U.S. measles cases continue to climb, with outbreaks across the country

Nearly two months after a deadly, massive measles outbreak in Texas was declared over, the highly contagious disease continues to spread across the country. The U.S. has now confirmed 1,563 cases this year, according to the Centers for Disease Control and Prevention — the highest annual number in more than three decades.

But the true total could be even higher, says Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia. "If you talk to people on the ground, including not only in Texas, but other states, they all say the same thing, which is that the numbers are much worse than that. Probably closer to 5,000 cases," Offit says. "And it's not done."

He points to the current outbreak in South Carolina, where more than 150 unvaccinated schoolchildren at two schools are now subject to a 21-day quarantine after being exposed to measles. The state Department of Public Health this past week reported the eighth confirmed measles case since Sept. 25. Public health officials say it's not clear if that new case, in Greenville County, is linked to seven cases that are part of a current outbreak in neighboring Spartanburg County, S.C.

"What this new case tells us is that there is active, unrecognized community transmission of measles occurring," Dr. Linda Bell, South Carolina's state epidemiologist, told reporters during a briefing Thursday. She said that makes it vital that the public receive the recommended measles vaccination.

Meanwhile, a large measles outbreak along the border areas of Arizona and southwestern Utah continues to grow, with Utah now reporting 55 cases this year as of Friday. Nearly all of the cases occurred in people who were unvaccinated. Six people required hospitalization. According to the Utah Department of Health and Human Services, wastewater testing shows that the measles virus is more widespread in the state than previously known. In neighboring Arizona, officials have reported 63 cases so far this year. And in Minnesota, officials last week reported that a small, recent surge in measles cases there has brought the state's total for the year to 20.

All told, the CDC has confirmed 44 measles outbreaks in 41 states this year. The agency defines an outbreak as three or more cases that are linked. The vast majority of cases were in people who were unvaccinated; 27% percent have been in children under the age of 5. About 1 in 8 measles cases have resulted in hospitalization.

Measles is one of the most contagious diseases known to humans. On average, an infected person will infect as many as 18 other unvaccinated people. A person with measles can emit infectious particles that linger in the air for up to two hours, long after they've left a room. That's why it's so highly transmissible.

Before widespread vaccination, pretty much everyone got measles in childhood. And 400-500 people in the U.S. used to die from it each year. The disease was declared eliminated in the U.S. in 2000. To protect communities against outbreaks, they need a vaccination rate of 95%, according to the CDC. Routine childhood vaccination rates have been dropping across the U.S. and vaccine-preventable childhood diseases have been rising.

Dr. Adam Ratner, a pediatric infectious disease specialist in New York City, says that in some communities, kindergarten vaccination rates are much lower than the national average. That's created pockets of opportunity for measles to spread. "If a measles case comes into a place with a low vaccination rate, it can really take off," Ratner says. "That's what we saw in Gaines County" — the epicenter of the Texas outbreak that saw 762 cases and two deaths — "and that's what we're seeing in lots of places."

The Department of Health and Human Services did not immediately respond to NPR's request for comment on Sunday.