An outbreak of mumps in Sweden, February-April 2004

Between 24 February and 26 April 2004, Västra Götaland county in Sweden reported 42 cases of suspected mumps. A descriptive study of the cases was undertaken. A questionnaire was administered by telephone and vaccine effectiveness was calculated using the screening method. Seventy four per cent (31/42) of the suspected cases were interviewed by telephone. Eight out of the 42 serum samples were positive or equivocal for mumps IgM by ELISA. Mumps virus genome was identified in 21/42 (50%) saliva samples. Eleven were selected for sequencing and all were confirmed to be mumps virus. Cases were predominantly from 2 small towns. Eighteen out of 19 cases that developed bilateral swelling could be linked to one small town. The median age of interviewed cases was 43 years (range 5 to 88). Six cases were admitted to hospital, 5 of which were older than 30 years. The highest incidence occurred in the 35 to 44 years age group. Vaccine effectiveness was estimated to be 65% for 1 dose and 91% for 2 doses. This descriptive study shows the increasing age of mumps cases with increasing vaccine coverage. Vaccine effectiveness was particularly high for 2 doses. Second-dose uptake must be ensured, as primary vaccine failure is well documented in mumps. Stronger precautions must be taken to avoid pools of susceptible older individuals accumulating due to the increased risk of complications.