A vaccine has been used to free a man who was trapped at home by a Covid infection that lasted for more than seven months.
It is the first time that a vaccine has been used to "treat" Covid rather than "prevent" it.
Ian Lester, 37, has a weakened immune system that was unable to defeat the virus on its own.
He says he became a prisoner in his home in Caerphilly, Wales, as he isolated for months on end.
Ian was born with Wiskott-Aldrich syndrome, which makes it harder for him to fight off infections. Even a common cold can linger.
He shielded during the first wave of Covid, but coronavirus eventually found him in December 2020. He had one of the classic symptoms - a slight loss of sense of taste and smell - which cleared up within a month.
For most of us that would be the end of it, but Ian's Covid journey was only just beginning. His doctors wanted him to keep on testing because his weakened immune system meant there was a risk he could be contagious for longer than normal.
But month after month, test after test came back positive. Ian had to give up work at the opticians where he'd be in close contact with others and stay at home.
Initially he tried to make the most of it, and the Christmas tree stayed up through January, but eventually the isolation took its toll.
"People would feel like it's going to be a long holiday, but after the three-month mark it wasn't," Ian told me.
Slowly every day began to feel the same - a pattern of cooking, TV, reading, playing guitar and just waiting for his wife Katie to get home.
Ian said: "It was like living in groundhog day, you just end up sitting there staring at the wall.
"It slowly became my prison cell, especially when it got to the summertime and the restrictions were lifting for everyone else, you could see family and friends starting to get back to a real life, and I was still getting these positive results."
Ian started to become ill again after the three-month mark. Fatigue, headaches, insomnia, and a tight chest all set in, he was struggling to concentrate and every three to four weeks there would be a build-up of sticky mucous on his lungs.
It was never enough to need hospital treatment, but it was clear his body wasn't easily shrugging off Covid.
"I was worried that I'd keep on getting worse and worse and worse and never get rid of it," he said.
Scientists and doctors were monitoring the battle between the virus and Ian's immune system at Cardiff University and at the Immunodeficiency Centre for Wales in the University Hospital of Wales.
The analysis showed Ian had a long-term infection, it was not just "dead virus" being detected, and his symptoms were not long-Covid.
"Ian really didn't have much of an immune response at all against the Covid virus," Dr Mark Ponsford, a clinician scientist at Cardiff University, told the BBC.
They could not find antibodies that would stick to and neutralise the coronavirus, and there were only limited signs of T-cells, another wing of the immune system, that could attack Covid.
At the time, in the early summer of 2021, there were limited treatments, so the medical team decided to try something radical. Instead of giving a vaccine to prevent an infection, they decided to use the Pfizer vaccine to treat one.
The difference in Ian's body "was like night and day", says Dr Ponsford. The first dose started to build his immunity, but it took a second dose to reach the point where his body could fight off the virus.
By the end of August, Ian was testing negative again.
So why was the vaccine able to clear the infection, when months of having the virus did not build up enough immunity?
Prof Stephen Jolles, clinical lead at the Immunodeficiency Centre, said: "This infection was burbling along, but with his [weakened] immune system it was just not enough to kick off a response sufficient to clear it.
"So the vaccine really made a huge difference, in antibodies and T-cells, and utilised and squeezed every last drop out of what his immune system could do."
On the day Ian was finally clear of Covid he was "ecstatic" and says "I couldn't believe it to tell you the truth". He celebrated with a day on the beach and a portion of fish and chips with Katie.
"Everything's back to normal now," he says.
The researchers think this approach can be used in more people with weakened immune systems who are struggling to fight off the infection. There are anti-viral drugs now that were not available when Ian had Covid, but the team think vaccines could offer a cheaper and more durable option. The findings were published in the Journal of Clinical Immunology.
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