Mumps outbreaks can occur even among highly vaccinated populations, according to a report on a summer camp outbreak in 2005.
The outbreak had 31 confirmed cases, and 55% were known to have had two doses of mumps vaccine. Analysis indicated two doses of mumps vaccine were 92% effective in preventing mumps. A single dose was 80% effective (Pediatrics 2007;120:doi:10.1542/peds.2006-3451).
Nine cases occurred in individuals who had never been vaccinated, said Dr. Joshua K. Schaffzin of the New York State Department of Health, Albany, and his colleagues.
According to a previous report on the outbreak, the index case was a 20-year-old male camp counselor from the United Kingdom who had never been vaccinated against mumps.
On June 30, 2005, about 11 days after his arrival, he was evaluated in the camp infirmary for a left-sided parotitis, sore throat, and a low-grade fever.
At the time, a mumps epidemic was occurring in the United Kingdom. But mumps was not considered, and the counselor continued work. During July 1523, an additional 25 cases of parotitis were seen. On July 26, the New York State Department of Health became aware of the outbreak, cases were isolated, and the camp was quarantined.
According to the new report, 541 persons were at the camp that summer, including 368 campers, whose median age was 12, and 173 staff members, whose median age was 21. Of the campers, all had received at least one mumps vaccination, and all but two had two doses. Of the staff, 21 had never been vaccinated.
Thirty-one cases of mumps were identified in the camp. Twenty-six met case definitions for mumps, and six were tested for mumps IgG and IgM. Five of those six were positive for both. Only one of the six had been vaccinated, and that person had received two doses.
Twelve of the patients were campers and the rest were staff.
Of the 29 cases with certain vaccine history, 16 were individuals who had received two doses of mumps vaccine. Four had received one dose. Nine had never been vaccinated. The investigators determined that the attack rate for those who had never been vaccinated was 43%.
For those who had received one vaccine, it was 9%. Among those who had been vaccinated twice, it was 4%. The investigators also found that, among those who had been vaccinated twice, the attack rates were lowest for those who had been vaccinated within 6 years (none of 72 individuals), and those who had been vaccinated more than 15 years before the outbreak (none of 3).
Among those in the middle, the attack rate was 4% for those vaccinated 610 years before the outbreak, and 7% for those vaccinated 1115 years before the outbreak.
While that finding might suggest waning immunity was playing a role in who contracted mumps, a regression analysis did not demonstrate any linearity in the attack rates by the years since last vaccination, Dr. Schaffzin said.
There was no difference in the clinical manifestations or in the duration of illness between those who had been vaccinated and those who had not, he added.