Vaccines intended to protect people against infection with COVID-19 appear to cure a mysterious condition called 'Long COVID', and no one's quite sure why.
An unknown number of people infected with the SARS-CoV-2 virus - possibly up to 30 percent, according to some research - suffer symptoms for longer than three months, well after they've recovered from the initial illness.
"The chest pain, the heart palpitations, the dizziness, the tingling, your body's going numb in different parts and no one can tell you anything, no one can help you," long-hauler Freya Sawbridge, 26, told Newshub in February.
Others have reported suffering brain fog, shortness of breath and muscle pains for months afterwards - even some who were completely healthy prior to catching the virus, and only had an initially mild illness.
There have been numerous reports of people catching the virus more than once, so even people who've had it before have been encouraged to get immunised, the hope being the vaccines provoke a stronger immune response than the virus itself.
But no one expected the vaccines to kill off Long COVID symptoms.
"I really felt back to myself," 34-year-old Brooklyn long-hauler Arianna Eisenberg told the Washington Post, after getting her second shot of the Moderna vaccine. "I didn't expect the vaccine to do this."
"I woke up and it was like 'Oh what a beautiful morning," Judy Dodd told the New York Times, after contracting COVID-19 mid last-year.
She said she felt miserable for three days after her second dose of the Pfizer-BioNTech vaccine, but on the fourth day everything changed.
"My head is clearer in the last week than it’s been the whole entire year," Los Angeles long-hauler Rebecca Neff told the Post after getting just a single shot of the Pfizer-BioNTech vaccine.
"It was like click, everything is fine," Alexandria nurse Bridget Hayward told the Times after getting her first. "It felt like a darkness lifted."
Doctors told the Times they've seen several similar cases. And a new British study suggests it could be more than some kind of placebo effect.
Forty-four long-haulers were given either the Pfizer-BioNTech or the AstraZeneca vaccine in an experiment to see if it was safe for them. Aside from the usual temporary side-effects, a month later very few reported feeling any worse - four times as many reported an improvement in their symptoms. Those who didn't receive a vaccine were split 50-50 on whether they felt better a month later.
Some informal surveys have had similar results, with about a third of long-haulers reporting improvements after getting their jabs.
Experts don't know exactly why the vaccines appear to help. Some feared Long COVID could be a long-running "overreaction" by the immune system, so beefing it up with a vaccine "could make them worse", infectious disease expert Michael Saag of the University of Alabama told the Post.
"I’m intrigued and puzzled by the reports and curious to see whether this pans out to be real and, if so, why is it happening."
One suggestion is Long COVID could be caused by a hidden reservoir of viral particles, much like how varicella (chicken pox) and HIV can hang around, and the vaccine is prompting an immune response strong enough to wipe it out.
The fact not all long-haulers improve after getting vaccines is a hint Long COVID might actually be two or more different conditions.
"It might be that there's a subset of people who have a certain type of Long COVID, who respond well to vaccines, but there might be other people who have a different subtype that we haven’t quite defined yet," virologist Adam Lauring of the University of Michigan told the Times.
None of the vaccine trials to date have triggered Long COVID.
The British study was published online, and is yet to be peer-reviewed.