How was the Yellow Fever Vaccine Created?

Mosquito-borne diseases have a way of creating mass panic throughout the world. An outbreak of Zika, West Nile or malaria will get headlines as people do anything to avoid the insects.

Those epidemics make it so much more satisfying when we find a vaccine against of these diseases.

Yellow fever had that breakthrough in 1937.

The race for a vaccine started 50 years earlier, when Walter Reed confirmed that yellow fever is a mosquito-borne virus. In 1901, the U.S. Army physician proved that the insects, and not a bacteria, were responsible for yellow fever.

The insect identified wasn’t just any mosquito, but Aedes aegypti mosquitoes. This is the species that carries dengue, chikungunya and most other deadly mosquito-borne viruses.

The initial push to stop yellow fever wasn’t focused on the virus itself. Instead, researchers put their time and money into eradicating Aedes aegypti.

At the forefront of these studies was the Rockefeller Foundation.

The foundation created an International Health Division (IHD) in 1913.

The IHD spent its first ten years working with the government and acting as a public health office. Like other researchers at the time, the division only attacked yellow fever by trying to eradicate the mosquitoes.

The IHD had success in some areas, but the Aedes aegypti survived in areas with heavy concentrations of the virus. It was only after appointing new director Frederick Russell in 1923 that the division made research a priority.

Finally, the IHD’s goal was to discover the the secrets to vaccination and viral transmission.

In the wake of World War I, it became clear that yellow fever was active in not just South America, but also Africa. The IHD sent scientists to Africa to test whether yellow fever was the same in both areas.

Tragedy struck the scientists, as three contracted the virus and died. The only silver lining came in the new African yellow fever samples they procured.

The IHD now had proof that immunity from South American yellow fever would protect from the African virus.

The division’s next big break came after hiring South African scientist Max Theiler in 1930. Theiler earned the IHD’s attention after successfully using mice for yellow fever studies. Before, scientists could only find the virus in monkeys. Mice proved to be a cheaper option and were much easier to control.

The combination of Theiler and mice provided a spark to vaccine development.

The IHD now had an easy method for growing yellow fever. In studying how yellow fever lived in the rodents, Theiler could grow his own weakened virus. The scientist then studied the weaker version to reduce the symptoms.

This is the normal procedure for vaccines. Scientists aren’t looking for a magic formula that repels a virus. Rather, they look for a version of the virus that doesn’t have the deadly effects. When injected with a harmless version, the body creates antibodies to fight the disease. The body is then ready to fight off a dangerous virus, if that time ever comes.

By 1937, Theiler discovered that perfect weakened strain. He created a watered-down yellow fever that wouldn’t affect the brain.

Trial testing for Theiler’s vaccine started the next year in Brazil.

The vaccine was given to roughly 50,000 Brazilian locals. Some experienced effects to the liver and brain that proved to be treatable. This left a bit more work for the vaccine. But, researchers at the local Oswaldo Cruz Institute tweaked the vaccine to make it stronger.

Not only was the vaccine then ready for use, but also for mass production.

Years later, and after widespread immunizations in Brazil, yellow fever cases in the area dropped.

Theiler was awarded the Nobel prize in 1951 for his work on the vaccine.

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