The good news is indicated by the findings of research which analysed immune memory in those who had recovered from Covid-19, and found that, eight months after infection, most people still had enough immune cells to fight off the virus and prevent another illness. 

The researchers reported a powerful, long-lasting response, and, while the findings have not yet been peer-reviewed, the New York Times described  it as “the most comprehensive and long-ranging study of immune memory to the coronavirus to date.”

“That amount of memory would likely prevent the vast majority of people from getting hospitalized disease, severe disease, for many years,” Shane Crotty, a virologist at the La Jolla Institute of Immunology who co-led the new study, told the paper.

The findings will come as a relief to policymakers who worried that immunity to the virus might be short-lived. The study echoes another recent finding regarding the SARS virus – that survivors still carried immune cells some 17 years after recovering.

The research also backs up findings from a study published in Nature last week which found that, even when antibodies are not detected, people who recovered from the coronavirus had powerful and protective immune cells to the virus.

In general, the human body will mount a strong and lasting protective response to a virus once the person has recovered from an infection.

Recent reports of declining antibody levels in post-Covid patients had created concern that immunity to the coronavirus might not be long-lasting, but immunologists have said that this drop naturally occurs and that antibodies are just one factor of the immune system.

Immune memory – when immune cells remember the virus and act to prevent it creating a serious illness is very effective since the immune system recognizes the invader and quickly extinguishes the infection. The coronavirus is considered slow to do harm, giving the immune system plenty of time to react strongly.

Alessandro Sette, an immunologist at the La Jolla Institute of Immunology and co-leader of the study told the New York Times that he and his colleagues recruited 185 men and women, aged 19 to 81, who had recovered from Covid-19. The majority had mild symptoms not requiring hospitalization; most provided just one blood sample, but 38 provided multiple samples over many months.

The team tracked four components of the immune system: antibodies, B cells that make more antibodies as needed; and two types of T cells that kill other infected cells. The idea was to build a picture of the immune response over time by looking at its constituents.

If you just look at only one, you can really be missing the full picture,” Dr. Crotty said.

He and his colleagues found that antibodies were durable, with modest declines at six to eight months after infection, although there was a 200-fold difference in the levels among the participants. T cells showed only a slight, slow decay in the body, while B cells grew in number — an unexpected finding the researchers can’t quite explain.

The study is the first to chart the immune response to a virus in such granular detail, experts said. “For sure, we have no priors here,” Dr. Gommerman said. “We’re learning, I think for the first time, about some of the dynamics of these populations through time.”

Worries over how long immunity to the coronavirus persists were sparked mainly by research into those viruses causing common colds. One frequently cited study, led by Jeffrey Shaman of Columbia University, suggested that immunity might fade quickly and that reinfections could occur within a year.

“What we need to be very mindful of is whether or not reinfection is going to be a concern,” Dr. Shaman said. “And so seeing evidence that we have this kind of persistent, robust response, at least to these time scales, is very encouraging.” So far, at least, he noted, reinfections with the coronavirus seem to be rare.

Exactly how long immunity lasts is hard to predict, because scientists don’t yet know what levels of various immune cells are needed to protect from the virus. But studies so far have suggested that even small numbers of antibodies or T and B cells may be enough to shield those who have recovered.

The participants in the study have been making those cells in robust amounts — so far. “There’s no sign that memory cells are suddenly going to plummet, which would be kind of unusual,” Dr. Iwasaki said. “Usually, there’s a slow decay over years.”

Some good news then in the bid to end the Covid-19 pandemic.