Rumours about COVID-19 reinfections have been circulating for months, and it's hard not to be concerned as patients who appear to recover from the virus suddenly test positive again, sometimes months later. They're rare, and doctors believe they can be put down to a few likely scenarios: faulty testing, or the virus lingering or "hiding" in the body only to resurface later.
It is true that scientists are still studying our immune system's response to coronavirus infection. As with many things during this pandemic, there's a lot we don't know, especially when it comes to immunity — how long it lasts, what it looks like in our bodies, and how that plays into the idea of herd immunity. Concerning rumours aside, there's good news on this front: scientists are optimistic that infected people do have long-term COVID-19 immunity. "All the pieces are there to have a totally protective immune response," University of Washington immunologist Marion Pepper, PhD, told the New York Times in an article about long-lasting immunity. There's some science to get through first, but bear with us — it's important.
Coronavirus Antibodies Are the First Line of Defence
Most of the news around immunity has centred around antibodies, proteins produced by B cells (more on those later) that protect your body against pathogens. COVID-19 antibodies do confer immunity, but the question is how long it lasts for — and science doesn't give us a concrete answer yet. Current research shows that antibody levels drop off within two to three months after infection, but doctors have stressed that this is a fairly normal immune response. "This is a famous dynamic of how antibodies develop after infection," said Michael Mina, MD, PhD, an immunologist at Harvard University, in a separate New York Times article on COVID-19 "reinfections." "They go very, very high, and then they come back down." This doesn't necessarily mean the body is defenceless to the virus.
Another significant point: scientists don't know what antibody level confers immunity for COVID-19. "Even small amounts of antibodies can potentially still be protective," said Mala Maini, MD, PhD, a viral immunologist at University College London, in an interview with Nature. In that case, even low numbers of antibodies, like the ones found in patients months after infection, could still provide some protection. And for the record, it's not just acute COVID-19 illness that can produce these lasting immune responses — some studies are reporting that even mild COVID-19 infections can do so.
T Cells and B Cells May Provide Long-Term Protection
Antibodies aren't the body's only form of protection: T cells are an important type of white blood cell that scientists are looking at. T cells can be split into a few different groups: the first kind, CD4 helper T cells, identify pathogens in the bloodstream and, when they find one, prompt B cells (another type of white blood cell) to begin the antibody-making process to deal with the virus. Another kind of T cell, called CD8 killer T cells, identifies infected cells through a slightly different process and kills them on the spot.
So these cells help to fight off an active infection, but what do they have to do with COVID-19 immunity? For starters, both B cells and killer T cells have the ability to mature into longer-lasting forms of protection. B cells can become plasma cells that keep producing antibodies for a few weeks, then relocate to the bone marrow for long-term protection; or they can morph into memory B cells, which are part of the body's "surveillance system," according to Medical News Today. If your body is infected again, the memory B cells remember the virus and flag down a helper T cell, which can restart the whole antibody production process. Killer T cells do something similar by turning into memory CD8 T cells, which "provide fast and long-lasting responses" if the virus returns.
For immunity, the good news here is that plasma cells, memory B cells, and memory CD8 T cells are all meant to stick around and protect the body for a long time, setting them apart from the antibodies that wane when the infection is taken care of. For COVID-19 specifically, a recent study in Cell isolated T cells from the blood of long-recovered patients and found that they responded actively to the presence of coronavirus pathogens, replicating into a protective line of defence. Notably, coronavirus-specific T cells were also found in the blood of people who'd been exposed, but tested negative in an antibody test — meaning that asymptomatic people may have some protection as well, even if they're no longer producing (and testing positive for) antibodies. Smita Iyer, PhD, an immunologist at UC Davis, described such results as "very promising," in the NYT immunity article. "This calls for some optimism about herd immunity, and potentially a vaccine," she continued, adding that this study shows evidence that "You can still get durable immunity without suffering the consequences of infection."
Antibodies themselves can also help us figure out whether B and T cells are lingering in the body. Specifically, the fact that they're still present months after the infection (as we talked about earlier) suggests that B and T cells are persisting, creating antibodies, and continuing to do their jobs. "Things are really working as they're supposed to," said Deepta Bhattacharya, PhD, an immunologist at the University of Arizona, in the NYT article.
Can You Become Immune to COVID-19?
To definitively prove immunity to COVID-19, scientists will need direct evidence that people can fight off the virus after a second exposure. We don't have that yet, though in two small studies, rhesus macaques were immune to the virus after being exposed once. All in all, right now the signs are good, showing evidence that our bodies can stay immune to the virus for at least a few months, and potentially longer.