Many COVID long-haulers report improved symptoms after getting the vaccine. A new study backs them up.

A significant number of the 30 million Americans infected with COVID-19 have lingering symptoms months after their initial illness. And many of these people with "long COVID," or COVID long-haulers, are reporting that their symptoms improved or cleared up after getting vaccinated. "That's not how vaccines work, normally," Tom Avril writes in The Philadelphia Inquirer. "The idea is to prevent disease, not treat it."

There were concerns that getting vaccinated would actually exacerbate long-haulers' symptoms, including fatigue, brain fog, shortness of breath, headaches, insomnia, and inability to smell. But "it's clear that vaccines have helped some people with long COVID with their symptoms," Yale immunologist Akiko Iwasaki wrote in a recent Medium post, laying out three possible explanations for how vaccines attack long COVID. The evidence for these improvements has been largely anecdotal or based on informal polls among long-hauler support groups.

But a new study by University of Bristol researchers, not yet published or peer-reviewed, found a statistically significant improvement among long COVID patients who got either the Pfizer or Oxford-AstraZeneca vaccine.

COVID long-haulers who got the vaccine experienced "no significant worsening in quality-of-life or mental wellbeing," the researchers wrote, and "when compared to matched unvaccinated participants from the same cohort, those who had receive a vaccine had a small overall improvement in long COVID symptoms."

It's possible the improvements for vaccinated long COVID patients could be due to the placebo effect, or the immune response sparked by the vaccine could wipe out lingering reservoirs of coronavirus, clean the body of "ghost" viral fragments, or distract the body from a harmful autoimmune response — all theories floated by Iwasaki and other immunologists and virologists.

Either way, immunologist Dr. Nancy Klimas tells ABC News, "my advice to long-haulers is to get the vaccine, not because of this, but because they should anyway, and if they get this as a bonus, I want to know."