How Long Could Covid-19 Immunity Last? It's Not Just About Antibodies

By Ed Cara on at

A recent study making the rounds everywhere is certainly unnerving: It found that previously infected people can lose almost all of a type of antibody to the coronavirus within three months. But the study’s findings are far too limited to assume that immunity to the virus is so short-lived.

The study was published last week in Nature Medicine. Using blood samples, researchers in China studied the immune response of 37 people who had tested positive for the coronavirus but never developed any symptoms. They were compared to 37 people who did get mildly sick, as well as a control group of people who tested negative for the virus.

Both groups who caught the virus had relatively similar immune responses at first. But two to three months later, over 90% of both groups experienced a significant reduction in covid-19 specific IgG antibodies; a majority of both groups also experienced a drop in the antibodies that block the virus from infecting cells, which are called neutralising antibodies. This decline in IgG and neutralising antibodies was more severe for asymptomatic people, with around 40% becoming seronegative for IgG, meaning their levels had dropped past the point of detection (12% of the symptomatic group also became seronegative).

The findings “might indicate the risks of using COVID-19 ‘immunity passports,’” the scientists wrote. But as they admit, we still need a lot more research to confirm these results and to know whether immunity could be so brief.

“Overall, it is a pretty well done study,” Otto Yang, a professor of medicine and associate chief of infectious diseases at the David Geffen School of Medicine at UCLA,” told Gizmodo. “But there are at least two big caveats that that haven’t been discussed very much that need to be kept in mind.”

One issue is that antibodies aren’t always the most important thing when it comes to fending off a disease we’ve had before, Yang said.

There are other immune cells that our bodies create when a virus or germ we’ve encountered before shows up again, particularly certain types of T-cells. Like antibodies, these cells are keyed specifically to recognise the returning pathogen. And so far, the evidence suggests that being infected with the coronavirus does tend to create a strong T-cell response. With some viruses, Otto noted, it’s cellular immunity that provides the most protection against reinfection or illness, not antibodies.

“The second caveat is that if antibodies do correlate with immunity, then it’s not clear what the cutoff would be,” Yang said.

We have some evidence from studies of other human coronaviruses that antibody levels tend to meaningfully wane after a year or two post-infection, and people can be reinfected around then. Even a year’s worth of protection would be more helpful than a few months, given ongoing efforts to create an effective vaccine as soon as possible.

For instance, a study recently published in Nature Medicine looked at blood samples collected from 149 recovered patients an average 39 days post-illness. Though most of these patients seemed to have relatively low levels of neutralising antibodies, the researchers still found that they provided “potent antiviral activity” in all patients tested.

Another consideration is memory B cells, which help quickly ramp up the immune system, including antibody production, in response to reinfection. Even if someone’s antibody levels to covid-19 do fade quickly, it’s possible that their bodies will still marshal a potent, rapid defence against the virus as soon it tries to infect them again. This response might not prevent reinfection, but it should make the second go-around easier to handle.

“I think it’s very reasonable to assume that reinfection would be milder,” Yang said. “Whatever balance of genetic or underlying factors that determined their disease the first time will probably be the same. And they will additionally have a head start in terms of whatever immune responses were used to clear the infection in the first place, either antibodies or T cells.”

Encouraging as that is, the findings definitely carry some important implications if they turn out to be valid. If people really are experiencing drastic drops in antibody levels a few months post-infection, then using antibody tests as a way to identify people supposedly “safe” from the virus will fall flat on its face. It could also mean that we have to screen people early for antibodies if we want to measure just how widespread the virus has been in an area and that later screening shouldn’t be considered reliable.

It also further throws into the question the notion of using herd immunity – the idea that enough immune people in a population will prevent the virus from spreading to vulnerable segments not yet infected – to get through this pandemic, according to Bruce Walker, an immunologist and director of the Harvard University Center for AIDS Research.

“This suggests that a significant number of people that generate antibodies are going to lose those antibodies. And then we’re left to speculate. Does that mean that those people are now infectable? Does that mean that there might still be some level of immunity that a repeated infection would be attenuated and perhaps not even noticed?” Walker told Gizmodo. “We just don’t know at this point.”

At the same time, Walker added, this study can only tell us something about natural immunity, not the sort of immunity that we’re hoping to get from a potential vaccine someday. Some vaccines do provide a more robust and longer-lasting immune response than a natural infection.

The question of reinfection and immunity is something of a moot point right now, though, since the vast majority of people in the world haven’t yet contracted the virus. Just this week, the US Centers for Disease Control and Prevention estimated that 5% to 8% of people in America, the country with the most cases of covid-19, have suffered from it so far. Those not yet infected include the millions of people over 65 or who live with chronic conditions like diabetes that make them more vulnerable to a severe or life-threatening infection. For most of us, it’s the first round of the virus that we should be worried about. But for people who have survived an infection, the findings may emphasise the need to still play it safe.

“One take home message is that if you have been infected already, don’t assume that you’re no longer infectable,” Walker said. “Use masks, distancing, and sanitising still.”

Featured image: Asaad Niazi / AFP (Getty Images)