Intestinal parasites may reduce covid-19 vaccine effectiveness

Covid-19 vaccines may be less effective in people who have intestinal parasite infections, or roughly a quarter of the world’s population. This is suggested by experiments in mice infected with parasitic worms, who developed significantly weaker immunity after covid-19 vaccines than mice without parasitic infections.

Previous research has shown that people with intestinal parasites have impaired immune responses to some vaccines, such as those for tuberculosis or measles. This is because the parasites suppress processes that vaccines trigger to confer immunity, such as the activation of pathogen-killing cells. Intestinal parasite infections are most prevalent in tropical and subtropical regions and often occur due to limited access to clean water and sanitation.

Scientists haven’t tested whether these pathogens reduce covid-19 vaccine effectiveness. Michael Diamond at Washington University in St. Louis, Missouri, and his colleagues inoculated 16 mice with a covid-19 mRNA vaccine, half of which had been infected 12 days before with an intestinal parasite that only occurs in rodents. They gave each mouse a booster shot three weeks after the first shot.

About two weeks after the booster dose, the researchers analysed the animals’ spleens to measure the concentrations of CD8+ T cells – a special type of white blood cell important for clearing other cells infected by the SARS-CoV-2 virus. On average, the spleens of mice with intestinal parasites had roughly half the number of these cells as those without parasites, suggesting an impaired immune response to the vaccine.

The researchers repeated this vaccine process in a separate group of 20 mice – half of which had been infected with intestinal parasites – and exposed them to a highly infectious omicron subvariant of the SARS-CoV-2 virus. Five days later, vaccinated rodents with intestinal parasites had about 20 per cent more of the virus in their lungs, on average, than those without them.

Together, these findings indicate that intestinal parasites may reduce the efficacy of covid-19 vaccines in people. However, different types of intestinal parasites are known to affect immunity in different ways, says Keke Fairfax at the University of Utah. So, it is unclear if those that infect people would have a similar effect on covid-19 vaccination as those in mice. Plus, people tend to have multiple types of intestinal parasites at once, further complicating the picture, she says.

Still, it is important to understand how parasitic infections alter immune responses to vaccination given their prevalence, and these findings suggest researchers may want to further evaluate vaccine efficacy in areas of the world where a high proportion of the population has intestinal parasites, says Fairfax.