Immunity to Covid-19 likely to last for years

Covid-19 patients who recuperated from the disease still have strong immunity from the coronavirus eight months after infection, a study says.

With the encouraging sign of the result, the authors interpret to mean immunity to the virus probably lasts for many years and it should remove fears that the Covid-19 vaccine would require repeated booster shots to protect against the disease, reports MIT Technology Review.

Shane Crotty, a researcher at the La Jolla Institute for Immunology in California and a coauthor of the new paper, said: "There was a lot of concern originally that this virus might not induce much memory. Instead, the immune memory looks quite good."

Published on January 6 in Science, the study contrasts with earlier findings that suggested the immunity to Covid-19 could be short-lived, putting millions of people, who have already recuperated, at risk of reinfection.

According to the new study, reinfection should only be a problem for a very small percentage of people who have developed immunity whether through an initial infection or by vaccination.

The study shows that a small number of recovered people do not have long-lasting immunity, but vaccination should resolve the problem by ensuring herd immunity in the mass people. 

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The new paper studied blood samples from 185 men and women who recovered from Covid-19 -- most from a mild infection, although 7% were hospitalized. 

Each individual provided at least one blood sample between six days and eight months after their initial symptoms, and 43 of the samples were taken after six months. 

The team that ran the investigation measured the levels of several immunological agents that work together to prevent reinfection -- antibodies (which tag a pathogen for destruction by the immune system or neutralize its activity), B cells (which make antibodies), and T cells (which kill infected cells).

The researchers found that antibodies in the body declined moderately after eight months though levels varied wildly among individuals. 

But T-cell numbers declined only modestly, and B-cell numbers held steady and sometimes inexplicably grew, which means that despite decreases in free-flowing antibodies, the components that can restart antibody production and coordinate an attack against the coronavirus stick around at pretty high levels. 

Shane Crotty said that the same mechanisms that lead to immune memory after infection also form the basis for immunity after vaccination, so the same trends ought to hold for vaccinated people as well.

In the case of Sars, a close cousin of the virus that causes Covid-19, a study published in August showed that T cells specific to Sars can remain in the blood for at least 17 years, bolstering hopes that Covid-19 immunity could last for decades.

However, the new study is yet to be termed perfect. It would have been better to collect multiple blood samples from every participant, the report said.

"Immunity varies from person to person, and uncommon individuals with weak immune memory still may be susceptible to reinfection," Crotty cautions. 

"And we can't make any firm conclusions about Covid-19 immunity until years have passed—it's simply too early. Nonetheless, this latest result is a good indication that if the vaccination rollout goes well [a big if], we might soon be able to put the pandemic behind us."