The surging number of measles cases around the world is a stark warning sign that outbreaks of other vaccine-preventable diseases could be next, the World Health Organization warned Friday.

“It’s crucial to understand why measles matters,” said Dr. Kate O’Brien, director of the WHO’s Department of Immunization, Vaccines and Biologicals. “Its high transmissibility means that even small drops in vaccine coverage can trigger outbreaks, like a fire alarm going off when smoke is detected first.”

That is, measles is often the first disease to pop up when vaccination rates overall drop.

"When we see measles cases, it signals that gaps are almost certainly likely for other vaccine-preventable diseases like diphtheria or whooping cough or polio, even though they may not be setting off the fire alarm just yet," O'Brien said at a media briefing Monday, ahead of the release of the WHO's Progress Toward Measles Elimination report, published Friday in its Weekly Epidemiological Record.

Indeed, whooping cough cases are also rising in the United States and are on track to be the most in a decade. More than 20,000 whooping cough cases have been reported so far in 2025, according to the Centers for Disease Control and Prevention.

In 2024, there were an estimated 11 million measles infections worldwide, according to the report, nearly 800,000 more than were recorded in 2019.

Last year, 59 countries reported large measles outbreaks. In 2025, the United States joined the list of countries.

Elimination status threatened

The ongoing outbreaks threaten the so-called measles elimination statuses of some countries.

Elimination means a virus has stopped spreading in a specific country or region. (Only one virus — smallpox — has been eradicated, or wiped out permanently, worldwide.)

In total, 81 countries had reached elimination status in 2024, according to the WHO. Canada eliminated measles in 1998. Two years later, the United States did the same.

Elimination status means a country has the capacity to stop an outbreak when measles cases arrive from abroad, O'Brien said. If vaccination rates are high enough, the virus won't have enough unvaccinated people to infect, halting an outbreak in its tracks.

But vaccination rates in the United States are falling: An NBC News investigation revealed that since 2019, 77% of counties and jurisdictions have reported declines in the number of kids getting routine childhood vaccinations like the measles-mumps-rubella shots.

The key determining factor for a country to lose its measles elimination status is the ongoing spread of the same strain of the virus for a full year.

Canada met that threshold this month. The United States could be next if scientists can trace current cases to a Texas outbreak that began in January.

Nearly all of the samples analyzed from those early cases were identified as a genotype of measles called D8, according to a CDC report published in April.

The D8 genotype was recently detected in a South Carolina outbreak.

Preliminary results from specimens sent from South Carolina to CDC labs "are the same type, D8, that is seen in other settings in the United States," Dr. Linda Bell, state epidemiologist for the South Carolina Department of Public Health, said at a news briefing Tuesday.

Additional genetic sequencing is needed to make a definitive link between the Texas outbreak and the one in South Carolina, as well as outbreaks in Utah and Arizona. A South Carolina Department of Public Health spokesman said the agency "expects those results in the next few weeks."

Bell said that as of Tuesday, 58 cases had been reported in South Carolina, mostly in Spartanburg County in the northwest part of the state.

An outbreak along the border of Arizona and Utah continues to grow. The Arizona Department of Health Services reported 153 cases this week, nearly all in Mohave County.

Cases in Utah have reached 102, according to the state Department of Health and Human Services. While the bulk of those cases are linked to the cluster at the Utah-Arizona border, case numbers are also rising near Salt Lake City. NBC affiliate KSL reported that eight students at a high school in Wasatch County had been diagnosed.

As of Wednesday, the CDC had reported 1,798 confirmed measles cases in 42 states in 2025. Three people, an adult in New Mexico and two little girls in Texas, have died.

This article was originally published on NBCNews.com