Pertussis control: where to now?

ertussis is a disease of significant morbidity and, in infants, mortality. Regrettably, even though there is greater than 20-fold reduction in pertussis burden with immunisation,1 it persists globally as a significant public health problem. For more than two decades, Australia has had the highest reported rates of pertussis in the world.2 In the 1990s, this was driven by the introduction of mandatory reporting by laboratories of positive test results for vaccine-preventable diseases to the National Notifiable Diseases Surveillance System and extensive use of serological tests for diagnosis, primarily in adults.3 Unlike many other countries, all positive test results in Australia are included in national data. Also, testing for pertussis by polymerase chain reaction (PCR) has qualified for reimbursement since 2008, after which a sevenfold increase in testing of children in general practice was documented.4 Pertussis epidemics occurred sequentially across Australia from 2008 to 2012 and, unlike previous epidemics, the highest notification rates were for children under 10 years of age. This raises the question of whether Australia’s “pertussis problem” is related to vaccines with poor effectiveness or is an artefact of testing. Observational methods are used to measure vaccine effectiveness (VE) (also known as “field efficacy”). The screening method enables estimation of VE if the vaccination status of patients with a case of the disease and population vaccine coverage are known — the more effective the vaccine, the lower the likelihood of patients with a case of the disease having been vaccinated compared with the source population.5 The screening method performs best when about 50% of the population is vaccinated. When vaccine coverage is over 90%, estimates of VE change substantially with small changes in population coverage estimates. In this issue of the Journal, Sheridan and colleagues use the screening method to estimate VE for acellular pertussis vaccine in Queensland children during an epidemic in 2009 and 2010.6 They found that VE for three doses in children aged from 1 to < 4 years was over 80%. However, similar to studies in the United States,7,8 VE fell significantly and progressively in children over 5 years of age, whether they had received four or five doses. It was previously reported that among Queensland children born in 1998, those who had received one or more doses of whole-cell pertussis vaccine were significantly better protected than those who had received only acellular vaccine, especially after 6 years of age.9 A national study, which included Queensland data from 2009, took a different approach — cases were individually matched by birth date to children on the Australian Childhood Immunisation Register and were limited to children younger than 4 years.10 Similar VE estimates were obtained for the first 2 years of life, but, in contrast to findings from the Queensland study, there was a significant and progressive fall in VE between ages 2 years and 4 years (the latter being the age at which children were eligible for the fourth dose). P