Most of the cases of mumps in this outbreak were male, were 15–24 years of age and were not fully vaccinated. The clustering of cases, particularly among people born between 1985 and 1991, reflects the susceptible cohort, as described previously. In addition, the active social lifestyle of this age group may have facilitated the transmission of the disease. The predominance of male cases was likely due to the settings in which they were exposed to the virus. These settings included athletic events, such as hockey tournaments, where there is frequent close contact between people.
The vaccine effectiveness of two doses of MMR vaccine was consistently higher than the effectiveness of one dose for each of the cohorts. The results of the sensitivity analyses show how important it is to accurately classify cases according to their vaccination status and to obtain accurate data on vaccine coverage within a population. Immunization status was unknown for 20 cases (14.9%) in this study. If cases for whom vaccination status was unknown were more likely not to have been vaccinated, then the estimates of vaccine effectiveness would be higher. For example, the vaccine effectiveness for two doses among people born between 1990 and 1991 would have increased from 88% to 95% if all cases with unknown vaccine status were unvaccinated (data not shown).
Knowledge of vaccine effectiveness can be used to estimate the level of coverage required to reach herd immunity thresholds. However, the basic reproductive number for mumps is not known with certainty because there are no longer populations fully susceptible to the disease. We therefore used a range of reproductive numbers to assess herd immunity.11,12
The two-dose coverage seen in the older cohorts in this investigation was far below the coverage needed to stop transmission of the disease. Targets for coverage set at the national level recommend that 97% of children should be vaccinated by their seventh birthday.13
However, achieving this target would result in herd immunity under only some of the circumstances outlined in (e.g., a reproductive number of four and a vaccine effectiveness of 80% or higher).
Mumps outbreaks have become relatively unusual in Canada and in other countries that have implemented two-dose MMR vaccination programs. In the past, it was not uncommon to have outbreaks among people who had received only one dose of a mumps-containing vaccine. However, that had not been the case since the implementation of the two-dose policies. In fact, outbreaks occurring predominantly among recipients of two doses of vaccine were rarely documented anywhere in the world until 2006.14
In the 2004–2005 outbreak in the United Kingdom that affected more than 5000 people, approximately two-thirds had not been vaccinated against mumps.15
Similarly, in the 2007 outbreaks of mumps in Canada, only 7.7% (45/586) of the cases whose immunization status was known had received two doses of vaccine.16
In contrast, between 62.5% and 75.2% of cases reported in more recent outbreaks in Canada, the US and Israel had received two doses of mumps-containing vaccine.1,2,6,17
Inadequate coverage, a susceptible cohort of people who were only eligible to receive one dose of vaccine and waning immunity have been cited as possible explanations for the resurgence of mumps in developed countries.3,4,6,15
Cohen and coauthors showed that the effectiveness of two doses of mumps vaccine declined from 98.8% in children 5–6 years of age to 86.4% in children 11–12 years of age.3
It has been 14 years since Ontario implemented a two-dose policy for MMR vaccine. Therefore we are now at the point in the evolution of the program where we may begin to see the effect of waning immunity among recipients of two doses of the vaccine.
Although the point estimates of vaccine effectiveness seen in this study must be interpreted with caution owing to the small size of the sample, they are generally consistent with the estimates reported in other outbreaks. In a recent review of 50 outbreaks of mumps, the effectiveness of one dose of mumps-containing vaccine was 72.8%–91.0%, whereas the effectiveness for two doses was 91.0%–94.6%.4
Estimates of vaccine effectiveness were slightly lower in the 2006 outbreak in the US, ranging between 64% and 84% for one dose, and between 76% and 88% for two doses.5
Mumps outbreaks that have affected older adolescents and young adults have previously been observed in Canada16–18
and in countries where two-dose policies were implemented during the 1990s.6,15,19
During the 2007 Canadian outbreak, 58.1% of cases were between 20 and 29 years of age;16
in the 2006 outbreak in the US, the highest attack rates were reported among people aged 18–24 years.6
The estimates of vaccine effectiveness were imprecise owing to the small number of cases in each birth cohort, as reflected by the wide confidence intervals. Although the screening method offers a quick and relatively simple means for assessing vaccine effectiveness, it requires accurate estimates of coverage. Because IRIS is not a comprehensive immunization registry, the coverage data used to derive these estimates have limitations.
Coverage varied among the selected health units; the weighted overall estimate may not have been fully representative of the population from which the cases were derived. Coverage may have been underestimated if immunization was not reported; conversely, coverage may have been overestimated if denominators failed to capture all eligible people.
Finally, we were not able to assess waning immunity due to the small number of cases.
Accumulating data on vaccine effectiveness and waning immunity may have important implications for immunization policy. All jurisdictions in Canada currently have a two-dose MMR vaccination policy; however, the timing of the second dose varies among the provinces and territories. As of 2009, 10 jurisdictions, including Ontario, offered the second dose at 18 months of age; the remaining three jurisdictions offer the second dose at four to six years of age.20
If all jurisdictions administered the second dose of the vaccine to children between the ages of four and six years, this could have an impact on waning immunity. However, it is unlikely that we will be able to eliminate mumps with the current vaccine and vaccination policies and at the current levels of coverage. Improving the coverage of two doses of MMR vaccine is critical to the prevention of further outbreaks of mumps.
Another policy question is whether a second dose of mumps-containing vaccine is needed for the susceptible cohort, notwithstanding the difficulty in accessing this population. Ontario implemented a province-wide mumps vaccination catch-up campaign that targeted students in post-secondary institutions between August 2008 and March 2009, but uptake was poor.21
Outbreaks of mumps in Canada and abroad serve as a reminder that we cannot become complacent about vaccination programs or maximizing vaccine coverage. Rapid assessment of vaccine effectiveness can occur as part of the response to an outbreak and is important in maintaining confidence in vaccination programs. Accurate and timely assessment of immunization coverage through the implementation of a comprehensive immunization registry would improve our ability to assess both vaccine effectiveness and waning immunity to the disease. Furthermore, closely monitoring waning immunity will help to ensure that we have the necessary data for making policy decisions, such as whether a third dose of MMR vaccine is necessary or whether a different vaccine should be considered, and for evaluating the cost-effectiveness of the program.