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xperts who advise the World Health Organization on immunizations have given their blessings to a plan aimed at stretching limited global supplies of yellow fever vaccine.

In a statement Friday, the WHO said the advisers had approved a plan that would permit the agency to recommend countries use one-fifth of a dose of vaccine per person. Although the WHO has not yet taken that step, officials are increasingly concerned that recent outbreaks in Africa and elsewhere will strain remaining stocks.

The WHO and the partners with which it is working to quell the multiple yellow fever outbreaks hasn’t yet decided to make the switch. But the evolving situation is being scrutinized constantly and the agency wanted to be ready in case it has to move quickly, said Dr. Joachim Hombach, a scientist in the WHO’s immunization department.

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“If WHO recommends further [mass vaccination] campaigns, and this can be imminent, then I think the issue will be on the table. Absolutely,” he told STAT.

That’s why the WHO asked the Strategic Advisory Group of Experts on Immunization — known as the SAGE — for advice on the plan.

Even though the use of fractional doses has only been studied in adults, the panel said smaller doses could be used in children 2 years of age and older. Below that, a full dose is still recommended, Hombach said, because there is some evidence even a full single dose may not work as well in children under the age of 2.

“It’s not a very strong observation. But to be on the precautionary side, the recommendation is to give children below 2 years the full dose,” he said.

The SAGE recommended that studies be done to see if fractional doses are protective in young children. Hombach said planning is already underway.

Using two different dosing regimens in mass vaccination campaigns is a complication public health officials generally try to avoid. When you need to vaccinate a lot of people quickly, a one-size-fits-all approach works best.

“It’s not ideal, but it is a precautionary measure. And our colleagues who have experience with campaigns tell us that it is feasible,” Hombach said.

The yellow fever virus, like the related dengue and Zika viruses, is spread by mosquitoes. It is deadlier, though, than its cousins, with a significant fatality rate.

People who contract the virus can experience fever, headache, backache, and muscle pains, loss of appetite, and nausea or vomiting. Symptoms typically last three or four days.

But about 15 percent of people then suffer a relapse, entering a so-called toxic phase in which they develop jaundice and more severe illness. Roughly half of that population dies.

A yellow fever outbreak that began in Angola last December has seeded a smaller but growing epidemic in the neighboring Democratic Republic of Congo.

Concurrent outbreaks in Ethiopia, Uganda, Peru, and a number of other countries are adding to the strain on supplies.

“Right now we have enough vaccines in the global stockpile to cope with the ongoing outbreaks if there are no further extensions,” said SAGE chairman Dr. Jon Abramson, who heads the department of pediatrics at Wake Forest University School of Medicine.

“However, given the wide spread of the disease in Angola and the potential for it to get out of control in the city of Kinshasa … WHO and partners are seriously considering the use of this dose-sparing strategy to prevent transmission through large-scale vaccination campaigns.”

One full dose of yellow fever vaccine is thought to provide protection for life. Four studies, all done in adults, have shown that low doses of yellow fever vaccine can provide sufficient protection, at least over the short term. The WHO’s proposal is to revaccinate people who get a fractional dose, when there is sufficient supply.

Vaccination approaches for children are often different than they are for adults, because their immune systems are still developing. Many childhood vaccines must be given in multiple doses — a first or priming dose followed by one or more “boosters.”

The WHO hasn’t declared that it will start recommending the smaller dose to countries. But Hombach acknowledged that when it does, there will need to be clear communications about the science supporting the move.

If people don’t trust they’ll be protected by a one-fifth dose, they may get back in line — which would undermine the dose-sparing efforts.

Said Homback: “It requires explanation and communication that this is in everybody’s interest in the current situation, in the emergency, to use this approach.”

Correction: An earlier version of this story misstated the age range that could be treated with the fractional dose of the vaccine.

Here's what you should know about the yellow fever outbreak happening in Angola right now. Hyacinth Empinado/STAT

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