Duodenal hookworms (Ancylostoma duodenale) cause one of the most common intestinal parasitic infections worldwide.
Katerina Conn/Shutterstock
COVID-19 vaccines may be less effective in people with intestinal parasitic infections, which represents about a quarter of the world’s population. This was suggested by experiments using parasitic worm-infected mice, whose immunity after COVID-19 vaccination was significantly weaker than that of mice without the parasitic worm.
Previous studies have shown that people with intestinal parasitic infections have a weakened immune response to vaccines for diseases such as tuberculosis and measles because the parasites suppress the processes that vaccines trigger to confer immunity, such as activating pathogen-killing cells. Intestinal parasitic infections are most common in tropical and subtropical regions, where they often occur because of limited access to clean water and sanitation.
Scientists have not tested whether these pathogens make COVID-19 vaccines less effective. Michael Diamond of Washington University in St. Louis, Missouri, and his colleagues vaccinated 16 mice with a COVID-19 mRNA vaccine, half of which had been infected 12 days earlier with an intestinal parasite that lives only in rodents. The researchers gave each mouse a booster shot three weeks after the first shot.
About two weeks after the booster shot, the researchers analyzed the animals’ spleens to measure concentrations of CD8+ T cells, specialized white blood cells that are important for eliminating other cells infected with the SARS-CoV-2 virus. They found that the spleens of mice infected with the intestinal parasite had about half the number of cells as mice without the parasite, suggesting a weakened immune response to the vaccine.
The researchers repeated the vaccination process in another group of 20 mice, half of which were infected with the intestinal parasite, exposing them to the highly infectious Omicron variant of the SARS-CoV-2 virus. After five days, the lungs of vaccinated rodents infected with the intestinal parasite had, on average, about 20% more virus than uninfected ones.
Together, these findings suggest that intestinal parasites may reduce the effectiveness of COVID-19 vaccines in humans. However, it is known that different types of intestinal parasites affect immunity differently, says Keke Fairfax of the University of Utah. Therefore, it is unclear whether parasites that infect humans will have a similar effect on COVID-19 vaccination as parasites in mice. Moreover, humans tend to carry multiple types of intestinal parasites at the same time, further complicating the situation, says Fairfax.
Still, understanding how to alter the immune response to vaccination is important given the prevalence of parasitic infections, and these findings suggest that researchers may need to further evaluate the vaccine’s effectiveness in parts of the world where a high proportion of the population is infected with intestinal parasites, Fairfax says.
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