Scientists See Signs of Lasting Immunity to Covid-19, Even After Mild Infections

Published by NYTIMES.COM

Summary generated on August 17, 2020

    Aug. 16, 2020, 5:40 p.m. ET.To the immune system, not all germs are equally memorable.

    Our body's cells seem to be seriously studying up on the coronavirus.

    Scientists who have been monitoring immune responses to the virus are now starting to see encouraging signs of strong, lasting immunity, even in people who developed only mild symptoms of Covid-19, a flurry of new studies suggests.

    Disease-fighting antibodies, as well as immune cells called B cells and T cells that are capable of recognizing the virus, appear to persist months after infections have resolved - an encouraging echo of the body's enduring response to other viruses.

    Although researchers cannot forecast how long these immune responses will last, many experts consider the data a welcome indication that the body's most studious cells are doing their job - and will have a good chance of fending off the coronavirus, faster and more fervently than before, if exposed to it again.

    "All the pieces are there to have a totally protective immune response."

    The findings could help quell recent concerns over the virus's ability to dupe the immune system into amnesia, leaving people vulnerable to repeat bouts of disease.

    The prospect of immune memory "Helps to explain that," Dr. Pepper said.

    In discussions about immune responses to the coronavirus, much of the conversation has focused on antibodies - Y-shaped proteins that can latch onto the surfaces of pathogens and block them from infecting cells.

    Antibodies represent just one wing of a complex and coordinated squadron of immune soldiers, each with their own unique modes of attack.

    Viruses that have already invaded cells are cloaked from antibodies, but are still vulnerable to killer T cells, which force infected cells to self-destruct.

    Another set of T cells, nicknamed "Helpers," can coax B cells to mature into antibody-making machines.

    Another sector of the immune system assails pathogens within minutes of their arrival, while sending out signals called cytokines to mobilize forces from elsewhere in the body.

    Antibodies also come with an expiration date: Because they are inanimate proteins and not living cells, they can't replenish themselves, and so disappear from the blood just weeks or months after they are produced.

    Hoards of antibodies appear shortly after a virus has breached the body's barriers, then wane as the threat dissipates.

    Most of the B cells that produce these early antibodies die off as well.

    Even when not under siege, the body retains a battalion of longer-lived B cells that can churn out virus-fighting antibodies en masse, should they prove useful again.

    Several studies, including those led by Dr. Bhattacharya and Dr. Pepper, have found antibodies capable of incapacitating the coronavirus lingering at low levels in the blood months after people have recovered from Covid-19."The antibodies decline, but they settle in what looks like a stable nadir," which is observable about three months after symptoms start, Dr. Bhattacharya said.

    Seeing antibodies this long after infection is a strong indication that B cells are still chugging away in the bone marrow, Dr. Pepper said.

    She and her team were also able to pluck B cells that recognize the coronavirus from the blood of people who have recovered from mild cases of Covid-19 and grow them in the lab.

    Multiple studies, including one published on Friday in the journal Cell, have also managed to isolate coronavirus-attacking T cells from the blood of recovered individuals - long after symptoms have disappeared.

    When provoked with bits of the coronavirus in the lab, these T cells pumped out virus-fighting signals, and cloned themselves into fresh armies ready to confront a familiar foe.

    Some reports have noted that analyses of T cells could give researchers a glimpse into the immune response to the coronavirus, even in patients whose antibody levels have declined to a point where they are difficult to detect.

    "This is very promising," said Smita Iyer, an immunologist at the University of California, Davis, who is studying immune responses to the coronavirus in rhesus macaques but was not involved in the new studies.

    Notably, several of the new studies are finding these powerful responses in people who did not develop severe cases of Covid-19, Dr. Iyer added.

    Some researchers have worried that infections that take a smaller toll on the body are less memorable to the immune system's studious cells, which may prefer to invest their resources in more serious assaults.

    People infected with the coronavirus typically produce immune molecules called antibodies, which are protective proteins made in response to an infection.

    These antibodies may last in the body only two to three months, which may seem worrisome, but that's perfectly normal after an acute infection subsides, said Dr. Michael Mina, an immunologist at Harvard University.

    There's no national policy on this yet, so check with your municipal school system regularly to see what is happening in your community... What has been observed in people who fought off mild cases of Covid-19 might not hold true for hospitalized patients, whose bodies struggle to marshal a balanced immune response to the virus, or those who were infected but had no symptoms at all.

    Good immune memory, she added, requires molecules and cells to be abundant, effective and durable - and scientists cannot yet say that all three conditions have been definitively met.