30,027 cases, reverting to levels not observed since the late 1980s, but strict pandemic control measures sharply disrupted this upward trend with a decline in the number of infant cases observed since 2018–21.
“Given the decrease in infant cases, associating the sudden surge in pertussis-related deaths with the circulation of highly virulent strains resistant to erythromycin is logical, and ongoing surveillance has identified a shift in dominance among clinical isolates,” he said.
“The ptxP3 variant of B pertussis is more virulent than ptxP1 strains. In China, the erythromycin-sensitive ptxP3 strain has been identified since 2002, with the first erythromycin-resistant ptxP3 strain (ERBP-ptxP3) identified in 2017, marking a shift in strain characteristics.
“ERBP-ptxP3 has a stronger ability to evade immune selection pressure as well as greater antigen variations and deficiencies, and the widespread prevalence of this strain poses a substantial challenge to both clinical therapies and the effectiveness of the pertussis vaccination program.
“To address the increasing prevalence of ERBP-ptxP3, we have advocated exploring alternative antibiotic options, and on May 25, 2024, the National Disease Control and Prevention Administration issued an updated pertussis prevention and control strategy, which could initially lead to an increase in reported cases.”
Whooping cough epidemics occur every 3–4 years in Australia and between 2008 and 2012, all states and territories experienced their largest pertussis epidemic since national reporting began in 1991.
During these epidemics, the highest rates of disease were in infants <6 months of age and children 5–9 years of age, and although the high number of cases identified was partly due to the increased availability of more sensitive tests, waning immunity was also a factor.
“Observational studies have shown that even a single dose of DTPa has a vaccine effectiveness of 51–55% against hospitalised pertussis, yet long-term follow-up of vaccinated adults has shown a rapid decline in levels of pertussis antibodies within the first two years,” Dr Yao said.
“Antibody levels then continued to decline steadily, although mean antibody levels remained above baseline 10 years after vaccination.”