Accounts of the deployed scientist's title differ. Some describe the specialist as an epidemiologist from the CDC's Epi-Aid program; others call the person a "disease detective" from the agency's Epidemic Intelligence Service. A CDC spokesperson said the specialist is scheduled to arrive later this week. Deployments of this kind typically last one to three weeks.

The outbreak's epicenter is Lancaster County, though cases have spread across the state. Doctors believe the true number of infections is considerably higher than the confirmed count, since measles cases are generally undercounted, and experts expect confirmed cases to climb further as respiratory virus season begins. The outbreak has been attributed to vaccination rates that have fallen below the threshold needed for herd immunity.

The death toll itself is disputed. Pennsylvania officials count five confirmed deaths, with a sixth case under investigation; health officials and county coroners have separately put the total at six deaths since August. The CDC's own dashboard listed just two measles deaths nationwide as of Thursday afternoon. Separately, the CDC has acknowledged two of the Pennsylvania deaths on its website, describing the remaining cases as "under review."

The federal deployment follows weeks of friction between Pennsylvania officials and Robert F. Kennedy Jr., the health and human services secretary. Dr. Debra Bogen, Pennsylvania's health secretary, had clashed with Kennedy over how to classify "measles-associated" deaths and said the state would accept CDC assistance only if federal officials acknowledged its death count. A separate dispute broke out in August between Governor Josh Shapiro and Kennedy on social media, after Shapiro announced that two unvaccinated people had died of measles complications and accused Kennedy of spreading vaccine misinformation. Kennedy responded that the deaths may have been "fabricated" for political profit.

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, initially characterized Bogen's condition for accepting federal help as political maneuvering. He later called the decision to accept the assistance "correct" and said he expected it to "augment" the state's response.

CDC Director Dr. Erica Schwartz has recommended the MMR vaccine as the most effective way to prevent measles.