The Centers for Disease Control and Prevention on Friday issued a health alert for doctors and health officials to be on the lookout for measles cases after a person with a confirmed, contagious case attended a massive religious event in Kentucky last month, potentially exposing an estimated 20,000 people to one of the most infectious viruses on the planet.

The event was a spontaneous "outpouring" at Asbury University, which drew tens of thousands of worshippers to the small, private Christian institution in Wilmore from February 8 to 23. Attendees came from around Kentucky, other US states, and other countries.

A case of measles was confirmed in an unvaccinated person who had recently traveled out of the country before attending Asbury University's outpouring. The person was at the event from February 17 to 18, the CDC reported. The university added that the person attended before developing symptoms, suggesting the person did not know they were infected. But the CDC notes that people with measles are contagious four days before measles' telltale rash develops, and state health officials said the person was contagious while they were in attendance.

It's unclear how many people may be at risk of the infection, but the CDC estimates that 20,000 attended during the same time period as the infected person. As such, the exposure can potentially spark an explosive outbreak among those who were unvaccinated or under-vaccinated at the event.

Kentucky has one of the lowest measles vaccination rates in the country, with only around 86.5 percent of kindergartners receiving two doses of the measles, mumps, and rubella (MMR) vaccine in the 2021-2022 school year, according to CDC data. In Jessamine County, which encompasses Asbury University, the kindergarten vaccination rate is 89.2 percent, a spokesperson for the Kentucky Health Department told Ars. Health officials target a 95 percent vaccination rate to prevent the dangerous infection from spreading.

Highly contagious

In a statement, the state health department spokesperson said that the agency is working closely with local health departments, Asbury University, and the CDC to get ahead of a potential outbreak. Though no other measles cases have been linked to the event so far, the public health response is likely to stretch on for several more weeks, at least.

"Individuals with a known exposure to the person with measles are being contacted and those who are not vaccinated against measles are being advised to quarantine," the spokesperson said. "There are no additional confirmed cases of measles connected with this exposure at this time. It typically takes seven to 21 days for someone who is exposed to measles to develop the characteristic measles rash, so we anticipate that this investigation will be ongoing for several more weeks."

Measles is a highly contagious virus, causing infections that start with fever, cough, runny nose, and pink eye before causing a characteristic red rash that typically develops three to five days after the initial symptoms. The infection can easily turn serious, particularly for younger children. In a recent outbreak in Ohio, 36 of 85 infected children (42 percent) were hospitalized.

The virus spreads through direct contact and the air, where it can linger for up to two hours after an infected person leaves an area. According to the CDC, up to 90 percent of unvaccinated people exposed to the virus will become infected.

Local, state, and federal health officials are now urging unvaccinated attendees to quarantine for 21 days and get themselves vaccinated.

"If you may have been exposed at Asbury University’s campus and develop any symptoms, whether previously vaccinated or unvaccinated, please isolate yourself from others and call your medical provider, urgent care, or emergency department to seek testing," Steven Stack, commissioner of the Kentucky Department for Public Health (KDPH) said in a news release. "Please do not arrive at a health care facility without advance notice so that others will not be exposed."