Scientists have found the strongest evidence yet that people who recover from Covid may mount a much faster and more effective defence against the infection if they encounter the virus again.
Researchers at Rockefeller University in New York found that the immune system not only remembered the virus but improved the quality of protective antibodies after an infection had passed, equipping the body to unleash a swift and potent attack if the virus invaded a second time.
“It’s very good news,” said Michel Nussenzweig, the head of molecular immunology at Rockerfeller and a senior author of the study. “The expectation is that people should be able to produce a rapid antibody response and resist infection in a large number of cases.”
It is unclear how long the immune system’s memory might last, but Nussenzweig said it could potentially provide some protection for years. The discovery may explain why verified re-infections from the virus are so far quite rare.
When people are infected with coronavirus, the immune system launches a multi-pronged attack. One form of protection comes from T cells, which seek and destroy infected cells, and so prevent the virus from spreading. A second front involves B cells, which release antibodies into the blood. Antibodies latch on to the virus and stop it from invading cells in the first place.
Once the infection has passed, the immune system stands down, but it remembers the virus by storing so-called memory T cells and memory B cells. Should the virus return, these are immediately called to action.
Many studies have shown that the first wave of antibodies to coronavirus wane after a few months, raising concerns that people may lose immunity quickly. In their study of 87 coronavirus patients, the US researchers confirmed that antibodies wane, falling to about a fifth of their peak level over six months, but this may not matter too much, they believe.
When the researchers examined the immune system’s memory, they noticed that six months after infection the antibodies made by memory B cells had evolved to become more potent. These highly honed antibodies could be unleashed within days of re-infection, rather than taking a couple of weeks to build up, as seen in primary infections.
The scientists went on to show that tiny amounts of coronavirus, or protein fragments from inactive virus particles, lurked in patients’ intestines and apparently helped to maintain the immune system’s memory. The remnants of the virus are not thought to be harmful.
“The take-home lessons are that people who have been infected, six months later have persistent B cell memory responses with antibodies that can neutralise the virus and can do it very well,” Nussenzweig said. That could mean wiping out the virus before it takes hold, he added. “We don’t know how long any protection will last, but it might be a really long time. It could be years.”
The study is preliminary and has not been peer-reviewed or published in a journal.
Charles Bangham, a professor of immunology at Imperial College London, who was not involved in the study, said the work suggested there was “a very good chance that if you’re re-exposed, you’ll make a brisker immune response” to the virus. “It remains to be proved that it’s protective, but you would be reasonably confident it would have some beneficial effect,” he said.
Arne Akbar, a professor of immunology at University College London, said: “This is good news for everybody who has been sick with coronavirus.” He said the immune system was like an army that stands down once the threat is over, but remains prepared for a future invasion. “You want the army to be generated again very quickly, and this is what these researchers have found.”