Seasonal variation in meningococcal disease in Denmark: Relation to age and meningococcal phenotype

Seasonal variation has been shown in meningococcal disease ( MD), but it is not known whether seasonal variation depends on age, gender and meningococcal phenotype. Based on complete registration of MD in North Jutland County, Denmark, during 1980 - 1999 ( n - 413 cases), a Poisson regression model of a sinusoidal form was used to examine: (i) whether the seasonal variation in MD follows a sinusoidal pattern; (ii) the magnitude of seasonal variation in MD; and (iii) whether seasonal variation is related to age and gender and meningococcal phenotype. The peak-to-trough ratio (PTR) was used as a measure of the magnitude of seasonal variation. An overall seasonal variation in MD was found [PTR 2.0, 95% confidence interval (95% CI) 1.5-2.6]. There were differences between age groups and between phenotypes, but not between genders. The highest PTRs were observed for the age groups 5 - 9 y ( PTR 4.9, 95% CI 2.1 - 11.9) and 10 - 14 y ( PTR 3.7, 95% CI 1.4 - 9.7). No seasonal variation was found for children <1 y ( PTR 1.4, 95% CI 0.6 - 3.2) or teenagers 15 - 19 y ( PTR 1.6, 95% CI 0.8 - 3.0). Among phenotypes the highest PTR was observed for C: 2a: P1.2,5 ( PTR 3.8, 95% CI 1.3 - 11.2). Thus, seasonal variation depended on age and meningococcal phenotype, the most pronounced seasonality being in 5 - 14- y-olds and in cases with phenotype C: 2a: P1.2,5.