Measles is one of the most contagious of the vaccine-preventable diseases, with reproduction rates (R 0) estimated at 12 to 18 — meaning that the average person with measles would be capable of infecting 12 to 18 other people if all his or her contacts were susceptible. The herd-immunity threshold (the population immunity level needed to interrupt transmission) is usually calculated as (R 0−1)÷R 0; for measles, this threshold is on the order of 92 to 94% to prevent sustained spread of the virus — higher than the thresholds for almost all other vaccine-preventable diseases.
The licensure of the first live attenuated measles vaccine in 1963 offered the opportunity to prevent this health burden. Current vaccines are highly effective — about 94% for a single dose, if it's administered in the second year of life. With two doses administered on or after the first birthday and at least 1 month apart, almost all immunocompetent children are protected against measles for life. But vaccines don't save lives — vaccinations do. Vaccines that remain in the vial are completely ineffective.
To prevent measles from being reestablished as an endemic disease in the United States, we must first do better in vaccinating our at-risk population. That means ensuring that vaccine is accessible to all who need it — especially to people traveling outside the Western Hemisphere and those traveling to the United States from countries with circulating disease — and convincing hesitant families both that the vaccine is safe and effective and that measles is not trivial and can result in serious illness.
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