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.