Yellow fever has been a feared disease for centuries, with its origins in Africa and its spread to the Americas through the slave trade. The virus, transmitted by mosquitoes, caused devastating epidemics characterized by fever, chills, and liver damage, which often resulted in a yellowing of the skin known as jaundice. For a long time, the cause of yellow fever was unknown, and it was not until the late 19th and early 20th centuries that significant progress was made in understanding the disease. The quest for a vaccine was driven by the urgent need to control the deadly outbreaks that plagued populations worldwide.
Early Research and Understanding
A key moment in the fight against yellow fever occurred in the early 1900s with the work of the U.S. Army Yellow Fever Commission. Led by Major Walter Reed, the commission conducted experiments in Cuba that proved the disease was not spread by direct contact with infected individuals or their belongings, a common belief at the time. Their research demonstrated that the Aedes aegypti mosquito was the vector responsible for transmitting the virus.
This discovery provided a clear target for controlling the disease’s spread. Efforts to control mosquito populations, such as eliminating standing water where they breed, became the primary method of prevention for several decades. While these measures were effective in reducing the incidence of yellow fever in some areas, they were labor-intensive and not always completely successful, so a more permanent solution was still needed.
The Breakthrough Development
The successful yellow fever vaccine was the result of years of research by scientists at the Rockefeller Foundation. In the 1930s, a team led by virologist Max Theiler made the breakthrough. Theiler and his colleagues developed the 17D vaccine strain, which was announced in 1937. The process involved attenuating, or weakening, the yellow fever virus by passing it through a series of chicken embryos.
By cultivating the virus in a non-human host, the researchers were able to reduce its virulence in humans while preserving its ability to elicit a strong immune response. The resulting 17D vaccine was both safe and effective, and it quickly became the standard for yellow fever prevention. Field trials were conducted in Brazil, and by 1939, over a million people had been vaccinated without serious complications.
The development of the 17D vaccine was a landmark achievement in public health, providing a tool for controlling outbreaks. The vaccine was produced on a large scale and was widely used during World War II to protect soldiers serving in areas where yellow fever was endemic.
Impact and Modern Use of the Vaccine
The 17D vaccine had an immediate impact on global health. Mass vaccination campaigns were launched in the 1940s, significantly reducing the incidence of yellow fever in many parts of the world. In recognition of his work, Max Theiler was awarded the Nobel Prize in Physiology or Medicine in 1951.
The 17D vaccine remains the only yellow fever vaccine in use today, a testament to its effectiveness. It is a live, attenuated vaccine that provides lifelong immunity in most individuals after a single dose. The World Health Organization (WHO) recommends the vaccine for people living in or traveling to areas where yellow fever is endemic, primarily in parts of Africa and South America. Proof of vaccination is often required for entry into certain countries to prevent the international spread of the disease.