The United States recorded 41 new cases of measles as of May 17, according to health officials.
The U.S. Centers for Disease Control and Prevention (CDC) said Monday the outbreak has now spread to 24 states, most recently to Oklahoma where one case was confirmed last week.
The 41 new infections bring the total number of individual cases in the U.S. so far this year to 880 - the highest number recorded since 1994, according to the CDC.
The CDC recommends vaccinations for everyone over the age of one, except those who contracted measles as children and have since become immune.
The vaccine, which first became available in the 1960s, is considered safe and effective by most public health experts. Measles spreads through the air when an infected person coughs or sneezes. The disease was considered eradicated in the United States in 2000.