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Most people who get sick with COVID-19 develop antibodies which help them ward off the disease. Scientists can't say for sure whether those antibodies will protect people from re-infection or how long they persist. NPR science correspondent Richard Harris looked into the latest studies on this subject and what they could mean for a vaccine.
RICHARD HARRIS, BYLINE: So many people are counting on a vaccine to end the coronavirus nightmare that any hint of bad news gets a lot of attention. And that's proving to be the case for a series of studies that have been examining how long antibodies last - one from China, one from Britain and now one from the United States. Dr. Otto Yang at UCLA tracked antibodies in 34 people with coronavirus. His team reports that the average antibody levels dropped to about half of where they started over two and a half months.
OTTO YANG: The big caveat is, of course, that this is just one short snapshot for a relatively short period of time, and so we don't know that it will continue at that same rate of drop over time.
HARRIS: Scientists have noticed that people with milder symptoms or no symptoms at all tend to have a weaker immune response. And Yang's letter to the New England Journal of Medicine focused on people with mild illness.
YANG: A majority of them just recovered at home with no hospital care, so their antibody titers were not extremely high. Indeed, we have seen other people that have very, very high antibody titers - on the order of 10 to 100 times higher than these.
HARRIS: A research group based at Mount Sinai in New York looked at nearly 20,000 people with mild to moderate illness. They have a more reassuring story. An unpublished research report on this finds that 90% of those people had antibody responses that lasted at least three months. And these antibodies neutralize the virus, at least in the lab. Florian Krammer one of the scientists on that study, says that bodes well for longer-term immunity.
FLORIAN KRAMMER: It is reasonable to assume that there will be protection for a timeframe of one to three years, but, of course, we are scientists. We have to prove that, right? And so that's why everybody's cautious about it. But I think that's a pretty fair assumption.
HARRIS: Scientists focus a lot on antibodies in part because they are easy to measure. They do tend to wane over time in many instances, but Krammer says when that happens your immune system doesn't necessarily have to start again from scratch.
KRAMMER: Your body also produces memory B cells, and so those are cells that don't actually make any antibodies unless the virus comes back and they see that virus again. But then they react very, very quickly and make antibodies very quickly. So there is many arms that the immune system has to tackle this virus.
HARRIS: Yet another line of defense is another kind of cell called a T cell. Dr. Kari Nadeau at Stanford University is one of many scientists working to figure out how they react in a coronavirus infection.
KARI NADEAU: T cells are very important in fighting viruses. We know that. And so one thing that we're looking at as the antibodies wane, do the T cell responses also wane because those T cell responses can kill the virus.
HARRIS: Scientists developing at least some of the coronavirus vaccines have noticed that their experimental products rev up the T cell response. Adrian Hill at the University of Oxford is leading a team working on one of the most advanced vaccines, and he is not fretting over reports of waning antibodies.
ADRIAN HILL: What we know from many other infections, the vaccine response can be much more durable than the natural infection response.
HARRIS: That's partly because people get a strong dose with a vaccine, which stimulates strong immunity.
HILL: I'm pretty confident that in COVID we're going to see that the vaccines are more durable than a natural COVID infection. But again, we don't know yet. We need to wait and see.
HARRIS: We should know a lot more in the coming months. Richard Harris, NPR News.
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