Mumps outbreaks in highly vaccinated populations What makes good even better?

In 2006 an outbreak of mumps occurred in the US and amazingly the majority of cases were individuals that had received two vaccine doses. This unexpected outbreak may be due to the low immunogenicity of the mumps vaccine, insufficient vaccination coverage or a combination of both. Both humoral and cellular immune responses induced by the mumps vaccine are considered to contribute to the vaccine-induced protection. The magnitude and persistence of these responses determine the overall efficacy of the vaccine and ultimately the degree of vaccination coverage that needs to be reached to eliminate the mumps virus. Although Finland and the US have used the same mumps vaccine for decades, a mumps outbreak has been reported in the US while in Finland this virus appears to be eliminated. In this commentary we speculate on the factors that may be responsible for this striking difference.