A spike in measles cases, largely caused by people not getting vaccinated, poses a "renewed threat" to the declaration in 2000 that the highly contagious airborne disease had been eliminated in the United States.
"The U.S. measles elimination status will continue to be threatened by global increases in measles incidence and decreases in global, national, and local measles vaccination coverage," the CDC said in a reportreleased Thursday. Elimination status means the disease is no longer constantly present in the country.
So far this year, the country has seen more than 120 cases of measles, according to CDC data. That's more than double the number reported for all of last year.
The CDC said "the rapid increase in the number of reported measles cases during the first quarter of 2024 represents a renewed threat to elimination."
The chances of widespread measles transmission in the U.S. remain low given the country's "high population immunity," the agency added. But the rise in cases is especially dangerous for infants and undervaccinated communities.
The uptick comes five years after the measles cases reached the highest level in over two decades. In 2019, the CDC tallied over 1,200 cases in 31 states, largely in undervaccinated communities in New York state.
Measles is also on the rise globally. According to the World Health Organization, there were about 9 million cases in 2022 — an 18% increase from 2021. The number of deaths rose 43% in 2022 compared to the prior year.
What the CDC data says
Nearly half of the cases so far this year occurred among children under 5. Nearly a third came from adults 20 years and older.
Cases are typically linked to people who have traveled abroad. But vaccine skepticism and communities with low vaccination rates have also contributed to the spike.
CDC data showed that more than 80% of cases this year involved people who were either not vaccinated or whose vaccination status was unknown. About 13% of cases involved people who only received one dose of the measles vaccine. Public health officials recommend two doses.
As of April 4, 17 states recorded cases, with Illinois and Florida seeing the most number of sick patients.
Why a measles outbreak is dangerous
Measles' symptoms include a fever, cough, runny nose, white spots inside the mouth, and rashes that spread across the body. Most cases are mild, but sometimes it can lead to brain swelling, pneumonia and death.
The danger is in how contagious measles can be. If an infected person coughed in a room, 90% of unvaccinated people in that room could get sick.
Federal health officials recommend that all children get two doses of the measles vaccine: the first dose at 12 to 15 months of age and a second dose at 4 to 6 years old.
Adults who are planning to travel abroad and women who are considering getting pregnant should check on their vaccination status, the CDC said.
What it would take for the U.S. to lose its measles elimination status
Before a vaccine was introduced, the illness killed hundredsof people and sickened most children in the U.S. before they turned 15. That all changed in 1963 when a vaccine became available.
In 2000, measles was declared eliminated from the U.S., meaning the disease was no longer "constantly present" the CDC said. But the U.S. is at risk of losing its elimination status if a measles outbreak continues for a year or more, the agency added.
Dr. Allison Bartlett, a professor of pediatrics at the University of Chicago's medical school, told NPR's Weekend Edition Saturday that the uptick in cases is a "very serious wake-up call."
"This is a very, very preventable illness by vaccination. But it requires very high levels of individuals being vaccinated," Bartlett said earlier this month.
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