Pertussis is a cyclical disease with increases occurring every 3-4 years. The third quarter (running from July to September) is usually the period of highest pertussis activity annually.
Pertussis epidemiology can be affected by a number of factors, including changes in laboratory techniques and heightened awareness of the disease. Overall increases in laboratory confirmed cases were observed in older age groups and subsequent to the introduction of new laboratory methods from 2001. A number of countries with longstanding vaccination programs and sustained high coverage have reported a resurgence of pertussis, particularly in older age groups, and more recently in some of those countries in young infants. Different explanations have been proposed for the rising pertussis incidence in these countries. The relevance of apparent increased pertussis among older age groups, in whom illness tends to be milder, atypical and underdiagnosed, relates to whether the increases are real or driven by improved case ascertainment. In England and Wales serologic testing, together with publications promoting higher awareness, has improved case ascertainment in older age groups.
Further increased pertussis activity was identified from the third quarter of 2011, however, with cases in those aged 15 years and older exceeding expected levels. This may be in part due to increased awareness amongst health professionals improving case ascertainment in older age groups. This is reflected by the increased demand for serology testing which is the predominant method of confirmation in adolescents and adults who typically present with milder features late in the course of the illness. However, waning immunity following vaccination and /or natural infection is also likely to be an important contributory factor. This increased activity has continued into the first quarter of 2012 and has extended into <3 month infants. Although a greater number of cases are being confirmed in older age groups, the incidence in these age groups remains relatively low.
When laboratory-confirmed cases are presented by laboratory method, testing is presented in the order; culture, PCR, serology. Therefore a patient who tests positive by culture and serology, for example, will be shown as culture-positive and a patient who is indicated as confirmed through serology testing will have been serology-positive only.
2012 Health Protection Reports for Pertussis