Health-care workers who have never treated someone with measles are having to become familiar with signs of the disease. Credit: Paul Hennessy/NurPhoto/Getty
Measles has been so rare in many countries that some physicians have never seen a case — but that is changing.
The United States recorded more than 2,000 cases last year, more than in any other year in three decades, and could surpass the 2025 total in 2026 (see ‘US measles surges’). The United Kingdom, Spain, Austria and three other nations all lost their official ‘measles free’ label in January. Canada lost its measles-free status in November, and the United States is projected to follow suit in April.
Source: US Centers for Disease Control and Prevention
The measles virus is highly contagious and can cause a fever, cough and rash, and even lead to death. Estimates show that each person with measles would infect, on average, 12–18 other people if everyone around them were susceptible to the disease. Up to 90% of people who are not immune will get measles if they encounter an infected person.
So how worried should vaccinated people be?
How protective are vaccines?
The measles vaccine is highly effective. After one dose, 93% of people will be protected from infection if they are exposed to the virus. Protection rises to 97% after two doses. For most people, this protection lasts for a lifetime.
When 92–94% of the population has immunity to measles, either through vaccination or previous infections, the virus can no longer spread, a phenomenon known as herd immunity. “You may see a very small outbreak or sporadic cases, but you won’t see sustained transmission,” says Nathan Lo, an infectious-disease physician-scientist at Stanford University in California. That is why the measles vaccination coverage target is set at 95%, he adds. In the United States, vaccination coverage among kindergarteners, who are generally aged between 5 and 6, dropped from 95.2% in the 2019–20 school year to 92.5% in the 2024–25, opening the door for outbreaks.
Are vaccinated people also at risk?
... continue reading