With the whole world counting on a vaccine to help end the coronavirus pandemic, any hint of bad news or clues attracts too much attention. A number of studies have looked at how long antibodies persist in people infected with coronavirus.
Antibodies help prevent infections. In some diseases, they can prevent reinfection, but scientists cannot say for sure whether this is true for coronavirus. It is also unclear how long the antibodies will last.
Three recent studies from China, the UK and now the United States have questioned how long people carry antibodies after infection. But the reality may be less dire than recent headlines suggest.
The latest of these papers is from a research group at UCLA. It tracked antibodies in 34 people with coronavirus and found that average antibody levels dropped to half of where they started over a period of about 2 1/2 months.
"The big caveat of course is that this is just a snapshot for a relatively short period of time," says Dr. Otto Yang of UCLA, "so we don't know if it will sustain the same rate of decline over time." He said.
The scientists noticed that people with milder symptoms or no symptoms at all tended to have a weaker immune response to the coronavirus. And Yang's letter, published in the New England Journal of Medicine, focused on people with mild illness.
"The majority recovered at home without hospital care, so their antibody levels were not extremely high," Yang says. "Indeed, we saw other people with very, very high antibody levels, 10 to 100 times higher than these." He said.
Mt. Icahn School of Medicine looked at about 20,000 people with mild to moderate disease. The group tells a more reassuring story in an unpublished research report.
It found that 90% of these people had antibody responses that lasted at least three months. And these antibodies neutralized the virus, at least in the laboratory.
"It's reasonable to assume that there would be protection for a time frame of one to three years," says Florian Krammer, one of the scientists in this study. "But of course, we are scientists. We have to prove it, right? That's why everyone is cautious about it. But I think it's a pretty reasonable assumption."
Scientists focus on antibodies as markers of immunity, partly because they are easy to measure. Antibodies tend to weaken over time in many cases, but when this happens, the immune system does not necessarily have to start from scratch. The memory of a past infection can be stored in a cell called a memory B cell.
"These are cells that don't actually produce any antibodies unless the virus comes back, and then they see that virus again," says Krammer. "But then they react very, very quickly and produce antibodies very quickly. So there are many arms of the immune system that have to fight this virus."
Yet another line of defense is a type of immune cell called a T cell.
"T cells are very important in fighting viruses," says Dr. Kari Nadeau of Stanford University. They can kill viruses. "And one of the things we're looking at is, as antibodies decline, do T cell responses also decline?"
Scientists developing some coronavirus vaccines have noticed that their experimental products trigger a strong T cell response. This includes the vaccine from Oxford University, which is in an advanced testing phase. "We know from many other infections that the vaccine response can be much more persistent than the natural infection response," says Adrian Hill, lead researcher of this vaccine study. This is partly because people get a strong dose with a vaccine that promotes strong immunity.
"I'm pretty confident that in COVID we'll see vaccines being more durable than a natural COVID infection," Hill says. "But again, we don't know yet. We need to wait and see."
And for now, people who are already infected with the coronavirus will also have to wait and see if they will remain immune, and if so, for how long. Immunity to the four milder coronaviruses that circulate widely and cause the common cold does not last for years. So the duration of an antibody response may depend, at least in part, on whether an infection leads to significant illness and a significant immune reaction.
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