T

he massive yellow fever vaccination campaign that got underway this month in Angola and the Democratic Republic of Congo (DRC) will help save lives in those countries. It is also serving as a dress rehearsal for how all nations can best cope with vaccine shortages, something we could see a lot more of in the years ahead.

This latest outbreak of yellow fever began in Angola six months ago. The disease is caused by a virus that’s spread primarily by Aedes aegypti, the type of mosquito that also spreads Zika, dengue, and other diseases. Although many people infected with the virus never develop any symptoms, some become severely ill and about half of them die. Worldwide, yellow fever kills between 30,000 and 60,000 people a year. There is no cure for yellow fever. That’s why vaccination, which is highly effective, is so important.

More than 18 million people in Angola and the DRC have been vaccinated against yellow fever since the outbreak started. That vaccination campaign temporarily depleted the world’s global emergency stockpile of 6 million doses. It has also diverted doses away from campaigns to prevent yellow fever elsewhere in the world. That means there isn’t enough yellow fever vaccine to handle the current epidemic and to prevent outbreaks elsewhere.

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We don’t have time to make enough vaccine to immunize everyone in Kinshasa — one of Africa’s megacities with more than 10 million people — before the rainy season starts in September, bringing out both Aedes aegypti and potentially yellow fever in full force.

To cope with this yellow fever emergency, the World Health Organization has taken the unusual step of recommending the use of fractional doses — each person will get one-fifth of the normal dose. Although this makes it possible for more people to get vaccinated, it also offers challenges such as finding enough smaller syringes and training health care workers in different methods of preparing and performing the injection.

This isn’t the first time that fractional dosing has been used. Much of the emergency smallpox vaccine stockpile in the United States is made up of fractional doses. With the rising risk of urban epidemics, and until emergency stockpiles reflect the size of that risk, fractional dosing may be the only way of ensuring that everyone is protected, not just those living in wealthy countries or countries with their own vaccine manufacturing facilities.

And yet there are still important unanswered questions about fractional dosing. In the case of yellow fever, we don’t know if fractional doses provide lifelong protection against the virus, as full doses do. Research suggests that fractional doses should provide protection for at least 12 months which, in the short term, may help prevent the spread of the yellow fever in Angola and the DRC and buy some time while vaccine stockpiles are replenished. The intensive vaccination campaign will provide crucial information on the duration of protection.

Yellow fever comes and goes in humans because the virus also lurks in monkey populations. New outbreaks tend to begin along the edge of natural forest habitats when unimmunized individuals come in contact with mosquitoes that have bitten infected monkeys. In such situations, rapid outbreak response campaigns can be conducted in affected communities that create a wall of immunity around those infected and bring the outbreak to a quick end.

But climate change is altering where and when disease-carrying vectors like Aedes aegypti live. Human settlements are encroaching on forested areas. Plus there’s a global trend towards urbanization, with more people living closer together. The combination of these three changes creates a very real potential for larger outbreaks than we’ve ever seen before. That’s why it is so important to vaccinate everyone in Kinshasa and elsewhere in Angola and the DRC.

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In addition to what we will learn about fractional dosing, another significant lesson we should take home from this vaccination campaign comes from the way these precious lifesaving doses are allocated. In 2009, during the H1N1 flu pandemic, we saw wealthy countries closing ranks as they scrambled to get hold of enough vaccine doses to protect their own citizens. While that was hardly surprising, it meant that people in many poorer countries went unprotected.

Fortunately, H1N1 proved to be far less harmful than originally feared. But in any outbreak of infectious disease, in the worst-case scenario, large amounts of global vaccine stocks are used to protect people who aren’t at risk, while others who are at risk go without. This can seriously compromise our ability stop infectious disease epidemics.

In the current yellow fever outbreak, we are witnessing a different practice. Brazil, a country that is itself at risk for yellow fever, is providing 2.5 million doses of vaccine for the Angola and DRC campaign, rather than keeping these doses for its own people. This is a positive step, and, as developing countries increase their production of vaccines, one we hope to see more of.

The changing disease landscape, driven in large part by shifting demographics caused by increasing urbanization and climate change, makes it all the more important that our yellow fever strategies take account of this new reality. We must also ensure that doses of the vaccine to fight yellow fever are not just available but that they are going to those who are most at risk.

Seth Berkley, MD, is the CEO of Gavi, the Vaccine Alliance.

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