Yellow Fever Vaccine: No Booster Needed After Initial Dose, New Research Confirms
Recent research published in The Lancet Microbe suggests that one dose of the yellow fever vaccine provides lifelong immunity, eliminating the need for a booster shot. This study, which reviewed cases over several decades, reaffirms the World Health Organization's position on yellow fever vaccination and provides insights into the rare occurrences of breakthrough infections.

Background
Yellow fever is a serious viral disease spread by mosquitoes (specifically Aedes and Haemagogus mosquitoes) common in parts of South America and sub-Saharan Africa. It can cause a high fever and, in severe cases, can lead to death. Each year, it leads to approximately 30,000 deaths, with up to 40% of those who show symptoms facing a fatal outcome.
Since no cure exists for yellow fever, prevention is key. The primary way to prevent yellow fever is through vaccination. The yellow fever vaccine, developed in the 1930s, is very effective. In 2015, the World Health Organization (WHO) announced that a single dose of this vaccine provides lifelong immunity, meaning no booster dose is needed. However, this decision sparked debates. Some research focused mainly on antibody levels (the body's first line of defense against infection), which suggested that a single dose might not be enough in some cases. But recent findings indicate that another part of the immune system, T cells, also plays a role in providing long-term protection.
About the Study
This study, published in The Lancet Microbe, aimed to summarize cases of yellow fever among vaccinated individuals to understand how often infections occur after the initial vaccine. The researchers searched multiple databases (Global Index Medicus, EMBASE, and Medline) for reports of yellow fever in people who had been vaccinated, covering research published between 1936 and 2023. They included various types of studies, such as clinical trials, reports on disease outbreaks, and individual case studies.
For a case to be considered "effectively vaccinated," the individual had to receive the vaccine at least 30 days before showing symptoms. Only moderate- to high-quality studies, as determined by the Newcastle-Ottawa Scale, were included. The study looked at the rate of infection in people who were vaccinated and the severity of those infections.
Key Findings
Out of 2,600 studies examined, 37 met the criteria for inclusion. Of these, 16 studies reported cases from Africa, while 24 reported cases from South America. Together, these studies reported a total of 40,850 suspected yellow fever cases, with 6,951 confirmed through lab tests. Only 537 cases were in people who had been effectively vaccinated, and most of these cases were identified in Brazil.
A detailed analysis showed that nine confirmed breakthrough infections occurred among those vaccinated at least 30 days prior. Notably, three of these infections happened within three years of vaccination, while none occurred in people vaccinated more than 10 years earlier. The study found that probable cases were more common in younger individuals who had been vaccinated as children.
In the meta-analysis, the overall proportion of confirmed and probable breakthrough infections was very low, about 3%. The study concluded that yellow fever infections in vaccinated individuals were rare, especially after 10 years post-vaccination.
Conclusion
This research supports the WHO's recommendation that a single yellow fever vaccine dose provides lifelong immunity, with breakthrough infections being extremely rare. However, it highlights that younger people vaccinated as children may have a slightly higher chance of infection, though the risk remains low. The authors suggest that further research is needed to examine the immune response to the yellow fever vaccine in younger populations and investigate the effectiveness of the vaccine over time.
Reference
Schnyder, J. L., Bache, B. E., Welkers, M. R. A., et al. (2024). Yellow fever breakthrough infections after yellow fever vaccination: A systematic review and meta-analysis. The Lancet Microbe. https://doi.org/10.1016/j.lanmic.2024.06.004
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