The worst yellow fever outbreak in decades has killed 250 people in Angola and is straining global vaccine supplies, posing a dilemma for health officials who fear it could spread further in Africa and possibly into Asia.
Some experts have called for a radical switch in strategy to use just one-tenth of the usual vaccine dose to conserve scarce stocks but the World Health Organization (WHO) says it can't be sure this would work.
Yellow fever is transmitted by the same mosquitoes that spread the Zika and dengue viruses, although it is a much more serious disease with death rates as high as 75 percent in severe cases requiring admission to hospital.
The condition, which takes its name from the jaundiced colour of some patients, has spread to the Democratic Republic of Congo and there is concern it could gain a foothold for the first time in Asia.
The WHO says cases of yellow fever imported into China, which has close commercial ties with oil-rich Angola, show that "this outbreak constitutes a potential threat for the entire world".
And it is warning that further spread elsewhere in Africa and Asia would increase the squeeze on vaccine supplies and could interrupt routine immunisation.
Adding to the problem is the complicated process of making yellow fever vaccine, which means there is no quick way to boost supplies. Manufacturers, including the Institut Pasteur, government factories in Brazil and Russia, and French drugmaker Sanofi, use a time-consuming method involving sterile chicken eggs.
A short-term solution could be to slash the dose, since research suggests just one-tenth can produce the same immune response as a full dose, according to Jack Woodall, a retired virologist who formerly worked for the WHO and the U.S. Centers of Disease Control and Prevention.
Woodall and colleagues laid out the case for the emergency use of a one-tenth dose in an article in The Lancet medical journal on April 16.
The WHO, however, is not convinced. A spokesman said more research was needed to measure immune response in children and duration of protection. The availability of suitable syringes for delivering a smaller dose was also a limiting factor, he said.