Measles is spreading across the US – here is what you need to know

The US is currently facing its most severe measles resurgence in years. On 26 March, Ohio became the fifth state to declare a measles outbreak this year, joining Texas, Oklahoma, New Mexico and Kansas.

How many measles cases are there in the US?

As of 27 March, the US Centers for Disease Control and Prevention (CDC) has confirmed 483 measles cases in the US this year. This is the highest number of infections since 2019, when there were more than 1200 confirmed cases. The CDC is aware of more potential measles cases but is waiting for confirmation before including them in the case count.

Which states are affected by measles?

So far, 19 states have confirmed at least one measles case this year: Alaska, California, Florida, Georgia, Kansas, Kentucky, Maryland, Michigan, Minnesota, New Jersey, New Mexico, New York, Ohio, Pennsylvania, Rhode Island, Tennessee, Texas, Vermont and Washington. Fourteen of these states have reported only isolated incidences, meaning the infection didn’t spread to more than one other person.

However, five states declared a measles outbreak, defined as at least three related measles cases. As of 28 March, confirmed measles cases totalled 400 in Texas and 44 in New Mexico. There were seven cases in Oklahoma as of 25 March. Kansas and Ohio have confirmed 23 and 10 measles cases, respectively, as of 26 March.

Has anyone been hospitalised or died?

The CDC reports that 70 people have been hospitalised for measles so far this year. One unvaccinated child also died from the virus on 26 February in Texas – the country’s first recorded measles death in a decade. A second possible measles-related death is under investigation in New Mexico.

When did the outbreak start?

Texas was the first state to declare a measles outbreak. The Texas Department of State Health Services issued an alert on 23 January after identifying measles in two unvaccinated adults who lived together in Houston and had recently travelled abroad. A week later, the state confirmed an additional two cases in unvaccinated children on the other side of the state in Gaines county. It isn’t clear if these two incidents were related.

Since then, measles has rapidly spread through Gaines county, located in western Texas, infecting at least 270 individuals. Gaines county has a large rural population and low vaccination rate: just under of children in kindergarten – which usually starts at age 5 – were vaccinated against measles. The real rate may be lower, as the data doesn’t include home-schooled children.

The outbreak has since spilled into neighbouring communities, including those across the state border in New Mexico, which announced its first case on 11 February, and Oklahoma, which announced its first two cases on 11 March. The New York Times reported that state health officials also believe the Kansas outbreak, which began on 13 March, is related to those in New Mexico and Texas.

The latest outbreak in Ohio is unrelated to those in other states. The Ohio Department of Health declared the first case on 20 March in an unvaccinated man who had contact with someone who had recently travelled abroad. All nine of the other confirmed cases are related to this first unvaccinated man.

Who is most affected by measles?

Nearly all of the confirmed measles cases – 97 per cent – have occurred in people who are unvaccinated or whose vaccination status is unknown. Only 2 per cent of cases are in people who have received two doses of the measles, mumps and rubella (MMR) vaccine. Two doses of the vaccine is 97 per cent effective against measles while one dose is only 93 per cent effective.

Children and adolescents are particularly susceptible to measles. The majority of cases – 75 per cent – are in people 19 years old and younger. Children are also at risk of developing serious complications from measles. About 1 out of every 20 children with measles develops pneumonia and about 1 out of every 1000 with the illness experiences brain swelling, which can lead to hearing loss, intellectual disability and death.

Why are there so many measles cases?

The US declared measles eliminated in 2000. However, occasional outbreaks still occur due to waning vaccination rates. Measles is a highly contagious virus. Each case can lead to an additional 12 to 18 cases if people aren’t protected against the disease. When more than 95 per cent of people in a community are fully vaccinated against measles, most people in the area are protected through herd immunity.

In the 2019 to 2020 school year, more than 95 per cent of US kindergarteners were vaccinated against measles. Yet that number fell to less than 93 per cent in the 2023 to 2024 school year, leaving roughly 280,000 kindergarteners at risk of infection.

What is the US doing to contain the outbreak?

Vaccination is the best protection against measles. “The fact that we now have a measles case in Ohio adds emphasis to the importance of being fully vaccinated,” Bruce Vanderhoff at the Ohio Department of Public Health said in a press release announcing the state’s first case. “This disease can be very serious, but it is preventable. I strongly encourage you to protect yourselves and your children by getting vaccinated.”

However, the US secretary of health, Robert F Kennedy Jr, has stopped short of calling on people to get vaccinated. “The decision to vaccinate is a personal one,” he wrote in a Fox News article on 2 March. “Vaccines not only protect individual children from measles, but also contribute to community immunity, protecting those who are unable to be vaccinated due to medical reasons.”