Patients who recover from Covid-19 may have immunity for several years, a study has suggested.
The finding will reinforce hopes an end to the pandemic is in sight amid the recent confirmation that two vaccines – Moderna and Pfizer – were effective in tackling the virus.
Researchers in the United States found that the recovered Covid-19 patients still had enough immune cells to fight the virus eight months on.
They usually decline slowly, suggesting protection against the virus could last long.
"That amount of memory would likely prevent the vast majority of people from getting to hospital with disease, severe disease, for many years," Prof Shane Crotty, a virologist at the La Jolla Institute of Immunology in California who co-led the study, told The New York Times.
Researchers are yet to determine how immunity to Covid-19 works. Previous studies have shown antibodies to the virus wane quickly.
However, a study of healthcare workers who were either asymptomatic or had mild infections, suggested that their T-cells, a type of white blood cell that attack cells infected with the virus, remained robust, even as antibodies fell.
Researchers at the La Jolla Institute of Immunology study looked at various parts of the body's immune response to Covid-19, in 41 people who had contracted the virus.
Antibodies were relatively stable over a period of time, they said. T-cells declined slightly and B-cells, which produce specific antibodies against infection, in this case SARS-CoV-2, the virus that causes Covid-19, actually grew in the eight months following infection.
Prof Crotty said the results of the study suggested “memory to SARS-CoV-2 appears to be robust”.
The research, which has not been peer-reviewed, is the most long-ranging study of its kind to date.
Concerns about the durability of Covid-19 immunity were sparked by research into other coronaviruses that cause colds.
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 within three months after the first bout, and in several cases the viral load actually increased, "revealing ineffective protective immune responses after initial exposure".
However, studies have shown those who were infected with Sars, also a respiratory disease, still have T-cells against the virus 17 years later.
Meanwhile, the Global Covid-19 Vaccines Forum hosted by Saudi Arabia this month, heard that people who caught Covid-19 twice probably had a mild or asymptomatic diagnosis the first time around.
A severe case, by contrast, meant recovered patients had significant number of antibodies and hence a reinfection would usually be milder.
Dr Naif Alharbi, director of vaccine development at King Abdullah International Medical Research Centre in Riyadh, said research on the body's immune response to the disease was still in its early stages.
“Reinfection is happening but it is rare,” he said. “I haven’t seen any severe cases so far so we cannot firmly determine whether immunity will be effective or not."