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Why did declining vaccination rates contribute to measles outbreaks?

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Infographic showing estimated lives saved by childhood vaccinations worldwide from 1974 to 2024, with syringe-shaped bars for each disease. Measles tops the list at 93.71 million lives saved, followed by tetanus, whooping cough, and others, for a total of 153.84 million lives saved. — vaccine Vaccines and the estimates of lives saved over the period 1974–2024.

Declining vaccination rates have contributed to measles outbreaks primarily by undermining herd immunity.12

Because the measles virus is exceptionally contagious, it requires a high proportion of the population to be immune—typically at least 95 percent—to prevent sustained transmission. When vaccination coverage drops below this threshold, the density of susceptible individuals increases, making it significantly easier for the virus to spread through communities.123

This resurgence has been exacerbated by growing vaccine hesitancy and disruptions to routine immunization schedules, such as those that occurred during the COVID-19 pandemic. As a matter of public health ethics, maintaining high vaccination rates is essential to protect not only the individuals who are vaccinated but also those who are vulnerable and cannot be immunized. When these rates fall, the protection afforded by herd immunity is lost, leading to the reemergence of the disease in areas where it was previously eliminated.2314