World Health Organisation advisers have recommended using a fifth of the standard dose of yellow fever vaccine in the event of a global shortage to combat the worst outbreak of the deadly disease in decades.
Fears of a widening outbreak of the mosquito-borne disease were fuelled this week by a spike in cases in the Democratic Republic of Congo (DRC), which now says it has seen more than 1000 suspected cases since March.
"Experts agreed to propose if necessary, if there is a shortage of vaccine, to divide the vaccine by five," WHO spokesman Tarik Jasarevic said on Friday, reporting on a meeting this week.
"One fifth of a dose according to their evidence would be sufficient to provide immunity for at least 12 months."
The normal full dose of the vaccine confers life-long protection and the WHO emphasised that the low dose endorsed by its independent experts was designed specifically for emergency mass vaccination, not for routine immunisation.
More research is also needed to see if low doses will work for young children, who may have a weaker immune response, and practical challenges remain over obtaining the right syringes.
The current yellow fever epidemic started in Angola but a major outbreak in the DRC's capital city of Kinshasa, which has a population of more than 12 million, is a big worry for healthcare officials.
The global stockpile of yellow fever vaccines has already been depleted twice this year to immunise people in Angola, Uganda and the DRC. It currently stands at 6 million doses but this may not be enough if there are simultaneous outbreaks in multiple densely populated areas.
Yellow fever is transmitted by the same mosquitoes that spread the Zika and dengue viruses, although it is a much more serious disease. The "yellow" in the name refers to the jaundice that affects some patients.
Although approximately 6 million vaccine doses are kept in reserve for emergencies, there is no quick way to boost output when there is a surge in demand since production, using chicken eggs, takes around 12 months.
Manufacturers include the Institut Pasteur, government factories in Brazil and Russia, and French drugmaker Sanofi.
The current outbreak of yellow fever was first detected in Angola in late December, 2015.
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