H Bernard, N G Schwarz, A Melnic, V Bucov, N Caterinciuc, R G Pebody, M Mulders, C Aidyralieva, S Hahné. Mumps Outbreak Ongoing Since October 2007 In The Republic Of Moldova. Eurosurveillance, Volume 13, Issue 13, 27 March 2008.

From the paper's introduction:

The Republic of Moldova is experiencing a nationwide mumps outbreak, with a total of 19,550 notified cases as of 23 March 2008.

The outbreak started in October 2007, with 105 cases notified in that month compared to an average number of 24 cases per month notified between January and September 2007. Between 1 October and 31 December 2007, 1,524 cases were notified.

In the Republic of Moldova, mumps monovalent vaccination was introduced in 1983 as a single-dose schedule targeting children aged 15-18 months-old. Since 2002, a second dose has been administered to six- to seven-year-olds (birth cohorts from 1995 onwards), using a combined vaccine for measles, mumps, and rubella (MMR). Reported mumps vaccination coverage has been high since its introduction, except for a period in the early 1990s when a vaccine shortage occurred due to political changes. Successful catch-up campaigns subsequently improved vaccination coverage in those birth cohorts affected by the vaccine shortage. According to routine national surveillance data, coverage with one dose of mumps vaccine is e94% in individuals born in 1989-1993. Coverage with two doses is e96% in individuals born in 1995-2000. Coverage in the 1994 birth cohort is 99% and 21% with one and two doses, respectively. The most recent large mumps outbreak in the Republic of Moldova occurred in 1996-1998, with 28,845 cases reported, predominantly from birth cohorts 1983-1990 (60%).

In February 2008, the country's Ministry of health invited an international outbreak investigation team coordinated by the World health Organization Regional Office for Europe, to identify the extent of the mumps outbreak and assess cases and their possible risk factors for acquiring mumps, and to provide recommendations for the current outbreak.

From the discussion:

The Republic of Moldova is facing a large ongoing mumps outbreak in teenagers and young adults, the majority of whom have previously been vaccinated with one dose of monovalent mumps vaccine. Mumps outbreaks comparable to this one with respect to size and affected age group have occurred recently in several countries, including the UK, the US, and Canada. In the outbreak in the UK, 79% of cases in 2004 occurred in individuals born between 1980-1989. Almost two thirds of the cases were unvaccinated, however, 30% had received a single dose of MMR. In the outbreaks in the US and Canada, 49% of the cases had been vaccinated twice. Mumps vaccine failure has often been attributed to primary vaccine failure, i.e. an insufficient immune response after vaccination.

More recent studies have suggested that secondary vaccine failure, i.e. decreasing antibody titres over time (waning immunity), contributes to the recurrence of mumps outbreaks. In the current outbreak, the high reported vaccine coverage with a single dose of mumps vaccine among the cases suggests that "failure to vaccinate" did not play a major role as the cause of the outbreak. Primary and secondary vaccine failure for one dose of mumps vaccine are currently being investigated in a further epidemiological study.