There is now an “imminent threat” of measles spreading to all parts of the world, the World Health Organization and the US Public Health Agency have said.
In a joint report, health organizations said there had been a decline in measles vaccines and less disease surveillance during the COVID pandemic.
Measles is one of the most contagious human viruses, but it is almost entirely preventable by vaccination, although it requires 95% vaccination coverage to prevent epidemics.
A record nearly 40 million children missed a dose last year due to barriers created by the pandemic, according to the report from WHO and the US Centers for Disease Control and Prevention.
This left millions of children vulnerable to disease.
“We are at a crossroads,” said Patrick O’Connor, WHO measles chief.
“It will be a very difficult 12 to 24 months to try to mitigate this.”
Although cases have not yet risen significantly from previous years, now is the time to act, he said.
Persistent social distancing measures and the cyclical nature of measles may explain why there has been no increase in cases, O’Connor said.
This could change quickly, however, as it is a highly contagious disease.
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There were around nine million measles infections and 128,000 deaths worldwide last year, officials said.
In February, health officials in England warned that vaccination rates had fell to its lowest level in a decade.
Measles is normally spread by direct contact and through the air by coughing and sneezing.
Unvaccinated young children are most at risk of measles and its complications.
It can cause pneumonia, encephalitis (inflammation of the brain) and damage to the immune system, which makes children more vulnerable to other infections.
Measles causes symptoms such as fever, muscle aches and a rash on the face and upper neck and can sometimes be fatal.
More than 95% of deaths occur in developing countries, mainly in Africa and Asia.
There is no specific treatment for measles, but the two-dose vaccine is about 97% effective in preventing serious illness and death.