More than 800,000 children are to be targeted for vaccination in the Democratic Republic of Congo, after a measles outbreak killed more than 3,500 people this year.
The World Health Organization (WHO) and the Congolese government aim to carry out the emergency programme across the country in slightly more than a week.
The WHO says the epidemic is the world's largest and fastest moving.
It has killed more Congolese people this year than Ebola.
Despite previous rounds of immunisations, the disease has spread to every part of the country.
Lack of routine access to vaccinations and healthcare has contributed to the problem.
"The DRC is experiencing a dire situation because too many children were missed by routine immunisation," said Dr Deo Nshimirimana, WHO representative to the DR Congo.
"[The outbreak] is deadly because the case management is not there," he told the BBC's Newsday programme.
"We don't have the resources really to prevent the disease and also to try to prevent the deaths so... measles is very deadly in this country. We are trying our best really to prevent the disease, but also to try to have resources to get supplies so that we can manage the cases."
Every one of the country's 26 provinces has reported cases of measles and is battling to control this outbreak, which the ministry of health declared on 10 June.
The campaign aims to vaccinate around 825,000 children in 24 regions, over a period of nine days, the agency said.
"As of 17 September, a total of 183,837 suspected measles cases (5,989 confirmed) had been reported in 192 of the 519 health zones nationwide, including 3,667 deaths - which exceed the number of deaths due to Ebola. Nearly all the deaths have been children," the WHO said in a statement.
In the country's east, Ebola has claimed more than 2,100 lives since erupting in August last year, and is the second largest outbreak of the disease on record.
The largest was the epidemic that ravaged parts of West Africa from 2014 to 2016, killing more than 11,000 people.
DR Congo has struggled to get the outbreak under control, given the insecurity in the east of the country and people's suspicion of treatments.
Measles is a highly infectious viral illness that can sometimes lead to serious health complications, including infections of the lungs and brain.
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