Wondering what's behind recent outbreaks of measles and whooping cough in the US? Perhaps not surprisingly, a new study published in JAMA finds that people who don't vaccinate—for non-medical reasons—contribute to the problem, Reuters reports. Since measles was declared eliminated in the US in 2000, there have been 1,416 cases of the disease reported, and the study finds that more than half of those cases were in people who hadn't been vaccinated for measles. Detailed vaccination data was available for 970 of the measles cases, and 405 of them involved people who skipped vaccinations without a medical reason to do so. Researchers also looked at more than 10,000 cases of whooping cough in patients with known vaccination status and found similar, though somewhat less dramatic, results.
In the pertussis cases, when looking at the five biggest statewide epidemics since 1977, 24% to 45% of the people affected were completely or partially unvaccinated. And in looking at the pertussis cases for which detailed vaccine data was available, 59% to 93% of cases were in people who were intentionally unvaccinated (as opposed to unvaccinated for medical reasons.) “If there are a high number of susceptible or unvaccinated individuals in the community the risk of getting infected—even for vaccinated children—goes up," says the study's senior author, because most vaccines are not 100% effective. Up to 2% of people who are fully vaccinated for pertussis and 3% of people who are fully vaccinated for measles are still at risk of getting the disease. Non-medical vaccine exemptions don't fully explain the resurgence of measles and whooping cough (researchers also found evidence of waning immunity against pertussis, for example), but another researcher says the study finds that "individuals who refuse vaccines not only put themselves at risk for disease," but others too.