In pre-vaccination times, 15-27% of mumps cases were asymptomatic. The number of asymptomatic cases today is unknown, as it is unclear how the vaccine modifies clinical symptoms. Orchitis (testicular inflammation) is the most common complication of mumps, but it is only possible in post-pubertal males. Orchitis is primarily unilateral. Infertility caused by mumps orchitis is quite rare, even in cases of bilateral orchitis.
Prior to introduction of the vaccine, cases of mumps were not recorded.
Monovalent mumps vaccine is virtually non-existent today, except for Japan, where MMR is still banned, mumps vaccine is not sponsored by the state, and very few people get vaccinated against it.
Here is the analysis of 2,482 cases of mumps hospitalization in 1958-1969 in 16 hospitals in England. They constitute the majority of mumps cases that required hospitalization in the country. Half of the patients were 15 years old or older. Complications were recorded in 42% of all cases. Three patients died, but two of them had another serious underlying illness and mumps might not have had anything to do with the death, and the third patient was most probably misdiagnosed and did not even have mumps. The only complication, which may have been permanent, was deafness in five patients (four of them were adults).
Meningitis in mumps happens so often that some people believe it should not even be considered as a complication, but rather an integral part of the disease. In any case, there is a consensus that mumps meningitis is not dangerous and rarely has any consequences. It is confirmed by this study.
What is usually most feared is orchitis. There is a general fear of infertility from orchitis, but its probability is overestimated. Even though it is impossible to exclude, a small retrospective study did not detect infertility as a consequence of orchitis.
The authors conclude that there is no need for mass vaccination against mumps. It might make sense to vaccinate post-pubertal teenagers on admission to boarding school or the army. Even then, however, it should be remembered that 90% of the boys have already had mumps by the age of 14, which is why they should be checked for antibodies first, and only those who do not have the antibodies should be vaccinated.
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.