Mumps Resurged in United States

Despite the widespread use of a second dose of mumps vaccine in the US, the largest outbreak for two decades occurred in 2006, prompting calls for a more effective vaccine or changes in policy.

Viral disease investigators from the US Centers for Disease Control and Prevention, and other colleagues from various state department health authorities, reported this finding in the April 10th issue of the New England Journal of Medicine.

US schoolchildren throughout the US started receiving a second dose of mumps vaccine in the early 1990s. This was followed by a period where there was a lull in reported mumps cases, and health authorities established a goal: to eliminate the disease by 2010.

But in 2006, the United States saw the largest mumps outbreak in 20 years.

This prompted an investigation by CDC viral disease experts and colleagues who examined national data on cases of mumps reported in the US in 2006. They also looked more closely at cases in the most affected states and reviewed the statistics on vaccination coverage drawn from three nationwide surveys.

They found that:

* 6,584 cases of mumps were reported in 2006.

* 76 per cent of these cases occurred between March and May.

* No deaths were reported and 85 of the cases were hospitalized.

* 85 per cent of the patients lived in eight midwest US states: Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota, and Wisconsin.

* Overall, the national incidence of mumps that year was 2.2 per 100,000.

* The highest incidence was among people aged 18 to 24 years, where the incidence rate was 3.7 times higher than all other age groups combined.

* 93 per cent of the patients in the 18 - 24 age group were college students.

* In 8 states with known vaccination status, 63 per cent of overall patients, and 84 per cent of those between 18 and 24 had received two doses of mumps vaccine.

* The coverage of one-dose mumps vaccine among pre-schoolers for the 12 years before the outbreak was 89 per cent nationwide and 86 per cent or more in the states highly affected by the outbreak.

* In 2006, the national coverage of two-dose mumps vaccine among adolescents was 87 per cent, the highest ever recorded in the US.

The investigators concluded that:

"Despite a high coverage rate with two doses of mumps-containing vaccine, a large mumps outbreak occurred, characterized by two-dose vaccine failure, particularly among midwestern college-age adults who probably received the second dose as schoolchildren."

They suggested there may be a need either for "a more effective mumps vaccine or changes in vaccine policy", in order to "avert future outbreaks and achieve the elimination of mumps".

Speculating on how the outbreak occurred, study co-investigator and deputy director of the CDC's viral diseases division, Dr. Jane F. Seward told WebMD they suspected the high transmission settings in colleges were a potential contributory factor, while some waning immunity may have been another. But exactly how mumps got into the country, they were not sure:

"Mumps virus got in somehow, possibly transported from the UK, although we don't know that for sure," said Seward.

In 2004 and 2005 there was a huge surge in mumps cases in the UK where many parents refused to have their children vaccinated with the MMR (measles, mumps, rubella) vaccine following a scare. The UK outbreak totaled over 10,000 cases, an infection rate 50 times that of the US in 2006, Seward told WebMD.

However, she added that:

"Our experience with this outbreak in the US in 2006 reassures us how effective the vaccine is."

"Two doses of mumps vaccine are highly effective but not completely effective. But without the high coverage rate, we would have seen a much larger outbreak," said Seward.

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