A large study conducted in the United States to determine whether vaccination protects against long-term covid found only a minor protective effect: Vaccination appeared to reduce the risk of lung and blood clot disorders, but did not appear to protect against the majority of other symptoms.

The new study, published in Nature Medicine on Wednesday, is part of a series of studies by the Department of Veterans Affairs on the impact of the coronavirus, and it is based on 33,940 people who had breakthrough infections after vaccination.

The findings support a large body of research indicating that vaccination significantly reduces the risk of death or serious illness. However, there was more ambiguity in the case of long covid. People in the study who were vaccinated had a 15% lower risk of developing long covid six months after their initial diagnosis of covid. The most significant benefit appeared to be in lowering blood clotting and lung complications. However, there was no difference in the longer-term risks of neurological issues, gastrointestinal symptoms, kidney failure, and other conditions between the vaccinated and unvaccinated.

Post-covid syndrome is defined by the World Health Organization as symptoms that last at least two months and cannot be explained by other diagnoses. It cited evidence indicating that up to 20% of the half-billion people worldwide estimated to be infected with coronavirus may suffer mid- and long-term consequences.

The Centers for Disease Control and Prevention released new estimates of the syndrome’s toll in the United States this week, indicating that it affects one in every five adults under the age of 65, and one in every four adults 65 and older. The CDC discovered that people in both age groups were twice as likely as uninfected people to develop respiratory symptoms and lung problems, including pulmonary embolism. The elderly were more likely to develop kidney failure, Type 2 diabetes, neurological conditions, and mental health problems.

The Veterans Affairs study, which is thought to be the largest peer-reviewed analysis of long-term covid based on medical records in the United States, looked at patients who received two doses of the Moderna or Pfizer—BioNTech vaccines, or one dose of the Johnson & Johnson vaccine. It did not assess the effectiveness of booster shots. While the study population represented a diverse range of ages and racial and ethnic backgrounds, it did skew older, Whiter, and more male than the general population of the United States.

There was also no way to tell how different variants might affect the risk of long covid in the VA study. These breakthrough infections, for example, occurred during a period when alpha, delta, and prior variants were prevalent in the United States. It does not include the time when the omicron variant and its subvariants first appeared in late 2021.

The findings add to the debate over similar studies from the United Kingdom, Israel, and other countries that have produced conflicting results regarding whether vaccines protect against long covid.

Based on self-reported data from an app, one British study published in the medical journal Lancet, for example, found a 50% reduction in risk among those who were vaccinated. A paper by University of Oxford researchers based on electronic records from the United States, on the other hand, discovered that vaccination did not appear to reduce the risk of long covid for most symptoms.

For doctors, the issue of vaccines and long covid has been critical. Some patients claim that a vaccine cured them, while others avoid shots for fear of triggering symptoms.

According to Igor Koralnik, chief of neuro-infectious diseases at Northwestern Medicine, recent research indicates that neither is true. Koralnik and colleagues discovered in a paper published on Tuesday in the Annals of Clinical and Translational Neurology that 77 percent of the 52 long-covid patients they are following had been vaccinated against the coronavirus, but the shots had no effect on cognitive function or fatigue.

Christina Martin, an advance practice nurse at Dartmouth Hitchcock Medical Center’s Post-Acute COVID Syndrome Clinic, said her staff has noticed a “worrying trend” of vaccinated people getting breakthrough infections and developing long covid since November.