Coronavirus immunity – who has it? Scientists still not sure, even as lockdowns are eased worldwide

Who among us is immune to the coronavirus? Six months and close to 7.9 million infections into the Covid-19 pandemic, scientists are inching their way towards answering that question. For those who have weathered a bout with the virus, there are promising signs that protection follows. But understanding the extent of true immunity is an exercise in humility, scientists caution. The stakes could not be higher: public health efforts to defeat coronavirus – from antibody testing and “immune passports” to the use of convalescent plasma and the design of a new vaccine – depend on understanding who is immune, and why. Dr Anthony Fauci, America’s leading expert on infectious diseases, called the emerging evidence on coronavirus immunity “confusing,” “perplexing” and a “a bit unusual”. “Here’s the deal,” Fauci says. “One can say that if you have recovered from infection, it is highly likely you’ll be protected from reinfection for a finite period of time, varying from person to person because of biological variability. We don’t know how long that finite period will be. It could be years, or a year.” Two newly published studies offer support for the hope that some people have earned immunity to coronavirus, and that it can be roughly measured. But as economies begin to reopen, their findings should temper the expectations of political leaders who are counting on armies of the recovered to lead a charge back to normality. How to stay healthy and happy during virus outbreak and flu season The new studies shed light on how many patients emerge from coronavirus infection with so-called neutralising antibodies – the kind that can, for an unknown stretch of time, protect a person from reinfection. A census of 382 sailors aboard the USS Theodore Roosevelt aircraft carrier has found that at least 60 per cent of those who were infected developed antibodies capable of protecting them to some extent. In a small group of study participants infected early, some still had the virus-blocking proteins in their blood 40 days after their symptoms first appeared. That is a reassuring sign on two fronts. It shows that even young and otherwise healthy people, who tend to have mild or even no symptoms when infected with coronavirus, are making antibodies that could protect them from reinfection. And it suggests that for some, that protection might have staying power. These servicemen and -women, caught up in a shipwide outbreak, have given scientists a glimpse of the coronavirus at work in a young, healthy population that lives and works closely together. Between mid-January, when the aircraft carrier set out from its home port of San Diego in California, and late March, when it pulled into the US Pacific Ocean territory of Guam for an emergency medical stop, about 1,000 of 4,800 aboard had been infected by coronavirus. A team from the US Centres for Disease Control and Prevention and the US Navy studied the sailors’ immune response. A second study has found that 83 per cent to 93 per cent of New Yorkers who had recovered from confirmed coronavirus infections and been well for at least two weeks had at least some neutralising antibodies in their blood. But the capacity of those antibodies to neutralise the coronavirus varied widely. The big underlying question is: How much neutralising activity you even need? This is the way immunology works. We are humbled Larry Luchsinger, a stem cell researcher In a little over half of the 370 recovered patients – between 52 per cent and 56 per cent – researchers found “relatively modest” antibody levels. There was better, and surprising, news too. Roughly one in 10 of those tested was found to have highly potent concentrations of infection-blocking antibodies. And the blood of recovered men had two to three times the coronavirus-neutralising activity as samples from recovered women. The findings suggest that this minority of recovered patients is likely to enjoy some protection from reinfection themselves. It also buoys hope that infusions of their blood plasma can protect others from infection, and reduce the risk of critical illness or death in Covid-19 patients. These reports confirm and amplify research released in late April by Chinese doctors in Shanghai. They found that, of 175 people who had been only mildly ill with Covid-19, a little over half produced relatively high levels of neutralising antibodies, while 30 per cent made low levels that scientists fear may not be helpful. However, the studies have left scientists with some maddening uncertainties. When it comes to coronavirus antibodies, does quantity or quality matter more? Could a recovered patient be immune even without neutralising antibodies? Do coronavirus patients who were very sick, or sick for a long time, have stronger immunity than young and healthy patients who had milder symptoms, or none at all? The human body produces many different kinds of antibodies in response to infection, and the neutralising variety may not be the only ones that hobble a virus. The immune system has other defensive weapons to muster for such a fight, including its killer T cells and B cells. Their powers against this coronavirus are not well understood – and even if they were, the antibody tests now being fielded aren’t designed to measure them, or other immune factors. Some of these overlooked antibodies may lie dormant, awakening only when challenged by the virus’s return, says Dr Jeffrey Henderson, an infectious diseases specialist at Washington University in St Louis, in the US state of Missouri. The body’s cellular immune response may have done such a good job that the humoral immune system that forges antibodies never even came into play, he says. With coronavirus, he added, “we’re keeping an open mind”. The same openness extends to other long-held assumptions. Scientists have observed that sicker patients make more and better antibodies. But that assumption may be upended by this coronavirus, says Larry Luchsinger, a stem cell researcher at the New York Blood Bank who spearheaded the study of recovered blood donors there. Now Luchsinger and his team hope to go back to the 370 blood donors and learn more about their reported symptoms to test the proposition. They may find that there’s no pattern at all: young people who scarcely noticed their infections may be just as immune as older patients who barely escaped with their lives. Or perhaps neither is immune. Or maybe one gained three months of immunity while the other got three years. And that is still only part of the story, Luchsinger says. “The big underlying question is: How much neutralising activity you even need?” he says. “This is the way immunology works. We are humbled.”