Division of Infectious Disease, Department of Pediatrics, Vanderbilt University School of Medicine, Nashville TN, USA
Correspondence: Kathryn M Edwards, CCC-5323 Medical Center North, Vanderbilt University School of Medicine, Nashville, TN 372322581, USA. E-mail Kathryn.edwards{at}vanderbilt.edu
Over the past two decades, several countries have reported increasing rates of pertussis disease. In the 1980s the UK and Japan documented increased pertussis illness associated with the decreased uptake of the reactogenic whole-cell pertussis vaccine.1 In the 1990s Canadian epidemiologists reported increased pertussis cases resulting from the administration of an ineffective whole-cell vaccine.2 In the past several years investigators in the US and France have suggested that pertussis is increasing in adolescents and adults.35 Although some have argued that the increase is simply a function of improved diagnostic methods and disease recognition, others believe that there has been a real increase.6 Finally, a recent report from The Netherlands suggests that the increase in pertussis incidence in that country is due to a mismatch between the vaccine strain and the currently circulating Bordetella pertussis organisms.7
In this issue of the International Journal of Epidemiology, Gzyl et al. summarize the epidemiology of pertussis disease in Poland from 1963 to 2000 and suggest a recent increase in reported cases of pertussis in their country as well.8 Using cases submitted to the Ministry of Health (MOH) and census data from the Central Office of Statistics, the Polish investigators determined the population-based incidence of pertussis disease by age. Pertussis disease was defined by:
the presence of the following symptoms: cough with whooping lasting more than 2 weeks following by vomiting in combination with apnoea, cyanosis, subconjunctival bleeding, leukocytosis, lymphocytosis, or contact with a confirmed or suspected pertussis case during the previous 3 weeks.In the majority of cases, culture, immunofluorescence, or serology was used to establish the diagnosis.
In Poland after the introduction of the whole cell pertussis vaccine in 1960, the number of reported pertussis cases per year decreased from a peak of 58 000 to a low of 122 reported in 1986. This represented a decline from 1000 cases per 100 000 population in the pre-vaccine era to only 1 case per 100 000 population in 1986. Similarly the total number of pertussis-associated deaths declined from 1000 in the 1950s to a single case in the 1980s. The last pertussis-related death was reported in 1991. However, beginning in the 1990s reported pertussis cases in Poland increased. From 1997 through 2000 the yearly incidence of pertussis increased to 5.4, 7.4, 2.3, and 5.9 cases per 100 000 population, respectively. Hospitalization rates also rose in the 1990s when compared with the prior two decades.
To understand the potential reasons for these increases, a review of the Polish pertussis immunization programme is needed. Conventional whole cell diphtheria, tetanus, pertussis (DTP) has been routinely administered at 2, 34, 5, and 1618 months of age since the beginning of the immunization programme. Using yearly reports submitted to the Provincial Health Laboratories, vaccine coverage for three and four doses has been 94.9% and 99.5%, respectively for most years studied. The authors suggest that the recently observed increase in reported pertussis cases is likely a reflection of waning immunity in older children and adolescents rather than vaccine failure. This theory is also supported when pertussis cases are stratified by age. During the early years of surveillance, children 04 years of age accounted for 73.9% of the total cases. However, the proportion of pertussis cases in children 04 years of age decreased to 65.9% and then 24.5% during 19761989 and 19901996, respectively. In contrast, pertussis cases in older children 59 years of age and in adolescents 1014 years of age increased 37 fold during the same study periods. As a result of these findings, Poland introduced an additional booster dose of acellular pertussis vaccine administered at 6 years of age.
The report from Poland illustrates the importance of monitoring disease burden of B. pertussis in each individual country. Depending on the country, an increase in pertussis cases may be due to a poorly protective vaccine, recent modifications of the vaccine schedule, waning vaccine-induced immunity, circulation of new B. pertussis strains that are not covered by the vaccine, or a combination of factors. By having data specific to each country the most effective strategy to reduce pertussis disease can be implemented.
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