New York , Oct 9 (IANS): People who survive serious Covid-19 infections have long-lasting immune responses against the virus, say researchers, adding that survivors may have four months of protection.

The study, published in the journal Science Immunology, offers hope that people infected with the virus will develop lasting protection against reinfection.

The research also demonstrated that measuring antibodies can be an accurate tool for tracking the spread of the virus in the community.

"The immune system produces proteins called antibodies in response to SARS-CoV-2, the virus that causes Covid-19. But there is a big knowledge gap in terms of how long these antibody responses last," said study author Richelle Charles from the Massachusetts General Hospital (MGH) in the US.

To find out, the research team obtained blood samples from 343 patients with Covid-19, most of whom had severe cases.

The blood samples were taken up to four months after a patient's symptoms emerged.

The blood's plasma was isolated and applied to laboratory plates coated with the receptor-binding domain (RBD) of the virus's "spike" protein, which attaches to cells, leading to infection.

The team studied how different types of antibodies in the plasma bound to RBD. The results were compared to blood samples obtained from more than 1,500 individuals prior to the pandemic.

The researchers found that measuring an antibody called immunoglobulin G (IgG) was highly accurate in identifying infected patients who had symptoms for at least 14 days.

Since the standard PCR (nasal swab) test for SARS-CoV-2 loses sensitivity over time, augmenting it with a test for antibodies in patients who have had symptoms for at least eight days (at which time 50 percent are producing antibodies) will help identify some positive cases that might otherwise be missed, Charles said.

The researchers found that IgG levels remained elevated in these patients for four months, and were associated with the presence of protective neutralizing antibodies, which also demonstrated little decrease in activity over time.

"That means that people are very likely protected for that period of time. We showed that key antibody responses to Covid-19 do persist," the authors wrote.

In another finding, the researchers showed that people infected with SARS-CoV-2 had immunoglobulin A (IgA) and immunoglobulin M (IgM) responses that were relatively short-lived, declining to low levels within about two and a half months or less, on average.

"We can say now that if a patient has IgA and IgM responses, they were likely infected with the virus within the last two months," the team noted.