Antibodies people make to fight coronavirus last at least four months after diagnosis and do not fade quickly as some earlier reports suggested, scientists have found.
A study involving tests on more than 30,000 people in Iceland makes for the most extensive work yet on the immune system's response to the virus over time and is good news for efforts to develop vaccines.
If a vaccine can spur production of long-lasting antibodies as natural infection seems to do, it gives hope "immunity to this unpredictable and highly contagious virus may not be fleeting", scientists from Harvard University and the US National Institutes of Health wrote.
Their commentary was published with the study in the New England Journal of Medicine on Tuesday.
One of the big mysteries of the pandemic is whether having had coronavirus helps protect against future infection and for how long.
Some smaller studies previously suggested antibodies may disappear quickly and some people with few or no symptoms may not make many at all.
The new study was done by Reykjavik-based deCODE Genetics, a subsidiary of US biotech company Amgen, with several hospitals, universities and health officials in Iceland.
The country tested 15 per cent of its population since late February, when its first COVID-19 cases were detected, giving a solid base for comparisons.
Three potential coronavirus vaccines at Novavax labs in Gaithersburg, Maryland in March, 2020. Source: AAP
Scientists used two types of testing: from nose swabs or other samples that detect bits of the virus, indicating infection, and those measuring antibodies in the blood, which can show whether someone was infected now or in the past.
Blood samples were analysed from 30,576 people using various methods and someone was counted as a case if at least two of the antibody tests were positive.
These included a range of people, from those without symptoms to people hospitalised with signs of COVID-19.
In a subgroup who tested positive, further testing found antibodies rose for two months after their infection initially was diagnosed and then plateaued and remained stable for four months.
Previous studies suggesting antibodies faded quickly may have been just looking at the first wave of antibodies the immune system makes in response to infection.
Those studies mostly looked 28 days after diagnosis.
A second wave of antibodies forms after a month or two into infection, and this seems more stable and long-lasting, the researchers report.
The results don't necessarily mean all countries' populations will be the same,or that every person has this sort of response.
Other scientists recently documented at least two cases where people seem to have been reinfected with the coronavirus months after their first bout.
The new study does not establish how much or which type of antibody confers immunity or protection - that remains unknown.
