While COVID-19 vaccines are very good—even unexpectedly good—at preventing disease, they are still unlikely to be good enough against transmission of the virus, which is key to herd immunity. On the whole, we should expect immunity to be less effective against transmission than against disease, to wane over time, and to be eroded by the new variants now emerging around the world. If vaccine efficacy against transmission falls below the herd-immunity threshold, then we would need to vaccinate more than 100 percent of the population to achieve herd immunity. In other words, it becomes downright impossible.
Even if herd immunity remains theoretically within reach, 15 percent of Americans say they will never get a COVID-19 vaccine, making that threshold all the harder to hit.
The role of COVID-19 vaccines may ultimately be more akin to that of the flu shot: reducing hospitalizations and deaths by mitigating the disease’s severity. The COVID-19 vaccines as a whole are excellent at preventing severe disease, and this level of protection so far seems to hold even against a new coronavirus variant found in South Africa that is causing reinfections. This, rather than herd immunity, is a more achievable goal for the vaccines. “My picture of the endgame is we will, as fast as we can, start taking people out of harm’s way” through vaccination, says Marc Lipsitch, an epidemiologist at Harvard. The virus still circulates, but fewer people die.
At the same time, we don’t need to hit the herd-immunity threshold before transmission begins to slow. With less transmission, fewer people will get exposed, and if those who do are vaccinated, even fewer will become seriously sick or die. The pandemic will slowly fade as hospitalizations and deaths inch down.
We likely won’t cross the threshold of herd immunity. We won’t have zero COVID-19 in the U.S. And global eradication is basically a pipe dream. But life with the coronavirus will look a lot more normal.
The variants are the newest and potentially most pressing challenge to herd immunity. As the virus evolves, our vaccines and our immunity will continually have to catch up. “The trillion-dollar question for where we go from here is this relationship we have with the variants,” says Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
For about a year, the coronavirus seemed to gain mutations at a steady and unspectacular rate. But recently, new variants have accumulated an unusually large number of mutations, and worrisome new data are now coming out every week.
The South Africa and Brazil variants, which are the most concerning for immunity, share several mutations, including a key one called E484K. These mutations change the shape of the virus’s spike protein, making it less recognizable to the immune system. In South Africa, the variant is reinfecting some people who had COVID-19 before. On Sunday, the country even paused its rollout of the AstraZeneca vaccine after data came to light suggesting that it does not protect against mild or moderate disease from the new variant. And the Johnson & Johnson and Novavax vaccines, which were trialed in parallel in multiple countries, also seem less effective in South Africa—falling from 72 to 57 percent efficacy and 89 to 49 percent efficacy, respectively. In Brazil, the region around Manaus is experiencing a huge second wave of COVID-19 despite high levels of previous immunity from a first wave last year. The new variant in Brazil may be responsible.