As with the many other questions that still remain about the coronavirus, no one is precisely sure of the answer to one of the key questions: how many people will need to be vaccinated to achieve herd immunity—the threshold at which there are enough people immunized that it indirectly protects the rest of the yet-unvaccinated public—and fully end the pandemic. In December, National Institute of Allergy and Infectious Diseases Director Anthony Fauci estimated that the cutoff was at least 75%, and closer to 85%.
But, with cases and deaths now spiking to record numbers by the day, a more immediate target is the threshold at which enough people receive the shot that the rate of transmissions begins to slow. Experts say it’s not yet clear when that might happen, especially when considering other factors, such as the order of vaccine priority and the appearance of more contagious variants of the virus.
At what level of vaccination could we see a downtick? “We don’t know that, but I would think it would have to be pretty high,” says William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, who guesses it will require more than 50% of the population. (The CDC estimates that about 9 million have now received both doses of the vaccine in the U.S., about 2.7% of the population.)
So far, we haven’t experienced much large-scale immunity from those who have had the virus and thus some antibody protection. Current estimates are that about 20% to 25% of the U.S. have had the virus (we still don’t know how long immunity lasts; most estimates right now assume it’s more than eight months). If it’s on the shorter end, we can’t rely on that being a considerable factor in adding to the greater population’s immunity. “That’s playing a role, but that’s a moving target,” says Keri Althoff, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.
That means vaccinations are fundamental to slowing transmissions, but that slowdown will also depend on which people are targeted. We may not see a downturn in infection rates until essential workers, who are generally in Phase 2 of vaccination rollout plans, get their shots. “Those are the people who are in the community who are still moving about,” Althoff says. “Those individuals becoming vaccinated, and building up a higher level of population immunity, will likely have a bigger impact on the number of infections.”
That’s not to say that the current phasing structure is wrong. Older populations, who are first in line to receive vaccines, aren’t likely to drive down cases, because they’re more isolated and don’t interact as much with the rest of the public. But it’s possible that deaths will start leveling off and dropping once more and more people over 75s are vaccinated. “So, it’s about who are you vaccinating, and when,” Althoff says, “as to whether or not you will see specifically deaths go down, as to whether or not you will see specifically infections go down.”
We could also see various subpopulations around the country gaining protection and thus driving down cases, while others remain high. If some geographic areas embrace vaccinations more than others, it may see a drop in cases sooner. “That could even extend down to micropopulations,” Schaffner says, such as within a school system, or a church community where a pastor encourages vaccination. “I think this won’t be uniform,” he says of the possible disparity.
There are other constantly changing factors—those “moving targets”—to consider. One is mutations of the coronavirus, such as the so-called U.K. strain, which could be 40% to 70% more infectious. This wasn’t taken into account in the CDC’s scientific modeling for the herd immunity threshold, and it will need to be for future models. But it could mean that the thresholds, for both herd immunity and the drop in cases, will be pushed higher by more infectious variants. “The more contagious the virus,” Schaffner says, “the higher the proportion of the population you have to vaccinate in order to reduce transmission.” Measles, for instance, is much more contagious, and its herd immunity threshold is around 95%, versus the seasonal flu’s, at around 33% to 44%.
Ultimately, vaccinations are only one part of a multifaceted public health strategy. We will need to continue wearing masks, and practice social distancing and good hand hygiene, to ensure that cases do start decreasing. Schaffner mentions a popular analogy: protection is like a series of slices of Swiss cheese packed together, where each one has holes that the virus can get through. We close those gaps with each one of these preventative measures, and they’re only completely effective in driving down transmissions if combined.
Althoff agrees. “Those are not things we throw by the wayside at all right now,” she says. “Vaccination is a tool we’re adding to our toolbox.” And she urges people to make sure they get that shot. “Making the choice to be vaccinated is being a part of that solution to help the population reach a level of protection,” she says, “so that we can get the virus stopped.”