Risk factors for transmission of measles during an outbreak: Matched case-control study

Background: In 2012, an outbreak of measles occurred in Merseyside, UK with 359 confirmed cases by 30 June. Numerous cases reported visits to healthcare and social settings. Aim: To identify risk factors associated with measles transmission during the outbreak. Methods: In April 2012, a retrospective matched case-control study was conducted. Fifty-five confirmed cases and 55 community controls, matched 1:1 for age and geography, were selected at random. Data on exposures in the two weeks before illness, including attendance at a healthcare setting, were collected via telephone interview. Univariate and multi-variate analyses were conducted and odds ratios were calculated. Findings: Forty-two cases and 42 matched controls were contacted successfully. Univariate exact conditional logistic regression analysis identified that cases were more likely to have attended an emergency department, been admitted to hospital and be incompletely vaccinated (for age). Multi-variate analysis found three factors to be independently associated with measles infection: incomplete/partial vaccination for age [adjusted odds ratio (aOR) 22.1, 95% confidence interval (CI) 3.8-∞, P < 0.001], under age for routine vaccination (aOR 20.4, 95% CI 2.0-∞, P = 0.009) and hospital admission (aOR 20.2, 95% CI 1.4-∞, P = 0.025). Conclusions: Incomplete/partial vaccination, under age for routine vaccination and hospital admission were associated with measles infection. These findings highlight the importance of timely vaccination of eligible individuals, early diagnosis, timely isolation of cases, and implementation of strict infection control measures.