A 24-year review on the epidemiology and control of measles in Singapore, 1981-2004.

The objective of our paper is to review the epidemiology of measles in Singapore and the impact of the measles vaccination program on the control of measles. Our review will form the basis for a critical appraisal of our future measles control program. We analyzed the trend of reported measles cases in relationship to measles vaccination coverage from 1981 to 2004 using routine measles notifications and measles vaccination data submitted to the Ministry of Health and the National Immunization Registry, respectively. We determined the measles vaccine efficacy using data from epidemiological investigations of reported institutional measles outbreaks. The herd immunity of the population against measles was accessed through three seroepidemiological surveys that we conducted in 1989/1990, 1993 and 1998. In addition, we collected blood specimens from every clinically diagnosed case of measles notified to the Ministry in 1998/1999 to for measles-specific IgM antibodies in order to evaluate the proportion of clinically diagnosed cases of measles that were laboratory confirmed. The incidence of measles has decreased significantly since 1981 as a result of increased vaccination coverage of 89-93% following implementation of compulsory measles vaccination in 1985. However, resurgences still occurred in 1992/1993 and 1997. With the implementation of the two-dose measles vaccination schedule, the annual number of laboratory confirmed cases of measles to date has been less than 150. Vaccine efficacy of the trivalent MMR vaccine based on institutional outbreak investigations was consistently above 92%. We also found that the overall seroprevalence of the population to measles has decreased from approximately 91.5% in 1989/1990 to 1993 to 77.9% in 1998 (mainly in children < 4 years old) and that only 7% of clinically notified cases of measles were serologically confirmed to be positive for measles. Achieving a vaccination coverage of more than 95%, tightening our MMR vaccine delivery system and strengthening surveillance of measles are essential components which must be addressed in order to interrupt measles transmission in Singapore.