Although most measles cases in England occur among unvaccinated individuals, the proportion of cases among those who were fully vaccinated with 2 doses of the measles, mumps, and rubella (MMR) vaccine increased between 2010 and 2019.

According to a in The Lancet Public Health, this uptick in breakthrough measles infections—primarily in young adults—was likely due to a slight waning of vaccine-induced immunity over time.

Using measles case data collected by the UK Health Security Agency, the researchers applied mathematical modeling to determine the most plausible scenario to explain increasing breakthrough infections.

The distribution of measles cases in fully vaccinated people by age group over time best matched models that accounted for decreasing immunity. This led researchers to conclude that although vaccination is the best way to protect against measles, the slow waning rate of about 0.04% each year is enough to increase the number of measles cases over time. The researchers noted that this waning of vaccine-induced immunity could worsen outbreaks primarily related to decreases in vaccine coverage.

Published Online: October 25, 2024. doi:10.1001/jama.2024.21822