Most COVID-19 cases are immune to the disease at least six months after infection, a new study has discovered - leaving scientists "cautiously optimistic" that reinfection isn't a significant threat.
The research, carried out by the UK Coronavirus Immunology Consortium (UK-CIC), saw the cellular (T-cell) immunity of 100 coronavirus-infected health care workers tested. Each of them were either asymptomatic or had mild or moderate symptoms.
Results showed T-cell levels - the cells that prevent reinfection - remain high in the majority of cases six months after they were infected with COVID-19.
In cases who had mild or moderate symptoms, T-cell immunity was significantly higher than those who were asymptomatic.
The UK-CIC says this may mean heightened cellular immunity may provide increased protection against re-infection, or that asymptomatic individuals are simply able to fight off the virus without the need to generate a large immune response.
Professor Paul Moss, the UK-CIC's lead on the study, said the research had given them an understanding of COVID-19 immunity that would prove "extremely important".
"Our knowledge of SARS-CoV-2 infection is increasing all the time," he said.
"While our findings cause us to be cautiously optimistic about the strength and length of immunity generated after SARS-CoV-2 infection, this is just one piece of the puzzle.
"There is still a lot for us to learn before we have a full understanding of how immunity to COVID-19 works."
The study, which is yet to be peer-reviewed, provides a less gloomy outlook on the risk of reinfection than previous research.
A King's College London study released in July found people could lose their immunity to the disease within months, with antibodies declining three weeks after recovery.
But the new UK-CIC study shows that while antibody levels wane quickly, T-cells outlast them and remain present several months after a COVID-19 case recovers.
Study author Dr Shamez Ladhani says the findings are a "potentially very significant piece of the COVID-19 puzzle", and could inform future vaccine strategies.
"A robust cellular memory against the virus persists for at least six months," Professor Fiona Watt, Executive Chair of the Medical Research Council said.
"This is promising news - if natural infection with the virus can elicit a robust T-cell response, then this may mean that a vaccine could do the same."
