A Small Mumps Outbreak in a Well-Vaccinated Population, Kosrae, 2017

On August 6, 2017, the Kosrae Department of Health Services in the Federated States of Micronesia confirmed a case of mumps in an 18-year-old resident who had two documented doses of measles-mumps-rubella (MMR) vaccine. The patient had recently visited Seattle, Washington, where a mumps outbreak was spreading among Pacific Islanders, and other Pacific islands were having large outbreaks at the same time, some over 500 cases. Officials feared the same could happen on Kosrae, an island of 6,600 people. By October 6 they had identified 17 cases — nine confirmed by laboratory and eight suspected, clinically diagnosed as parotitis.

The outbreak

By December 15, 23 cases with onset between August 5 and November 1 had been found.

• 52% of patients were male, and the median age was 14 (range 1–26).
• Common symptoms were parotitis and fever, each in 20 patients (95%), and headache in 18 (86%). One young patient was hospitalized.
• Seven patients (30%) had contact with the first patient. For the 20 whose links were traced, spread happened mainly through the island's schools.
• 21 patients (91%) had the recommended two or more MMR doses; the other two had one dose each.
• Of 21 tested, 19 were positive by RT-PCR or IgM antibody. In 19 of 20 specimens tested for IgG avidity, the antibodies were high-avidity, so these were classified as secondary vaccine failures — vaccine-induced immunity that had waned — and one result was indeterminate.

Suspected and confirmed mumps cases by date of symptom onset, Kosrae, August–December 2017. CDC.

A smaller response than planned

During a large measles outbreak in 2014, Kosrae gave 4,360 MMR doses, covering 90% of people aged 6 months to 57 years, and health officials first planned a similar mass campaign for mumps. But the records of the 21 fully vaccinated patients showed that 76.2% (95% confidence interval 58%–94%) had received their last MMR dose before the 2014 campaign, a median of 12 years earlier. Studies of recent outbreaks suggest that waning vaccine immunity can help mumps spread even where vaccination coverage is high.

The findings suggested the 2014 dose had probably prevented more cases and that another mass campaign wasn't needed. Instead, officials ran a catch-up campaign for people aged 1–24 with fewer than two documented MMR doses.

The result

The outbreak was declared over on December 15, 2017. Unlike outbreaks on other Pacific islands, it stayed small. The time since the last MMR dose appears to have contributed to infection, and the 2014 campaign dose may have limited spread. Active case-finding and checking vaccination status saved an estimated 1,000 MMR doses, showing the value of accurately assessing who is at risk before choosing the most efficient, cost-effective response.

Sources

Based on McKay SL, Kambui A, Taulung LA, et al., "Notes From The Field: Mumps Outbreak in a Recently Vaccinated Population — Kosrae, Federated States of Micronesia, August–December, 2017," MMWR Vol. 68, No. 4, CDC; a work of the United States government in the public domain.