Environment & Science Editor
People who recover from Covid-19 are protected against the virus for at least six months, and likely much longer, according to a study from the United States that suggests the immune system continues to evolve long after infection and retains the ability to fight it.
Researchers at the Rockefeller University in New York believe this immunity may even block mutant forms of the virus, such as the South African variant. Their study, published in the science journal Nature, found antibodies are produced by immune cells that keep evolving.
This may be due to continued exposure to remnants of the virus hidden in gut tissue.
As the number of people who have recovered from Covid-19 increases, the critical question has been: how long will their immunity last?
The findings provide the strongest evidence yet that the immune system “remembers” the virus and continues to improve the quality of antibodies after infection has waned.
Antibodies produced months after the infection showed increased ability to block Sars-CoV-2, as well as its mutated versions.
Other studies suggest, however, some mutations may dampen vaccine effectiveness.
Moderna said on Monday early tests suggest its vaccine appears to work against new, more infectious variants found in the UK and South Africa.
Antibodies triggered by the vaccine can recognise and fight the new variants, it added.
The scientists in the Rockefeller antibody response study suspect that when the recovered patient next encounters the virus, the response is both faster and more effective, preventing reinfection.
"This is really exciting news. The type of immune response we see here could potentially provide protection for quite some time, by enabling the body to mount a rapid and effective response to the virus upon re-exposure," said Prof Michel C Nussenzweig, head of the laboratory of molecular immunology, whose team has been tracking and characterising antibody response since the early days of the pandemic.
Antibodies, which the body creates in response to infection, linger in the blood plasma for several weeks or months, but their levels significantly drop with time. The immune system has a more efficient way of dealing with pathogens: instead of producing antibodies all the time, it creates memory B cells that recognise the pathogen and can quickly unleash a new round of antibodies when they encounter it a second time.
But how well this memory works depends on the pathogen. To understand the case with Sars-CoV-2, the team studied the antibody responses of 87 individuals one month after infection and again six months later.
They found that although antibodies were still detectable after six months, their numbers had markedly decreased, which was to be expected.
Lab experiments showed the ability of participants’ plasma samples to neutralise the virus was reduced fivefold.
In contrast, the patients’ memory B cells, specifically those that produce antibodies against Sars-CoV-2, did not decline in number and even slightly increased in some cases.
"The overall numbers of memory B cells that produced antibodies attacking the Achilles' heel of the virus, known as the receptor-binding domain, stayed the same," said Dr Christian Gaebler.
“That’s good news because those are the ones that you need if you encounter the virus again,” he told ScienceDaily.
Focusing on memory B cells revealed they had gone through numerous rounds of mutation even after the infection was resolved and as a result the antibodies they produced were much more effective than the originals.
Lab experiments showed this new set of antibodies were better able to latch on tightly to the virus and could recognise even mutated versions of it.
“We were surprised to see the memory B cells had kept evolving during this time,” Prof Nussenzweig added. “That often happens in chronic infections, like HIV or herpes, where the virus lingers in the body. But we weren’t expecting to see it with Sars-CoV-2, which is thought to leave the body after infection has resolved.”