Most of the people infected with measles during recent outbreaks were unvaccinated, a review of 18 studies has reported. Moreover, most of those who caught the disease were children whose parents decided not to vaccinate them, rather than forgetting or lacking access. Anti-vaccination sentiment was a smaller, but still important, contributor to the spread of pertussis (whooping cough).

Measles, once declared eliminated in the United States, has returned recently as imported cases have set off chains of infection. The fact that such outbreaks are becoming more common has been blamed on the rise of the anti-vaccination movement, but the accusation has involved a lot of guesswork.

A paper in the Journal of the American Medical Association has changed that. The authors acknowledge that a “formal assessment of study quality was not performed,” and some of the studies include several known to be prone to biases. Nevertheless, the results were striking.

Combining nine outbreak reports and nine annual disease summaries, they analyzed 1,416 measles cases in recent years. Vaccination history was unclear or incomplete for some of the patients. However, among the 970 infected individuals where detailed information was available, 574 had not been vaccinated against measles, and 405 of these had nonmedical exemptions, indicating their parents were opposed to vaccination in principle. It is unclear how many of the other 169 had missed out on vaccinations through lack of consistent access to healthcare, and how many were also the consequence of deliberate parental decisions.

Considering that 91.9 percent of Americans have been vaccinated for measles, though, those whose parents refused to vaccinate make up a hugely disproportionate share of those who both get sick, and infect others.

“Central to any justification to restrict individual freedom by mandating vaccines to prevent harm to others is an understanding of the nature and magnitude of these risks and harms,” the paper noted. “However, the risks of vaccine refusal remain imperfectly defined, and the association between vaccine refusal and vaccine-preventable diseases may be both population- and disease-specific."

This study provides a start to measuring the risks parents take when they refuse to vaccinate their children, providing a basis for legislators weighing the merits of policies to boost rates of coverage.

Pertussis is far more common than measles, and the paper collated data on 10,609 infected individuals with a reported vaccination status. The pertussis vaccine is not as effective as the jab for measles, so the majority of those who caught the disease had actually been vaccinated.

Nevertheless, the paper reported, “The five largest statewide epidemics had substantial proportions (24-45 percent) of unvaccinated or undervaccinated individuals.” Smaller outbreaks occurred in vaccinated populations but the presence of a substantial unvaccinated population in which the disease could spread easily allowed these epidemics to spread further. “Although pertussis resurgence has been attributed to waning immunity and other factors, vaccine refusal was still associated with an increased risk for pertussis in some populations,” the authors concluded.