According to CDC data, children under five account for 18 percent of confirmed cases, while those aged five to 19 make up another 41 percent.

In total, children and teenagers represent approximately 59 percent of reported infections.

Of the 5,182 confirmed cases, 4,993 were associated with outbreaks.

The surge comes as measles vaccination coverage among US kindergarteners has declined from 95.2 percent in the 2019-2020 school year to 92.4 percent in 2025-2026.

The 2026 total is also more than double the 2,289 confirmed cases reported in 2025.

The CDC has reported four measles-related deaths in 2026, but its national tally is lower than the number reported by state authorities, particularly in Pennsylvania, the state hardest hit by the outbreak.

Pennsylvania’s Department of Health has reported six deaths and 1,487 measles cases this year.

The discrepancy has fueled a dispute between Pennsylvania Governor Josh Shapiro and US Health Secretary Robert F Kennedy Jr over how measles-related deaths should be classified.

Under Pennsylvania’s reporting criteria, a death is classified as “measles-associated” when a person with confirmed measles dies within 30 days of developing symptoms, unless the death is attributed to an unrelated event.

In August, Pennsylvania reported that two unvaccinated people had died of measles.

The CDC initially included the deaths in its national tally but later removed them after Kennedy, a vaccine sceptic, questioned whether measles had caused or contributed to the fatalities.

The CDC has said it is working with the US Council of State and Territorial Epidemiologists to establish a standardized definition for measles deaths.

Infectious disease experts have warned that the dispute over death classifications puts the CDC at odds with a longstanding approach to tracking the consequences of measles.

Epidemiologists have told Al Jazeera that tracking “measles-associated” deaths has long been standard practice for both the World Health Organization and the CDC.

The United States declared measles eliminated more than two decades ago, but infections have increased in recent years, particularly in communities with vaccine skepticism or limited access to healthcare.

Measles cases have also risen globally, with major outbreaks reported in several countries.

Experts warn that declining vaccination rates in one location can leave populations thousands of miles away vulnerable, underscoring the importance of international efforts to prevent the spread of the disease.