Welcome to the Great Reinfection

If you’re unfortunate enough to have had an intimate encounter with the dreaded Sars-CoV-2 virus, I’m afraid your dalliance with it might not have been your last. Get ready for round two (and three, and maybe four—maybe ad infinitum). Welcome to the Great Reinfection.

In the early months of the pandemic, reinfections were a remarkable rarity, even making global news when discovered. “When the pandemic first started, everybody assumed that once you got it, you were done,” says Juliet Pulliam, director of the South African DSI-NRF Centre for Epidemiological Modelling and Analysis at Stellenbosch University.

Two years and some change in, that novelty has largely evaporated. A perfect storm of waning immunity, loosened restrictions, and an extremely transmissible variant making the rounds has meant reinfections are the new normal for many. But even setting aside these factors, it makes sense that there are now more reinfections than ever. At this stage of the pandemic, repeat infections would always have been more common than before, owing to the sheer number of people who’ve had Covid-19. You can’t get reinfected unless you’ve already been infected in the first place. 

Beyond that basic math, it’s not really surprising that reinfections are happening, says Aubree Gordon, an infectious disease epidemiologist at the University of Michigan. “The virus has changed a lot,” she says. If you were infected with an earlier variant, Omicron is like that variety wearing a wig and makeup—making it largely unrecognizable to our bodies’ immune defenses and harder to stave off.

But if reinfections are now part and parcel of the future of the pandemic, just how common are they? An exact number is hard to pin down, thanks to a nosedive in testing and reporting that has made tracking all kinds of Sars-CoV-2 infections much trickier. Plus, not everyone defines a reinfection the same way; health authorities in the UK, for example, require at least 90 days to elapse between a first and second infection for this to count as a reinfection. Others, like the European Centre for Disease Prevention and Control, use a shorter 60-day minimum between infections. 

In England, close to 900,000 possible reinfections have been identified since the beginning of the pandemic. Of those, over 10,000 were a third infection, and almost 100 were a fourth. 

Pulliam’s own work has tried to put a number on how many infections are actually reinfections. She and her team found that as of last week, around 15 percent of current infections in South Africa are reinfections. “And that is almost certainly an underestimate,” she cautions, “because our surveillance isn’t great, and we probably missed a lot of people’s first infections.” But to answer just how prevalent reinfections are—in the grand scheme of things—Pulliam uses two words to sum it up: fairly rare.

She and her team have also investigated just how much Omicron has shaken things up. They started monitoring reinfections towards the end of the Beta wave in South Africa (which peaked in January 2021), looking at over 100,000 suspected reinfections. They found that the protection an initial infection offered against reinfection stayed the same all through the Beta wave and all through the Delta wave that peaked the following July. And then Omicron hit. The risk of reinfection steadily rose and stabilized at a higher number.