Self-collected buccal swabs and rapid, real-time PCR during a large measles outbreak in Wales: Evidence for the protective effect of prior MMR immunisation

Abstract

Background: We describe the laboratory response to a large measles outbreak that occurred during 2012-2013 centred in mid and west Wales, UK.

Objectives: To demonstrate the impact of rapid measles testing on the management of a large outbreak, to show the complex molecular epidemiology and determine the role of previous MMR immunisation on a large cohort of exposed people.

Study design: Results from oral fluid antibody testing and self-collected buccal swabs tested by real-time PCR were reconciled and analysed to determine level of agreement and to calculate MMR vaccine efficacy during the outbreak.

Results: During the outbreak 1435 notifications of measles were received from across Wales. Samples were received from 70% of notified cases with a positivity rate of 56% within the outbreak compared to 15% for the rest of Wales. Measles RNA was detected in 53 cases with previous history of MMR immunisation, but viral loads were lower than those detected in unimmunised cases. The molecular epidemiology showed at least two distinct D8 strains of measles virus were introduced into Wales along with a separate introduction of a B3 strain outside the outbreak area.

Conclusion: Molecular testing of all notified measles cases offers the most rapid way of confirming the introduction of measles into a population potentially before secondary transmission has already occurred. The outbreak data confirms the protective effect of the MMR vaccine with vaccine efficacy calculated at 96% for one dose and 99% for two doses supporting the WHO recommendations for a two dose MMR immunisation schedule.