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.”