Perplexities of pertussis: recent global epidemiological trends and their potential causes

Abstract

Recent much-publicized increases in pertussis case reports in some countries with high vaccine coverage have raised concerns about its current and future control. The ubiquity of this trend, however, remains unexamined. In an attempt to paint a global picture, we used case counts to determine which countries experienced statistically significant trends in incidence over the past two decades and to map changes in incidence during this period. These data reveal that pertussis resurgence is not a universal phenomenon. The heterogeneity in incidence trends, even in countries with superficially similar demography, socioeconomic conditions and vaccination programmes, is striking and requires explanation. In this opinion piece, we review and assess the multifaceted proposed explanations incorporating evolution, population dynamics, and the details of immunization programmes. While we do not solve the riddle that is pertussis epidemiology, we highlight critical aspects that are likely to hold the key to understanding its worldwide epidemiology.

Fig. 1

[colour online]. Boxplot of annual rates of change of pertussis incidence between 1990 and 2010 for countries that met our inclusion criteria: mean DTP3 vaccine uptake of >80% between 1990 and 2010; populations of >5 million individuals; and >80% complete case count records for this time period. The pink area indicates countries with significantly positive trends in incidence based on Kendall's tau rank correlation (P < 0·05), white indicates countries for which trends were not significant, and blue indicates countries with significantly decreasing trends. Note that rates of change are plotted on a log scale. Representative time-series of countries with increasing (Australia), stationary (Argentina), and decreasing (Spain) trends are shown on the right.

Fig. 2

[colour online]. Absolute incidence of pertussis per country for the years 1990, 2000, and 2010, binned logarithmically. Data are smoothed using 5-year moving averages to account for the known periodicity of pertussis.

Fig. 3

[colour online]. Changes in incidence of pertussis per country, 1990–2000 and 2000–2010, binned logarithmically. Data are smoothed using 5-year moving averages to account for the known periodicity of pertussis. Blue shading indicates decreases in incidence; maroon shading indicates increases in incidence.