In 2013, 15 mumps outbreaks were registered in France. 72% of the cases had been vaccinated twice. The effectiveness of the vaccine was 49% for one dose and 55% for two doses.
Among those who had been vaccinated once, the risk of getting mumps increased by 7% for every year that had passed since the vaccination.
Among those who had been vaccinated twice, the risk of getting mumps increased by 10% for every year that had passed after the second dose.
Orchitis was observed in five men. One of them was unvaccinated, two had been vaccinated with one dose, and another two had been vaccinated twice.
Mumps is a mild disease, which passes on its own, but sometimes it can cause severe complications, such as orchitis, meningitis, pancreatitis or encephalitis, especially in adults. Complications from mumps are observed more often in adults and they are more severe than in children, especially among the unvaccinated.
In other countries, mumps outbreaks are also observed among the vaccinated. The reason for this is the decreasing effectiveness of the vaccine and the lack of natural boosters. Other reasons for the outbreaks could be the initial overestimation of the vaccine effectiveness, insufficient vaccination coverage, or existence of a strain that is not covered by the vaccine.
Outbreaks occurring among the vaccinated and the decreasing effectiveness, both lead to thoughts about a third dose of the vaccine. This kind of an experiment was conducted in the US during the outbreaks in 2009 and 2010. Both times the outbreak subsided a few weeks after the administration of the vaccine. However, the outbreaks always subside at some point, so it was unclear whether it had anything to do with the vaccination. Nonetheless, this and other experiments hint that a third dose of the vaccine might not be a bad idea. Moreover, during the vaccination campaigns in the US, the third dose had few side effects.
They wanted to introduce a third dose of MMR into the national immunization schedule in Netherlands, but changed their mind, since mumps rarely causes complications, and the vaccination coverage among adults is unlikely to be satisfactory.
Mumps outbreaks among the vaccinated, along with this study, led the Ministry of Health of France to recommend a third dose of MMR at times of outbreaks. Even though it is unknown whether the vaccine is effective for those already infected with the virus, it is quite possible that the vaccine will cause a decrease in the contagious period of the vaccinated patients.
The Dutch study determined that two thirds of cases during outbreaks are asymptomatic. The role of asymptomatic patients in the transmission of the disease remains unknown.
Future observations in France, and possibly other countries that would adopt the same recommendation, will help determine whether the third dose of MMR is effective during outbreaks.