The United Kingdom has lost its measles-free status after a rise in cases and deaths, a decision the World Health Organization linked to declining vaccination coverage. Public health experts say the situation reflects a broader challenge: when vaccination rates slip, highly contagious diseases like measles can quickly return.

Aimee J. Palumbo, PhD, MPH, an epidemiologist at Temple University’s Barnett College of Public Health (CPH), says vaccines can become victims of their own success.

“People often have no functional memory of a world before vaccines and no first-hand experience with the effects of these diseases, especially the severe outcomes,” Palumbo said. “Because the vaccines have been so effective, the real and perceived risks of vaccination can start to outweigh the real risks of the disease in people’s minds.”

Those perceptions, she said, don’t spread randomly.

“They circulate through social networks, which is why we see pockets of communities where fears of vaccination take hold,” Palumbo said. “When people see recommendations change or weaken, it can also exacerbate those fears.”

Measles is especially difficult to control once vaccination rates dip, she added, because it is the most contagious infectious disease known.

“Even a small drop in vaccination rates can push a community below the herd immunity threshold,” Palumbo said. “It takes very little virus to cause infection, people are contagious for a long time—even before the rash appears—and the virus can linger in the air. An unvaccinated person can catch it just by being in the same room as someone who was infected.”

Kirsten E. Wiens, PhD, an infectious disease epidemiologist at CPH, said the developments in the UK are a reminder of how fragile measles elimination can be.

“Measles is highly contagious and entirely vaccine-preventable, so any increase in cases is concerning,” Wiens said. “The lesson is universal: maintaining high vaccination coverage requires consistent public health effort and community trust.”