There are nearly 900 measles cases in the US. Here's what you need to know

The number of measles cases in the U.S. has risen to 884, according to new Centers for Disease Control and Prevention data published Friday.

Cases have been confirmed in 29 states including Alaska, Arkansas, California, Colorado, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, New Jersey, New Mexico, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, Vermont, Virginia and Washington.

At least six states including Indiana, Kansas, Michigan, New Mexico, Ohio and Texas are reporting outbreaks, meaning three or more related cases.

In Texas, where an outbreak has been spreading in the western part of the state, at least 646 cases have been confirmed as of Friday, according to the Texas Department of State Health Services.

Dr. Marschall Runge, dean of the University of Michigan Medical School and CEO of Michigan Medicine, said the number of cases -- at the national level and in Texas -- is likely an undercount.

"I think it's likely that there are a lot of unreported cases in children who weren't particularly sick or didn't come to medical attention," he told ABC News.

Here's what you need to know about measles in the U.S.

Measles is potentially deadly

Many people think measles is a benign virus but "complications from measles are common and often severe," Runge said.

Measles is most well-known for causing a fever and a rash, but complications can be severe, including blindness, pneumonia, respiratory distress, swelling of the brain, severe diarrhea, dehydration and death, according to the CDC.

About one in five unvaccinated measles patients are hospitalized and one to three of every 1,000 children infected with measles will die from measles-related complications, the CDC says.

In Texas, two unvaccinated school-aged children have died of measles. Both were previously heathy with no underlying conditions, according to Texas DSHS.

Measles can be especially dangerous for infants, children, pregnant women and people with weakened immune systems.

"People who are immunocompromised are a population that is at risk all the time, which is a very compelling reason to have people vaccinated as they are relatively unprotected," Runge said.

Why are we still hearing about measles outbreaks?

Measles was declared eliminated from the U.S. in 2000 due to a highly effective vaccination program, according to the CDC.

The CDC currently recommends that people receive two doses of the measles, mumps, rubella (MMR) vaccine, the first at ages 12 to 15 months and the second between 4 and 6 years old. One dose is 93% effective, and two doses are 97% effective, the CDC says.

Runge said outbreaks are still emerging because vaccination levels have dropped in the U.S.

During the 2023-24 school year, 92.7% of kindergartners received the MMR vaccine, according to CDC data. This is lower than the 93.1% seen the previous school year and the 95.2% seen in the 2019-20 school year, prior to the COVID-19 pandemic.

A recent study published in JAMA predicted the U.S. could see millions of measles cases over a 25-year period if vaccination rates decline by 10%.

Lagging vaccination rates means people are often left unprotected against measles, which is very contagious, Runge said.

"You don't have to be highly symptomatic to be contagious with measles," he said. "It's one of the most transmissible illnesses."

Runge also said there have been many false claims about the MMR vaccine including that it causes autism and that there are more complications from the vaccine than from measles itself.

The myth that the MMR vaccine causes autism was born out of a now-debunked paper from the U.K. in 1998.

The paper has since been discredited by health experts, retracted from the journal in which it was published, and its primary author, Andrew Wakefield, lost his medical license. More than a dozen high-quality studies have since tried to find a link and have not been able to do so.

As for the second claim, "it's like many, many, many thousands-fold more complications with measles than with vaccinations, and this is a very safe vaccination," Runge said.

Does vitamin A help treat measles?

In multiple interviews, Health and Human Services Secretary Robert F. Kennedy Jr. has claimed that vitamin A and cod liver oil are effective treatments for measles.

There is no scientific evidence that vitamin A supplementation can prevent measles or is a cure for measles.

In fact, too much vitamin A can lead to nausea, vomiting, blurry vision, liver damage and potentially increased pressure inside the skull that press on the brain.

Covenant Children's Hospital, which has treated dozens of measles patients in Texas amid the outbreak, told ABC News in a statement last month some parents appear to have given their unvaccinated children vitamin A for "treatment and prevention." Some of those children now show signs of vitamin A toxicity.

Vitamin A can be used as part of supportive treatment for those who are already sick with measles, with the World Health Organization recommending two doses of vitamin A in children and adults to restore low vitamin A levels, which can help prevent eye damage and blindness.

"Vitamin A in significant doses, but not huge doses, can lessen the duration of the disease, lessen side effects, and even reduce mortality by up to 50% so that's an important effect," Runge said. "Now most of those data come from countries where measles is very prevalent and there's also a great deal of malnutrition."

Kennedy has also suggested that steroids or antibiotics can treat measles.

Runge said an antibiotic may help treat a bacterial infection acquired after contracting measles, such as pneumonia, and a steroid might be helpful in treating airway disease in asthmatic patients with measles, but neither are cures for the viral infection.

How do I know if I'm protected against measles?

The CDC considers those who received two doses of the MMR vaccine as children protected for life and not ever in need of another dose.

If someone is unsure if they are immune to measles, they should first try to find their vaccination records. If they cannot find written documentation, there is generally no harm in receiving another dose of the MMR vaccine, according to the CDC.

A health care provider can also test blood to determine whether someone is immune, but this is generally not recommended.

Dr. Jennifer Miao is a board-certified cardiologist and critical care fellow at Yale School of Medicine/YNHH, and a member of the ABC News Medical Unit.