Five reasons why COVID herd immunity is probably impossible

New variants change the herd-immunity equation

Even as vaccine roll-out plans face distribution and allocation hurdles, new variants of SARS-CoV-2 are sprouting up that might be more transmissible and resistant to vaccines. “We’re in a race with the new variants,” says Sara Del Valle, a mathematical and computational epidemiologist at Los Alamos National Laboratory in New Mexico. The longer it takes to stem transmission of the virus, the more time these variants have to emerge and spread, she says.

What’s happening in Brazil offers a cautionary tale. Research published in Science suggests that the slowdown of COVID-19 in the city of Manaus between May and October might have been attributable to herd-immunity effects (L. F. Buss et al. Science 371, 288–292; 2021). The area had been severely hit by the disease, and immunologist Ester Sabino at the University of São Paulo, Brazil, and her colleagues calculated that more than 60% of the population had been infected by June 2020. According to some estimates, that should have been enough to get the population to the herd-immunity threshold, but in January Manaus saw a huge resurgence in cases. This spike happened after the emergence of a new variant known as P.1, which suggests that previous infections did not confer broad protection to the virus. “In January, 100% of the cases in Manaus were caused by P.1,” Sabino says. Scarpino suspects that the 60% figure might have been an overestimate. Even so, he says, “You still have resurgence in the face of a high level of immunity.”

There’s another problem to contend with as immunity grows in a population, Ferrari says. Higher rates of immunity can create selective pressure, which would favour variants that are able to infect people who have been immunized. Vaccinating quickly and thoroughly can prevent a new variant from gaining a foothold. But again, the unevenness of vaccine roll-outs creates a challenge, Ferrari says. “You’ve got a fair bit of immunity, but you still have a fair bit of disease, and you’re stuck in the middle.” Vaccines will almost inevitably create new evolutionary pressures that produce variants, which is a good reason to build infrastructure and processes to monitor for them, he adds.