The number of measles cases in the United States has reached a 25-year peak, propelled by the spread of misinformation about the vaccine that can prevent the disease, federal health officials say.

Key points: The majority of cases have occurred in children who have not received the MMR vaccine

The majority of cases have occurred in children who have not received the MMR vaccine US Health and Human Services Secretary Alex Azar said the increase in cases was "completely avoidable"

US Health and Human Services Secretary Alex Azar said the increase in cases was "completely avoidable" The current outbreak has been concentrated in New York City, where officials have linked anti-vaxxers to more than 390 cases recorded since October

The Centres for Disease Control and Prevention (CDC) said 695 cases had been reported in 22 states since January, making it the nation's worst year for measles since 1994, with eight months still to go.

The vast majority of cases have occurred in children who have not received the measles-mumps-rubella (MMR) vaccine, which confers immunity to the disease, officials said.

"The suffering we are seeing today is completely avoidable," US Health and Human Services Secretary Alex Azar said.

"We know vaccines are safe because they're among some of the most studied medical products we have."

Public health experts say some communities have low vaccination rates because of the spread of bad information — especially the now-debunked notion that the MMR vaccine is linked to autism — through social media, pamphlets, hotlines and other means.

"Many parents are afraid," said Dr Jonathan Fielding, former head of the Los Angeles County Department of Public Health.

"If you want to believe your kid doesn't need that many shots, there's plenty of places to find people who agree with you.

"It's not so easy to discern what is real and what is not."

Travellers bringing measles into the United States

Measles in most people causes fever, a runny nose, a cough and a rash all over the body.

However, a very small fraction of those infected can suffer complications such as pneumonia and dangerous swelling of the brain.

None of the victims of the recent outbreak have died, but 3 per cent have contracted pneumonia and 9 per cent have been hospitalised due to complications from the disease, CDC director Robert Redfield said.

The current outbreak has been concentrated in New York City, where officials said more than 390 cases have been recorded since October, mostly among children in Orthodox Jewish communities in Brooklyn.

"This outbreak is being fuelled by a small group of anti-vaxxers in these neighbourhoods," New York City health commissioner Dr Oxiris Barbot said.

The national outbreak has escalated since 82 people in 2018 and more than 40 people in 2019 brought measles to the United States from other countries, most frequently Ukraine, Israel and the Philippines.

Up to 10 per cent of patients in the current outbreak are adults who had received one or two doses of the vaccine, the CDC said.

Some adults may need a new dose depending on whether they received the recommended two doses of the live virus or if they are travelling into and out of outbreak areas.

Although the virus was eliminated from the country in 2000, meaning it was no longer continually present year-round, outbreaks still happen via travellers coming from countries where measles is still common.

Reuters/AP