This study shows that clusters of unvaccinated individuals can lead to sustained measles virus transmission within a community, even if the community at large has a high immunization rate.
Long-Lasting Measles Outbreak Affecting Several Unrelated Networks of Unvaccinated Persons. Dallaire F, De Serres G, Tremblay F-W, et al. Journal of Infectious Diseases 2009; 200: 1602-5.
Explanatory Note: Mathematical modeling suggests that to protect a community from sustained measles virus transmission, more than 94% of the community must be immune to measles. These types of models assume that unimmunized persons are randomly distributed in the population. However, studies have shown that unimmunized persons tend to cluster together. In 2007, Canada experienced a prolonged outbreak of measles largely among unimmunized persons but within a community with immunization levels higher than 94%.
Did the clustering of unimmunized persons promote the sustained transmission of measles within a highly immunized community?
After the detection of measles in Quebec in May 2007, suspected cases were investigated; history of measles immunizations was confirmed from the immunization records.
There were multiple (12-17) “generations of measles cases” (assuming a generation is 10-14 days, the incubation period for measles) documented over 25 weeks.
At least 94 individuals were affected. Most cases occurred in unimmunized or under immunized (those who had had only one dose of measles vaccine) persons.
The outbreak included a number of persons without exposure to a known case, including 2 of 3 schools although molecular studies demonstrated that the infections were due to the outbreak strain of virus.
Clusters of unvaccinated individuals can lead to sustained measles virus transmission within a community, despite community immunization levels exceeding 94%.
This measles outbreak suggests that even small numbers of unimmunized individuals within a highly immunized population can place their community at risk for sustained measles outbreaks most likely because unimmunized individuals tend to congregate together.