Patterns of exposure and severity of measles infection. Copenhagen 1915-1925

Using data on children hospitalized with measles in Copenhagen from 1915 to 1925, I found that secondary cases (infected at home) exposed to two or more index cases had a higher case fatality rate than did children exposed to a single index case (relative risk (RR) = 1.90; 95% confidence interval (CI): 1.12 to 3.22). Compared with surviving cases, fatal secondary cases had a shorter interval between their own rashes and rash in the index case (P < .02), suggesting a shorter period of incubation for severe cases. Secondary cases infected by a severe case of measles had higher mortality (RR = 3.87; 95% CI: 1.65 to 9.08) than did secondary cases infected by an index case without pneumonia. These observations suggest that differences in patterns of exposure, possibly due to the dose of infection, may be important for understanding variation in measles mortality.