Doctors Identify Groups at Highest Risk as U.S. Measles Cases Surge
With more than 3,200 measles cases reported in the United States this year, infectious disease specialists say infants under one year, unvaccinated pregnant people and immunocompromised individuals are the most vulnerable.
The United States has recorded 3,294 confirmed measles cases across 47 states and jurisdictions in 2026, according to the Centers for Disease Control and Prevention. Pennsylvania accounts for 731 of those cases, with outbreaks reported in 38 of its 67 counties and four deaths linked to the disease.
In a briefing, infectious disease experts highlighted the populations most at risk. > "Anyone with an infant under 12 months should be most concerned," said Dr. Amesh A. Adalja of Johns Hopkins, noting that two of Pennsylvania’s measles-related deaths were infants. He added that infants are too young for the routine first MMR dose and therefore have limited protection.
Dr. Adalja also warned that unvaccinated pregnant individuals face heightened risk of severe complications and cannot receive the live MMR vaccine during pregnancy. Dr. William Schaffner of Vanderbilt University explained that the vaccine is contraindicated for pregnant people because of the theoretical risk of the weakened virus crossing the placenta.
Immunocompromised persons who have not been vaccinated are similarly vulnerable, as the live vaccine could cause illness in those whose immune systems cannot respond adequately, Dr. Schaffner said.
Dr. Mark Hicar, chief of pediatric infectious diseases at the University at Buffalo, summarized the overall risk profile: "Those who are not vaccinated or lacking prior infection should be concerned." He noted that a single MMR dose provides about 93% protection, while two doses raise effectiveness to 97%, and that individuals born before 1957 are generally considered immune from natural infection.
During outbreaks, health officials may administer the first MMR dose as early as six months of age and advance the second dose to protect children sooner, Dr. Hicar said. While breakthrough infections can occur in vaccinated people, they are typically milder than infections in unvaccinated individuals, Dr. Schaffner added.
The physicians also emphasized the seriousness of measles complications, including pneumonia, encephalitis and a phenomenon called immune amnesia that can diminish immunity to other diseases. They urged anyone without documented two doses of MMR to get vaccinated, and advised that a single dose can begin to confer protection within 72 hours of administration. If symptoms develop, they recommended contacting a medical provider before visiting a clinic to reduce the risk of exposing others to the airborne virus.