The number of measles cases has hit a 25-year high in the US, where health experts are blaming the spread of misinformation for turning parents against vaccines.
The Centres for Disease Control and Prevention (CDC) said 695 cases had been reported in 22 states so far this year, as of Wednesday.
That makes 2019 the country’s worst year for measles since 1994 – when there were 963 cases – with eight months still to go.
The virus can lead to deadly complications, but no measles deaths have been reported in the latest outbreaks.
It comes after new figures revealed more than half a million children in the UK are at risk of the virus because they have missed their recommended immunisations in the last eight years.
Analysis by children’s charity Unicef warned that a rising number of infants worldwide were missing out on the measles jab, with 21 million a year needlessly put at risk of infection since 2010.
Measles is a highly contagious virus and can be spread through the air by coughing and sneezing.
Public health officials claim the outbreak in the US has been fuelled in part by misinformation being spread about the measles, mumps and rubella (MMR) vaccine, which some parents believe can cause autism.
The largest outbreak has been in New York City where at least 390 cases have been recorded since October, mostly among children in Orthodox Jewish communities in Brooklyn, making it the city’s worst outbreak since 1991.
That total included 61 cases recorded in the last six days, of which two were pregnant women, the city’s health department said on Wednesday.
Nationwide, the number of children getting vaccinated has remained “high and stable” for several years, the CDC said.
The New York City health department took the unusual step earlier this month of issuing an emergency order requiring unvaccinated people in affected neighbourhoods to get the MMR vaccine unless they could otherwise prove they had immunity.
It has issued civil summonses to 12 people it said defied the order.
They will each face a fine of up to $1,000 if found to be noncompliant at a hearing.
The CDC recommends the vaccine for everyone over a year old, except for people who had the disease as children since they will be immune.
Dr Amesh Adalja, an infectious disease expert with the Johns Hopkins Centre for Health Security, called the resurgence a “completely preventable occurrence”.
He said: “We are fighting a disease now in 2019 that should have been off the table in the 1960s with the development of the vaccine.
“It should be viewed as an embarrassment that so many Americans have turned away from vaccines that we are having a record year for measles.”
US health secretary Alex Azar urged greater vaccination, saying that the vaccine’s “safety has been firmly established over many years”.
He added: “The United States is seeing a resurgence of measles, a disease that had once been effectively eliminated from our country.”
Additional reporting by Reuters