Washington (AFP)

The number of children vaccinated in California has increased significantly following a 2015 law that removed the possibility for parents to obtain exemption from vaccination for non-medical reasons. The result seems obvious, but researchers wanted to confirm it in the data using a rigorous method.

The proportion of children returning to school at five years of age and up to date with vaccination rose from 94.5% in 2015 to 97.8% after the reform that came into force in 2016, according to this analysis by researchers from University of California, San Francisco, and covering until 2017. The study is published Monday in the journal PLOS Medicine.

The rate of children obtaining medical exemption from a doctor increased from 0.2% to 0.6%. However, this increase was largely offset by the reduction to zero of exemptions for non-medical reasons (religious or personal), which formerly represented 2.4% of five-year-old children.

California had removed non-medical exemptions after a measles epidemic at Disneyland in late 2014. The measles epidemic that hit the United States in 2018/2019, the largest since 1992 with more than 1,600 patients, has led several States, including those of New York and Washington, to reduce or eliminate the possibilities of exemptions.

Previous studies had observed a general increase in vaccinations after California law, but these observations did not take into account the general trend in the country. The method used in this study is more conclusive because it consisted in comparing "real" California to a California which would not have changed its law, by comparing it to the other American states.

The California counties that had the most immunization exemptions before, and were the most vulnerable to an epidemic, saw the largest increases in vaccination, according to this study.

Authors' conclusion: "Removing non-medical exemptions can be effective in increasing immunization coverage".

To date, 45 of the 50 American states continue to allow exemptions on religious grounds, and 15 keep personal, moral or philosophical exemptions, according to the National Conference of State Legislatures. Throughout the United States, a child can get a medical exception, for example in the rare case of a weakened immune system, or if an allergy to a particular vaccine has been noted before.

© 2019 AFP