As coronavirus cases increase in the United States and around the world, scientists are finding evidence that the immunity for those who have had COVID-19 may last at least six months, if not longer.
After people with COVID-19 have largely recovered, immune proteins called antibodies are still detectable six months later. What’s more, proteins have improved their ability to fight coronavirus, researchers report in a preliminary study published Nov. 5 on bioRxiv.org. The remnants of the virus that remain in the gut after the symptoms have gone away can help the immune system improve that response.
The finding also predicts how long a vaccination can protect. Immunity from a vaccine is expected to last as long or longer than natural immunity.
Antibodies, which are immune proteins that bind to microbes to fight an infection, are part of the body’s immune defense cache. People usually make a wide variety of antibodies during an infection. These proteins can recognize different surfaces in viruses, such as a Swiss Army knife capable of working on various parts of the virus, and evolve over time to better recognize their target (SN: 28/04/20).
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Six months after a coronavirus infection that causes COVID-19, called SARS-CoV-2, people appear to build up an arsenal of antibodies that are not only more potent than those initially developed, similar to what has been seen in other infections. These antibodies can also recognize mutated versions of the virus, the researchers found. In addition to antibody updates, long-lasting antibody-producing immune cells, called memory B cells, stick to the blood, ready to initiate a rapid response if people are exposed to the virus again.
“The main message is that the immune response persists,” says Julio Lorenzi, a viral immunologist at Rockefeller University in New York City. "We see that these B cells survive over time and the antibodies six months after infection are even better than the onset of the infection."
In the study, Lorenzi and colleagues analyzed the antibodies that made 87 people against the coronavirus one to six months after developing symptoms. Although antibody levels in the blood decreased, immune proteins were still detected after six months. It is important to note that memory B cell levels were stable, showed an evaluation of 21 of the 87 participants, a sign that these cells may remain in the body for a while.
Other studies have suggested that B cells may persist for more than six months in patients recovered with COVID-19. Preliminary results from a study found that memory B cells – as well as other cells involved in immune memory known as T cells – slowly decline in the blood, the researchers reported on Nov. 16 at bioRxiv.org. That slow decrease may mean that immunity can last for years, at least in some people (SN: 19/10/20).
In addition, Lorenzi and his team discovered, B cells refined the antibodies they made over a five-month period to generate proteins that would better recognize the coronavirus. In a six-person cell analysis, the researchers found changes in the genetic instructions that B cells use to make antibodies, a sign that B cells were making new variations.
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Some of the new antibodies were better at preventing the viruses from infecting new cells, and some could even bind to viruses with mutations in the ear protein, which helps the coronavirus break down in host cells. Such bound antibodies could make it difficult for the immune system to escape recognition.
The findings are encouraging, experts say, although it is still unclear whether people with signs of immunity such as antibodies are completely protected against reinfection (called sterilizing immunity) or whether they would only become seriously ill if they were reinfected.
“When the first studies on antibody responses to SARS-CoV-2 began to come out, everyone was in turmoil because the response was potentially defective,” says Nina Luning Prak, an immunologist at the University of Pennsylvania. The previous results suggested that antibody-generating B cells were poorly trained to make immune proteins, perhaps because structures called germ centers that teach cells what parts of a virus antibodies should bind to did not form properly.
This may leave other immune signals in addition to the germ centers to activate B cells, leading to the production of less effective antibodies that could attach to weak parts of the virus. “As a result, (some scientists thought that) maybe (B cells) made antibodies that weren’t that big,” says Luning Prak.
But that may be part of a normal immune response, says Luning Prak. Or defective germ centers may appear in the more severe cases of COVID-19 where “it’s a practical-style immune response” with a lot of inflammation. When people survive the infection, researchers are now beginning to find that “when it is observed (in patients with COVID-19) six months out, antibody responses seem much more conventional,” he says.
B cells can learn how to make better SARS-CoV-2 antibodies over time, with the help of a store of viral proteins that remain hidden in the gut after the virus is removed from the rest of the body. Since the early days of the pandemic, researchers have documented the presence of coronavirus genetic material in the feces of some infected people.
In the new study, seven of the 14 patients recovered with COVID-19 had evidence of coronavirus in their intestinal tissue, the researchers found. Electron microscopy images of a sample of a patient revealed what look like intact virus particles adorned with a crown of spike proteins, a distinctive feature of coronaviruses.
Right now, it’s unclear whether the viral pieces seen in the gut actually help the immune system evolve to better recognize the coronavirus, let alone whether those pieces come from infectious or dead viruses, Lorenzi says. “That’s a possibility,” but researchers need to study more people to find out.
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