When stockpiles of yellow fever vaccine run low, partial doses are effective, according to a new study.

The report on the vaccine, which currently is in short supply, comes as officials in Brazil attempt to contain an outbreak with what they describe as the largest-ever mass vaccination campaign using partial doses.

Yellow fever is a mosquito-borne viral disease found in tropical Africa and South and Central America. Severe cases can cause jaundice and death, but most cases involve fever, muscle pain and vomiting.

Congo outbreak, experiment

During a major outbreak in the Democratic Republic of the Congo in 2016, the government aimed to prevent the disease from spreading in the capital, Kinshasa. Health officials launched a mass vaccination campaign targeting 7.6 million people.

But the outbreak had depleted vaccine stockpiles. Hoping to stretch the available supply, the World Health Organization reviewed the small number of available studies on using reduced doses and recommended using one-fifth of a dose per person.

It seemed to work.

Researchers took blood samples from more than 700 people before and after they received the partial dose. In the new study, published in the New England Journal of Medicine, nearly all of those vaccinated with the lower dose developed enough antibodies to the virus to prevent infection.

“That was the encouraging thing, that this can be done as a potential way — when there’s supply limitations on the vaccine — to help potentially control an outbreak,” said study co-author Erin Staples at the U.S. Centers for Disease Control and Prevention.

Hundreds infected

More than 350 people have become infected with yellow fever in Brazil since late last year, and health officials have launched a campaign targeting nearly 24 million people with a one-fifth dose of the vaccine.

Staples says the new study is good news for controlling outbreaks like Brazil’s in the short term. But, she notes, “We still need some information about how long immunity will last.”

A full dose of yellow fever vaccine provides lifelong immunity. Researchers will continue to study how long people who received partial doses are protected.