The relationship between age and risk for classic dengue fever has never been quantified. We use data from clinical patients to show that the relative risk of having classical disease after primary dengue virus infection increases with age. This relationship has implications for strategies aimed at controlling dengue fever.

Dengue fever has emerged as a serious international public health threat with almost half of the world’s population at risk for infection (

Clinical dengue incidence data that have been serologically confirmed were abstracted from a survey conducted in 1997 in the communities of Belém and Ananindeua in Pará State, Brazil (

Estimated minimum proportion of the population, by age, with laboratory-confirmed classical dengue, showing exact 95% binomial confidence intervals. A) Fitting a logistic regression model (not shown) to the absolute proportion produced a significant age estimate: McFadden R^{2} = 0.762, χ^{2} = 5,196.13, df = 1, p<0.001. B) Relative risk, by age, of having classical dengue after primary infection. Black circles, observed; line, model fit. See text for details of statistical analysis.

We further investigated the relationship between age and probability of having clinical disease by calculating the risk for each age class relative to the age class that had the highest proportion of clinical cases. Unlike the absolute proportion, the relative risk is independent of transmission intensity. In calculating relative risk, we assumed that the risk for dengue infection was independent of age, which was corroborated by a seroepidemiologic study performed in Fortaleza, Brazil, in 1994 (

The logistic model provided a significant fit to the data (McFadden R^{2} = 0.512, χ^{2} = 4.86, df = 1, p<0.028) and described a clear positive relationship between age and relative risk for clinical disease (β = 0.164; bootstrap 95% confidence interval 0.1470–0.1769), as shown in the

To our knowledge, this is the first time data have been used to empirically derive the quantitative relationship between age at time of primary dengue infection and risk of having clinical dengue fever. These findings are consistent with results of earlier studies that suggest that adults are more likely than young children to have clinical dengue (

Several factors should be considered when interpreting these results. First, because dengue virus serotypes l and 2 were circulating in the population during the study period, some persons may have been infected with both serotypes during the 1-year period and, therefore, clinical signs may have resulted from a secondary infection. This proportion is probably small. Second, several factors other than age are thought to influence severity of classic dengue illness, including viral serotype and strain (

Despite the complexities of dengue epidemiology, these findings provide strong empirical evidence that age is an important factor in determining risk for disease severity after primary dengue virus infection. As such, these findings have important implications for initiatives aimed at controlling dengue. Interventions focused on reducing the number of

Mr Egger is a doctoral candidate in the Department of Infectious and Tropical diseases at the London School of Hygiene and Tropical Medicine. His research focus is the epidemiology of dengue.

Dr Coleman is an honorary senior lecturer in the Department of Infectious and Tropical Diseases at the London School of Hygiene and Tropical Medicine. He is an epidemiologist with a particular interest in the dynamics of vectorborne disease transmission.