Outbreak of measles in primary school students with high first dose MMR vaccination coverage

Ong G, Rasidah N, Wan S, Cutter J
Correspondence: Dr Gary Ong, gary_ong@moh.gov.sg

ABSTRACT
Introduction Indigenous cases of measles continue to occur in Singapore despite the implementation of a two-dose mumps, measles and rubella (MMR) vaccination policy in 1998. We investigated a measles outbreak that took place in a primary school from April 17 to May 6, 2004 to identify all cases, evaluate vaccine efficacy (VE) and implement outbreak control measures.
Methods A case of measles was defined as anyone having generalised rash and fever with or without cough, coryza or conjunctivitis during the outbreak period, and who had either laboratory-confirmed acute measles infection or was epidemiologically linked to a patient with laboratory-confirmed measles infection. Vaccination status was obtained from the studentos health booklet and confirmed with the National Immunisation Registry. Attack rates in unvaccinated (ARU) and vaccinated (ARV) students were calculated and VE was evaluated using the formula: VE (percent) = [(ARU-ARV) / ARU] x 100 percent.
Results Nine students, aged between eight and 14 years, from five classes in primary three and primary six, were epidemiologically linked to have measles. None of them had received the second dose of the MMR vaccine. 93 percent of students in the affected classes (n = 184) had prior documented evidence of receiving at least one dose of MMR vaccination, as compared to 96.5 percent for the entire school enrolment (n = 1,309). The attack rate was 1.2 percent in the vaccinated group and 53.8 percent in the unvaccinated group. The VE for the primary dose of MMR in the affected classes was 97.8 percent.
Conclusion It is important to achieve a high coverage for the primary dose of MMR vaccine in order to prevent any potential outbreaks prior to receiving the booster dose.

Keywords: measles, measles outbreak, vaccine efficacy, vaccination coverage
Singapore Med J 2007; 48(7): 656–661