The effectiveness of the mumps component of the MMR vaccine: a case control study
Introduction
Mumps is a potentially serious viral infection, the incidence of which has declined dramatically since the introduction of the MMR vaccine in October 1988. In England and Wales the number of notified cases fell from 20,713 in 1989 to 1587 in 1998 [2], and similarly dramatic declines in have followed the introduction of mumps vaccines in the US [3], Sweden [4] and Finland [5]. However, outbreaks of mumps occur where vaccination coverage is low. Mumps has re-emerged as an important infection of childhood and early adulthood as uptake of the MMR vaccine has fallen in England and Wales [2], [6], [7].
Between June 1998 and May 1999, an outbreak of mumps occurred in North East London due to genotype C virus, thought to have been imported from abroad [1]. As the outbreak became apparent, the local Consultant in Communicable Disease Control (CCDC) wrote to all local general practitioners (GPs) to encourage statutory notifications of clinical cases of mumps. Of the 144 cases notified, 142 were children of a single religious community and were notified by two general practices, A and B, which served this community. The community was concentrated into a relatively small geographical area and had its own schools and amenities. MMR vaccination coverage was low, between 67 and 86%. A case control study was conducted to identify unreported cases and to assess the effectiveness of the mumps component of the MMR vaccine.
Section snippets
Selection of cases and controls
A case was defined as a clinical or laboratory diagnosis of mumps with date of onset from 18 June 1998 to 2 May 1999. All cases in children aged between 1 and 18 years old from the religious community were included. Cases were identified in three ways. Firstly from statutory notifications of mumps from general practices A and B to the local CCDC. Secondly from searches of the electronic practice lists. Both practices used the EMIS system and patients’ notes were updated following all
Results
A total of 161 cases aged between 1 and 18 years were identified with dates of onset during the outbreak period (Fig. 1). One hundred and forty-two cases were notified by general practices A and B, 12 were identified through electronic searches of the practice lists, and seven were reported by parents. Forty-three (30.3%) of the notified cases were confirmed in the laboratory by IgM radio immunoassay, detection of mumps RNA by PCR, or both. Six cases (3.7%) of complications associated with
Discussion
The mumps component of the MMR vaccine used in the UK, which contains the Jeryl Lyn strain, is generally accepted to confer protection to around 90% of recipients [8]. This figure is based mainly on trials that have examined the immunogenicity of mumps vaccines [9], [10], [11], [12]. Observational studies conducted during mumps outbreaks have generally found the effectiveness of mumps vaccines to be lower. Our observed vaccine effectiveness of 64% for a single dose of MMR is similar to the
Acknowledgements
Dr. Bernard Cohen, HPA-CDSC, 61 Colindale Avenue, London, NW9 5EQ. All staff of the two General Practices who contributed data to the study.
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