For the immune system, not all germs are equally memorable. But the cells of our body seem to be seriously studying about the coronavirus.
Scientists who have been monitoring immune responses to the virus are now beginning to see encouraging signs of strong, lasting immunity, even in people who develop only mild symptoms of COVID-19, a flurry of new research suggests. Disease-fighting antibodies, as well as immune cells called B cells and T cells that are able to recognize the virus, appear months after infections are resolved – a stimulating echo of the body’s continued response to other viruses. .
“Things are really working out the way they should,” said Deepta Bhattacharya, an immunologist at the University of Arizona and an author on one of the new studies, which has not yet been peer-reviewed.
Although researchers cannot predict how long this immune response will last, many experts consider the data to be a welcome indication that the body’s most studied cells are doing their job – and will have a good chance of destroying the coronavirus, faster and faster than rather, if exposed to it again.
“This is exactly what you would be hoping for,” said Marion Pepper, an immunologist at the University of Washington and an author on another of the new studies, which is currently being reviewed in the journal Nature. “All pieces are there to have a fully protective immune response.”
Protection against reinfection cannot be fully confirmed until there is evidence that most people who encounter the virus a second time are actually able to keep it, Pepper said. But the findings may help raise recent concerns about the virus’ ability to double its immune system in amnesia, making people vulnerable to recurrent diseases.
Researchers have yet to find conclusive evidence that coronavirus reinfections occur, especially within the few months that the virus has been transmitted by the human population. The chance of immune memory “helps to explain that,” Pepper said.
In conversations about immune responses to the coronavirus, much of the conversation has focused on antibodies – Y-shaped proteins that can attach to the surfaces of pathogens and block them from infecting cells. But antibodies represent only one wing of a complex and coordinated squadron of immune soldiers, each with its own unique attacks. Viruses that have already invaded cells are covered by antibodies, but are still vulnerable to killer T cells, forcing infected cells to self-destruct. Another set of T cells, nicknamed “helpers”, can coax B cells to mature into anti-antibody machines.
Antibodies also come with an expiration date: Because they are inanimate proteins and not living cells, they cannot replace themselves, and they disappear as weeks or months after producing from the blood. Hardened antibodies appear shortly after a virus has broken down the body’s barriers, then disappear as the threat disappears. Most of the B cells that produce these early antibodies also die.
But even if it is not under siege, the body retains a battalion of longer-lived B cells that can churn a lot of virus-fighting antibodies, they could prove useful again. Some patrol the bloodstream, waiting to be re-triggered; others find their way back into the bone marrow, generating small amounts of antibodies that are active for years, sometimes decades, after an infection has passed. Several studies, including those led by Bhattacharya and Pepper, have found antibodies that are able to hang the coronavirus at low levels in the bloodstream after humans recover from COVID-19.
“The antibodies decrease, but they establish themselves in what appears to be a stable nadir,” which is observed about three months after the onset of symptoms, Bhattacharya said. “The response looks perfectly sustainable.”
Looking at antibodies this long after infection is a strong indication that B cells are still going away in the bone marrow, Pepper said. She and her team were also able to pick B cells that recognize the coronavirus from the blood of people who have recovered from mild cases of COVID-19 and grow them in the lab.
Several studies, including one published Friday in the journal Cell, have also succeeded in isolating T cells from coronavirus attacks from the blood of recovered individuals – long after symptoms have disappeared. When provoked with pieces of the coronavirus in the lab, these T cells pumped out virus-fighting signals, and sounded themselves in fresh armies ready to confront a trusted enemy. Some reports have suggested that T cell analyzes may give researchers a glimpse into the immune response to the coronavirus, even in patients whose antibody levels have dropped to a point where they are difficult to detect.
“This is very promising,” said Smita Iyer, an immunologist at the University of California, Davis, who is studying immune responses to the coronavirus in rhesus macaques but was not involved in the new studies. “This requires some optimism about herd immunity, and potentially a vaccine.”
Notably, several of the new studies have found these potent responses in people who have not developed serious cases of COVID-19, Iyer added. Some researchers have worried that infections that take a smaller toll on the body are less memorable for the tough cells of the immune system, who may prefer to invest their resources in serious attacks. In some cases, the body itself could spread the viruses so quickly that it did not catalog them. “This article suggests that this is not true,” Iyer said. “You can still get lasting immunity without suffering the effects of infection.”
What has been observed in people who fight mild cases of COVID-19 may not be true for patients in hospital whose bodies struggle to mars a balanced immune response to the virus, such as those who were infected but had no symptoms had. Research groups around the world are studying the full range of answers. But “the vast majority of cases are these mild infections,” said Jason Netland, an immunologist at the University of Washington and an author on the paper that checks at Nature. “If these people are going to be protected, that’s fine too.”
This new disk of studies could also address fears more about how and when the pandemic will end. On Friday, updated guidance released by the Centers for Disease Control and Prevention was misinterpreted by several news outlets suggesting immunity to the coronavirus may last only a few months. Experts responded quickly, notes the dangers of propagating such statements and points to the wealth of evidence that people who previously had the virus are likely to be protected from reinfection for at least three months, if not much longer.
In line with other recent reports, the new data reinforces the idea that, “Yes, you develop immunity to this virus, and good immunity against this virus,” said Dr. Eun-Hyung Lee, an immunologist at Emory University who was not involved in the studies. “That’s the message we want out.”
Some diseases, such as the flu, can plague populations several times. But this is at least partly attributed to the high mutation rates of influenza viruses, which can quickly make the pathogens unrecognizable to the immune system. Coronaviruses, on the other hand, tend to change their appearance less easily from year to year.
Still, much remains unknown. Although these studies indicate the potential for protective ability, they do not prove protection in action, said Cheong-Hee Chang, an immunologist at the University of Michigan who was not involved in the new studies. “It’s hard to predict what’s going to happen,” Chang said. ‘People are so heterogeneous. There are so many factors at play. “
Animal research could help fill a few gaps. Small studies have shown that one bolt of the coronavirus rhesus-macaques seems to protect against re-contracting.
But tracking human responses in the long run will take time, Pepper said. Good immune memory, she added, requires molecules and cells to be numerous, effective and durable – and scientists can not yet say that all three conditions are definitively met.
As human bodies move to the post-coronavirus state, “we just reach the point of relevance” to take the long view of immunity, Bhattacharya said. Things can usually change in a few months or years. Or they may not.
“There are no shortcuts here,” Bhattacharya said. “We just have to follow it.”
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