Cellular immunity to Covid-19 could last for up to six months after infection, a study has found.
Researchers evaluated 100 health care workers who suffered either asymptomatic or mild infections in March and April.
Tests six months later showed that while the antibody levels had dropped for some, their T-cell response, a type of white blood cell that attacks cells infected with the virus, remained robust.
The researchers said the results suggest T-cells could play a more important role than antibodies in combating the infection.
"These results provide reassurance that, although the titre of antibody to Sars-CoV-2 can fall below detectable levels within a few months of infection, a degree of immunity to the virus may be maintained," said Charles Bangham, chair of immunology at Imperial College London.
Levels of T-cells were 50 per cent higher in those who experienced symptoms at the time of their infection.
The reasons for that are not yet clear, but the researchers said it could either mean people with more severe disease are left with better protection, or asymptomatic Covid-19 patients are able to fight the virus without generating a large immune response.
But experts caution it is still not clear how immunity to Covid-19 works.
"This bodes well for the long term, in terms of both vaccine development and the possibility of long-term protection against re-infection," said Eleanor Riley, an immunology and infectious disease professor at Edinburgh University.
She added, however, that "we don't yet know whether the people in this study are protected from re-infection."
The study, led by the UK coronavirus immunology consortium, has not yet been reviewed.
The immune system is complex, and involves multiple parts.
If the innate immune system does not halt an infection, B-cells and T-cells mount a defense against it. Some T-cells act as slayers against infected cells, while others stimulate B-cells to generate antibodies.
A recent large-scale study, also conducted by Imperial College London, added to mounting evidence suggesting antibodies to Covid-19 wane quickly, dropping by a quarter in three months.
At the time, researchers said the results could point to a decline in the immunity of the population, leading to a heightened risk of reinfection.
But the drop, which many experts have said is expected, would be much less of a concern if T-cells ultimately prove more important in combating a Covid-19 infection.
There have been a handful of cases of reinfection so far.
Of the four cases studied by researchers, symptoms were worse in two patients, suggesting no clear pattern.
But experts have said if SARS-Cov-2, which causes Covid-19, follows the same pattern as other coronaviruses, reinfections will become common.
Research has shown it is possible to catch a cold caused by the same strain months later as immunity fades quickly.
In one study from Kenya in 2018, almost 30 per cent of those who caught one variant of a coronavirus experienced a second reinfection. About 10 per cent caught it a third time and one person was infected four times.
A number of reinfections occurred only three months after the first bout, and in multiple cases the viral load actually increased, “revealing ineffective protective immune responses after initial exposure”.
