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Why did decreasing vaccine coverage increase the risk of measles outbreaks?

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Diagram showing the West Nile virus transmission cycle: a mosquito bites an infected bird (the primary reservoir host), then transmits the virus to humans or horses (incidental hosts) through subsequent bites. — infectious disease Birds are the primary reservoirs of West Nile virus. The virus is transmitted to humans and horses by mosquitoes that have bitten infected birds.

Decreasing vaccine coverage increases the risk of measles outbreaks primarily by undermining herd immunity.12

Herd immunity occurs when a sufficiently large proportion of a population is immune to a disease, which limits the ability of the pathogen to spread from person to person. Because the measles virus is exceptionally contagious, it requires a very high threshold of population immunity—typically around 95 percent—to prevent sustained transmission.23

When vaccination rates drop below this critical threshold, the density of susceptible individuals in a community increases, making it significantly more likely that an infected person will come into contact with someone who is not protected. This decline in coverage is often driven by vaccine hesitancy, misinformation regarding vaccine safety, and logistical interruptions to routine immunization schedules. Consequently, areas with high concentrations of unvaccinated individuals become vulnerable to outbreaks, allowing the virus to reemerge even in regions where it was previously considered eliminated.2134