The number of children vaccinated in the US state of California has markedly increased since a law eliminating non-medical exemptions took effect, according to a study published Monday.
Although the result seems obvious, researchers wanted to confirm it with data using a rigorous method.
The proportion of children returning to school at five years of age and up to date with their vaccinations went from 94.5 percent in 2015 to 97.8 percent after the California law took effect in 2016, according to the study by researchers at the University of California San Francisco.
They published their findings Monday in the journal PLOS Medicine.
The rate of children obtaining a doctor's exemption, for medical reasons, rose from 0.2 percent to 0.6 percent. But this increase was largely compensated for by the reduction to zero of non-medical exemptions due to religious or personal reasons.
California ended non-medical exemptions after a measles epidemic at Disneyland in late 2014.
Another measles epidemic struck the United States in 2018-19, leaving 1,600 people ill. It was the largest outbreak since 1992, and many of those affected were unvaccinated members of Orthodox Jewish communities, primarily in New York.
An anti-vaxxer movement in the US and elsewhere has surged with the social media-fueled rise of online conspiracy theories.
Measles is one of the world's most contagious viruses. Spread by coughing or sneezing, the virus can linger in the air long after an infected person leaves a room.
The study published on Monday found that counties in California that previously had the most exemptions, and were the most vulnerable to an epidemic, saw the greatest increase in vaccinations.
Evidence supports "the adoption of similar governmental policies eliminating non-medical exemptions to help address the growing public health challenge of vaccine hesitancy," the authors said.
Forty-five of the 50 American states continue to allow exemptions for religious reasons, and 15 allow personal, moral or philosophical objections to vaccinations, according to the National Conference of State Legislatures.
Exemptions for medical reasons, such as a weakened immune system, can be obtained anywhere in the United States.