Universal childhood vaccination has proven to be an essential tool to prevent or ameliorate morbidity due to infection and therefore to control, eliminate or eradicate vaccine-preventable infectious diseases causing illness, disability or death throughout the world. Although mass vaccination is largely responsible for the worldwide eradication of smallpox and the reduction of the disease burden associated with diseases such as polio and measles, achieving and maintaining high vaccination coverages is quintessential for vaccination strategies to be effective (Anderson and May, 1990; Fine, 2004). However, outbreaks of vaccine preventable diseases still occur even in countries with high vaccine uptake due to, for example, primary and secondary vaccine failure (see, e.g., Vandermeulen et al., 2004).
Trivalent measles, mumps and rubella (MMR) immunisation has been widely implemented throughout Europe. Nevertheless several small- and large-scale measles outbreaks have been reported in recent years (see, e.g., Bernard et al., 2008; Smithson et al., 2010; Grgic-Vitek et al., 2015). A large-scale measles outbreak in France initiated in 2008, with more than 20,000 reported measles cases by 2013 (see, e.g., Antona et al., 2013). Furthermore, in 2012 and 2013 large-scale measles outbreaks have been reported in the Netherlands (Knol et al., 2013) and the UK (Vivancos et al.,2012; Baugh et al., 2013). To date, no large measles outbreaks have been reported in Belgium since the start of the two-dose vaccination program in 1995, although small outbreaks occurred in Orthodox Jewish communities in Antwerp, 2007-2008 (Lernout et al., 2009) and in a day care center in Ghent, 2011, before spreading to anthroposophic schools, where vaccination coverage was low (Braeye et al., 2013). Recently, one noticed an outbreak in a kindergarten in Antwerp affecting primarily unvaccinated children aged 3 to 15 months (De Schrijver et al., 2014).
In addition to measles outbreaks, mumps outbreaks have been reported in highly vaccinated populations during recent years (Briss et al., 1994; Cohen et al., 2007; Eriksen et al., 2013). In 2012, a large mumps outbreak occurred in Belgium, initially affecting mainly the region around Ghent before spreading throughout Flanders (Flipse, 2012; Flipse and De Schrijver, 2013). The highest attack rate was reported among university students who had been vaccinated with two doses of MMR. This was also observed during other recent mumps outbreaks in high-income countries (see, e.g., Date et al., 2008), raising concerns about mumps vaccine failure. Whereas there are no recent reports on rubella outbreaks in Belgium, 15,000 cases of rubella and 43 cases of congenital rubella syndrome were observed as a result of the 2012-2013 rubella outbreak in Japan. This rubella resurgence mainly affected unvaccinated adult men aged 35-51 and men and women aged 24-34 years for which vaccination coverages were relatively low (Ujiie et al., 2014). On the other hand, Finland was documented to be the first country in which indigenous rubella is eliminated as a result of a 12 year, 2-dose MMR vaccination program (Peltola et al., 1994).
Despite MMR vaccination efforts throughout the world, it is of interest to determine whether measles, mumps and rubella resurgence is possible in countries with high vaccination coverage (e.g., Belgium). Serological survey data is typically used to determine the age-specific susceptibility to infection in the population of interest. However, a proper quantication of the susceptibility, and consequently of the risk of a possible resurgence, based solely on serological data is only possible if these data were collected recently and under the assumption of age-homogeneous transmission. Since recent Belgian serological data on MMR is absent, we introduced a multi-cohort model that allows for the use of available serological data, not necessarily collected at the calendar time of interest. Our methodology combines vaccination coverage information, serological survey data and data on social contact behaviour and supplements these data sources with estimates of the duration of maternal immunity, and of primary and secondary vaccine failure, obtained from extensive literature reviews. The objective of our approach is to identify regions of high outbreak potential for measles, mumps and rubella in Belgium based on regional estimates of the effective reproduction number R. Upon availability of the data sources mentioned above, the methodology is generally applicable to assess outbreak risk for other vaccine-preventable diseases in highly vaccinated populations.
Hens, N., Abrams, S., Santermans, E., Theeten, H., Goeyvaerts, N., Lernout, T., Leuridan, E., Van Kerckhove, K., Goossens, H., Van Damme, P., and Beutels, P. (2015). Assessing the risk of measles resurgence in a highly vaccinated population: Belgium anno 2013. Eurosurveillance, 20(1):pii=20998.