Over 1,000 cases of measles have been recorded so far this year in Pennsylvania alone, health officials in the northeastern US state said on Monday, the largest outbreak in a US state since the early 1990s.

The US Center for Disease Control (CDC) has put the number of cases recorded nationwide so far this year at 3,869, in what is seen as the worst year for measles since 1991.

What do we know about the latest measles outbreak?
The current outbreak started west of Philadelphia in April, health officials said. Nearly 200 people have been hospitalized and five have died. Nearly all of the cases involved people who were not vaccinated, according to the Pennsylvania Department of Health.

In neighboring New York on Monday, Governor Kathy Hochul declared a month-long state disaster emergency to "strengthen ongoing efforts to confront" measles.

Over 100 cases have been recorded this year in New York State outside of New York City, with 92 of those reported in "under-immunized" rural areas since July 15.

The executive order suspends regulations on administering vaccines, expanding permission to paramedics and midwives, while allowing nurses to order testing and lowering permission barriers.

"No one should get seriously ill or die from a vaccine-preventable disease," Hochul said.

US vaccine coverage dips
Since last year, measles has broken out in several US states, including Texas, Utah and South Carolina, which has reported just under 1,000 cases.

Measles includes symptoms like high fever, cough, and rash. It can cause serious complications, especially in young children. It is highly contagious but can be prevented with vaccines.

However, skepticism toward vaccines in the US has risen in recent years, with more parents waiving routine vaccinations amid disinformation about vaccine safety.

In 2000, the US had eliminated the domestic spread of measles with high vaccination rates, but coverage has recently dipped below the 95% necessary to prevent outbreaks.

US Health and Human Services (HHS) Secretary Robert F. Kennedy, Jr, who has expressed views skeptical of vaccines that are not backed by science, has characterized vaccinations as a "personal choice."

Although Kennedy maintains that the measles vaccine is the best defense against the highly contagious and potentially fatal infection, he has also advocated unproven "alternative treatments" like taking vitamins.