Investigation and Control of Measles Outbreak in a Prison, Samut Prakan Province, January-March 2011

Abstract

Background: In Thailand, the Measles Elimination Program was launched in early 2011. Measles vaccine is administered to 9-month children as recommended by the national EPI since 1984. Following a notification of measles outbreak in Prison S, an investigation was initiated to describe epidemiological characteristics, determine factors associated with measles in young adult prisoners and deliver control measures.
Methods: Review of measles vaccine coverage and incidence of measles in Thailand was done. Active case finding was conducted in the prisoners and prison guards. A suspected case was a prisoner or guard in Prison S who developed fever with rash and at least 1 of the following symptoms: cough, rhinorrhea and conjunctivitis between 14 January and 31 March 2011. A confirmed case had positive ELISA-IgM or measles viral isolation. Paired serum samples of 30 healthy prisoners aged lower than 25 year were collected before MMR vaccination and 3 weeks after for ELISA-IgG assay. Prisoner’s activities and prison environment were inspected and association between ward area per prisoner and ward-specific attack rate was estimated by Pearson’s correlation.
Results: In early phase of measles vaccine program in Thailand during 1984-1996, the coverage of measles vaccine were between 5 and 90%. Meanwhile the incidence rates of measles had been reduced below 20 per 1,000 populations after the vaccine coverage was raised over 90%. Overall, 44 measles cases met case definitions (11 confirmed and 33 suspected cases) with an attack rate 8.4 per 1,000 populations. Male to female ratio was 43:1 with the median age 24 years (Range: 18-35). Age-specific attack rate was highest (25.4 per 1,000 populations) in prisoners aged 18 to 25 years. Of 30 prisoners surveyed 22 (73.3%) were tested positive for measles IgG. During the investigation, we observed sharing of uniforms and prisoners lived in overcrowded condition, infected prisoners who work as guard assistants and a proxy of non-immune population were possible risk factors of the measles outbreak. Inverse correlation between ward area per prisoner and ward-specific attack rate was observed with high correlation coefficient -0.79. Of 2,026 prisoners aged 18-25 years, 1,953 (96.4%) received MMR vaccine during the investigation.
Conclusions: The measles outbreak in young adult prisoners reflected the low percentage of immune populations (73.3%) in the prisoners aged 18-25 years which was significantly below the herd immunity level at 92%-95%. Sharing of uniforms, living in overcrowded condition, infected prisoners who assisted guards and a proxy of non-immune population possibly contributed to the measles outbreak. The outbreak subsided after the MMR vaccine coverage reached 96.4% in prisoners aged 18-25 years. Birth cohorts in 1983-1995 should be stressed as a target group for measles elimination program in Thailand by strengthening surveillance and providing measles vaccines when outbreaks occur.