A fourth person has died from complications related to measles in Pennsylvania, say state officials, in an outbreak that has infected hundreds of people.

An 18-year-old died from acute encephalomyelitis, a rare and severe neurological complication of measles, the Mifflin County Coroner’s Office in Pennsylvania said.

The Keystone state has seen over 600 cases of measles this year, primarily in Lancaster County.

The US eliminated measles in 2000 with the help of vaccines, but it has this year experienced the highest number of deaths from the virus since the 1990s.

Last year, three people – including two children – died from a measles outbreak in Texas that spread predominantly among the Mennonite community.

The rising number of cases has come as vaccine hesitancy in the US has increased in recent years, leading to lower immunisation rates.

In Pennsylvania, state and local officials have said the 18-year-old and a 40-year-old – both unvaccinated – died from measles-related complications over the past month.

Two infants have also died. US children typically do not receive their first MMR (measles, mumps, and rubella) vaccine until 12 months of age.

The measles virus – with its signature blotchy red rash – used to infect millions of children a year before a vaccine was invented.

The highly contagious virus can be fatal, and some patients can experience long-term complications including encephalitis and immune amnesia.

To achieve herd immunity, around 95% of the population must have the vaccine, say health officials.

Democratic Governor Josh Shapiro posted on X on Tuesday: “This disease was declared eliminated in the United States in 2000 – yet decades later, we are seeing it surge due to lower vaccination rates.”

The reporting of the deaths in Pennsylvania has led to tensions between Shapiro and Health Secretary Robert F Kennedy Jr.

Shapiro has accused Kennedy of spreading misinformation about vaccines and has said he would withhold information about the deaths from the top health official in order to protect the families of the deceased.

Kennedy has questioned whether the infant deaths were in fact from measles.

A 40-year-old woman died in Jefferson County, Pennsylvania, on Saturday, with the coroner saying the cause of death was respiratory failure due to measles.

Lancaster County Coroner Stephen Diamantoni said a six-week-old born with a rare genetic condition, Amish lethal microcephaly, died of measles on 18 August.

Diamantoni has said another infant who tested positive for measles died soon after birth on 14 August, with the primary cause of death listed as a spleen laceration.

Measles infections can cause the spleen to enlarge, raising the risk of rupture. State health officials have said the death was from a measles infection that resulted in a spleen rupture.

In a post on X after the first two deaths in the state, Kennedy accused Shapiro of “political grandstanding”.

The health secretary said the US Centers for Disease Control and Prevention would only update its information on deaths once reviews were complete.

The Pennsylvania Department of Health said it had conducted “rigorous investigations” with the coroner’s offices to confirm that the latest two deaths were indeed from measles.

“These investigations included confirming clinical evidence of a measles infection, a positive laboratory test, and that the death was not due to an unrelated cause,” the health department said.

In November, a panel is set to decide whether the US will lose its status of having eliminated measles, which Canada lost last year.

Kennedy has been criticised by public health experts who argue his remarks and remaking of US vaccine policy has hindered efforts to stop the outbreaks.

A vaccine sceptic, Kennedy has shared mixed messaging on the MMR vaccine, which has proven safe and effective.

The health secretary has said the worsening measles outbreak is a global problem.

Speaking before a Senate committee last month, he said: “A lot of nations have lost their elimination status. The whole world had their worst measles year.”