Health officials said Monday that 704 people have now been infected with the highly infectious measles virus, the most in 25 years.
Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, told reporters in a phone call Monday that most infections were among children who had not been vaccinated, and other officials warned that people opposed to vaccinations were spreading misleading information about the vaccine that prevents measles to dissuade parents from getting their children vaccinated.
“Vaccine-preventable diseases belong in the history books, not our emergency rooms,” Health and Human Services Secretary Alex Azar said in call with reporters to mark National Infant Immunization week.
At least 22 states have been affected, but most cases are in New York state, New York City, and New Jersey, primarily among unvaccinated people in Orthodox Jewish communities. Of those infected, 66 have been hospitalized, and no deaths have been reported.
Children are supposed to get the measles vaccine, known as MMR, around the age of 1 and again before they enter kindergarten. The first vaccine has a 93% effectiveness rate, and the booster has a 97% protection rate.
[Related: How the measles virus is infecting hundreds, after being eliminated in 2000]
Redfield said that when people aren’t vaccinated they risk infecting those who are more vulnerable, including infants under age 1 and people with serious illnesses. The latest tally is the highest since 1994, when 963 people had the virus. It was officially declared eliminated in 2000, meaning that it’s considered no longer to be an illness that is circulating among the population, even though occasional flare-ups occurred after people traveled to parts of the world where measles hadn’t been eliminated and then came home and infected others.
Before widespread vaccinations, measles was very common in the United States, infecting about 4 million people a year and identifiable through a red rash that spreads all over the body. People are infectious for measles four days before the rash appears, meaning that when they develop symptoms such as a runny nose and red eyes, they don’t know that they are carrying a highly infectious virus and risk spreading it quickly to others.
People who are skeptical of vaccinations today tend to raise a debunked study claiming that the vaccine caused autism. Officials on Monday stressed that the vaccine is safe and does not cause autism.
[Also read: NYC declares health emergency amid measles outbreak]
President Trump used to spread the message that the MMR vaccine caused autism when he was running for office but said last week that it was very important for people to get vaccinated.
“They have to get the shot,” Trump said Friday morning when asked about the measles outbreak. “Vaccinations are so important. This is really going around now. They have to get their shots.”
People who get measles become immune for life, but the illness can be much more dangerous and even deadly for some people. Before the vaccine, 48,000 people were hospitalized in the U.S every year from the disease, and 500 died. Another 1,000 developed brain swelling, known as encephalitis.
Schools require children to get vaccinated, but some states allow exceptions for religious or philosophical reasons, and some parents choose not to vaccinate their children because they fear vaccines are unsafe. In other instances, parents try to space out vaccines and fall behind. The CDC estimates that roughly 100,000 children under the age of 2 in the U.S. aren’t vaccinated.
Adding to the difficulty of containing the virus is all of the adults that are vulnerable to infection. The U.S. didn’t start recommending two vaccines until 1989, meaning that many adults haven’t had the booster shot not because they are actively opting out but because they were never urged to get a second vaccine. Health officials urged people of all ages to check with their doctors about whether they have been vaccinated.
[Related: Anti-vax organization sues New York over measles vaccine mandate]
