Pennsylvania health officials said Monday that the state has recorded 1,004 measles cases, including 198 hospitalizations and five deaths, since an outbreak began west of Philadelphia in April.

The outbreak is now the largest reported in any U.S. state in more than three decades. The last comparable outbreak occurred in New York City in the early 1990s, when more than 4,500 people were infected over multiple years.

The nationwide measles outbreak has also reached levels not seen in decades. Nearly 3,900 cases have been reported across the country this year, according to the Centers for Disease Control and Prevention, while eight measles-associated deaths have been reported over the past two years.

Health officials have linked the recent outbreaks to declining vaccination rates. The U.S. eliminated ongoing domestic transmission of measles in 2000 after maintaining vaccination coverage high enough to prevent sustained outbreaks, but coverage has fallen in recent years. Health authorities generally cite about 95% vaccination coverage as necessary to maintain community protection.

Pennsylvania is among several states that have experienced major outbreaks since last year. Texas and Utah have reported significant outbreaks, while South Carolina previously recorded 997 cases, making it the largest state outbreak before Pennsylvania surpassed that figure.

Neighboring Ohio has also reported more than 200 cases this year. Some of the hardest-hit counties in both Pennsylvania and Ohio have significant Amish populations, where vaccination rates tend to be lower.

Measles is highly contagious and can spread through coughing, sneezing and breathing, as well as through contaminated surfaces. The virus can remain in the air for up to two hours, and the CDC estimates that as many as nine out of 10 susceptible people exposed to measles will become infected.

Pennsylvania's outbreak has also drawn political attention because of a dispute over how the state's measles deaths have been reported. State officials announced in August that two unvaccinated residents had died after contracting the virus. Three additional deaths have since been reported by the state.

The CDC initially did not include the Pennsylvania deaths on its national measles dashboard, saying it needed additional information. The agency later added one Pennsylvania death to its tally two weeks ago and a second last week.

The issue became part of a broader dispute between Democratic Pennsylvania Gov. Josh Shapiro, who has promoted vaccination, and Health Secretary Robert F. Kennedy Jr., a Trump administration appointee who has previously questioned aspects of established vaccine policy.

International health officials are also scheduled to meet in November to determine whether the United States and Mexico retain their measles-free status.