Risk Factors Associated With Secondary Measles Transmission in Index Cases

BACKGROUND: Limited information has been shown quantitatively assessed factors associated with the risk of secondary transmission in measles index cases. METHODS: We collected data on measles index cases reported in Osaka Prefecture between November 2018 and March 2019, along with their close contacts. Secondary attack rates (SARs) in contacts of index cases were calculated and compared depending on the vaccination status, classical or modified measles and other risk factors in index cases. For the multivariate analyses, we used binary logistic regression to adjust for these potential confounding factors on the secondary transmission and calculated the adjusted odds ratio (aOR) for the SARs. RESULTS: A total of 105 index cases were included, involving 9,846 close contacts, with 72 secondary cases identified. A significant negative association between the number of vaccinations received by index cases and the risk of secondary infection in his/her close contacts was observed (P < 0.01). Index cases who had received one vaccine dose had an aOR of 0.22 (95% confidence interval [CI], 0.08-0.65), while those with two or more doses had an aOR of 0.25 (95% CI, 0.07-0.85). Index cases who did not exhibit all three classic measles symptoms had a significantly lower risk of causing secondary infections compared to those who presented with all three symptoms (aOR 0.12; 95% CI, 0.03-0.39). CONCLUSION: The indirect protective effect of measles vaccination would present important evidence supporting the further strengthening of vaccination policies.