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Previous vaccination modifies both the clinical disease and immunological features in children with measles.

INTRODUCTION
Measles that develops in previously vaccinated cases has been reported to be associated with modified disease, although severity has usually been assessed by the presence or absence of symptoms. To date no studies have attempted to subjectively grade the severity of the clinical features.


AIM
To investigate both the objective and subjective severity of measles in vaccinated and unvaccinated cases in the context of a community outbreak.


METHODS
A retrospective observational cohort study conducted in Christchurch in 2009 using notified data compared the presentation of measles in 14 confirmed cases that had received at least one MMR (measles, mumps, rubella) vaccination and 14 age-matched unvaccinated confirmed cases. Additional details on the subjective and objective severity of the illness were obtained from parents/guardians using a standardised telephone questionnaire.


RESULTS
The vaccinated group had significantly fewer clinical features on presentation (p=0.01, RR=1.3, 95% CI 1.1-1.6) and a less severe illness objectively, as measured by height and duration of fever, the number of days needing medication other than paracetamol and days required in bed. Unvaccinated cases were 2.8 times more likely to have more severe clinical features than vaccinated cases (OR=2.8, 95% CI 1.5-5.0). Unvaccinated cases were 3.0 times more likely to develop IgM antibody (RR=3.0, 95% CI 0.9-9.3).


DISCUSSION
Previously vaccinated children who develop measles are likely to have less severe disease and serology results that may be inconclusive, particularly for IgM antibody if tested in the first few days after the rash onset.