January had the distinction of having the highest number of measles cases since the U.S. achieved elimination status in 2000, according to county health officials.
The majority of those infected are people who are unvaccinated or have unknown vaccination status, officials said.
Large swaths of the country are at an increased risk of contracting the highly contagious disease due to falling vaccination rates, according to a study published this year in the journal Nature Health. The measles vaccine was first used in 1963 and combined with mumps and rubella vaccines in 1971.
The researchers analyzed county-level MMR vaccination estimates to determine the level of measles risk locally. In Southern California, two counties are in the “very high risk” category.
Measles is a highly contagious viral illness that spreads through the air when an infected person breathes, coughs or sneezes, officials said. It can also spread through direct contact with infectious droplets. Symptoms typically include fever, cough, runny nose and red eyes, followed by a rash that begins on the face and spreads to the body.