Measles Ring Vaccination in Dormitories: An Outbreak Containment Measure and Simulation Study

Abstract

Background: Two measles outbreaks occurred in migrant worker dormitories in Singapore in 2019. We report the outbreak investigation and use of ring vaccination to contain transmission. Methods: We collected clinical and epidemiological data from laboratory-confirmed measles cases and conducted field investigations to examine the interactions between dormitory residents. Two measles seroprevalence studies were performed using 1136 residual sera obtained from Bangladeshi, Chinese and Indian workers previously enrolled in 2016-19 for a study on latent tuberculosis and 101 samples prospectively collected from healthy workers residing on the same level as the index cases. We administered measles, mumps and rubella vaccine to close contacts and dormitory residents residing on the same level as a case (ring vaccination). The effectiveness of ring vaccination in a dormitory of varying contact patterns and population immunity was also evaluated using simulation models. Findings: From Jun 14 to Jul 29, 2019, there were 12 laboratory-confirmed cases of measles from two distinct dormitory outbreaks and 214 close contacts along with 1532 dormitory residents were vaccinated. Measles Immunoglobulin G was detected in 99.5% (95% CI 98.9 – 99.8) of the residual sera and in all blood samples. Outbreaks simulated in a dormitory of 24 194 people, with 90% immunity and an average of 25 contacts per person, revealed a significant reduction in the final outbreak size from 2026 (IQR 1986–2058) cases to 8 (IQR 3– 26) cases when ring vaccination was implemented. In addition, vaccinating each person would avert two new cases. At higher population immunity of 93% and an average of 15 contacts per person, the maximum outbreak size was reduced by 50% with ring vaccination. Interpretation: Measles outbreaks can occur in densely populated areas with high contact rates despite high population immunity. Ring vaccination may be a strategy to protect susceptible contacts to minimise the risk of large outbreaks. Funding Statement: The authors stated: None. Declaration of Interests: The authors declare no competing interest. Ethics Approval Statement: All epidemiological investigations and outbreak containment measures were implemented under the Infectious Diseases Act, which provides for the use of outbreak investigation data for analysis and evaluation.