Half of Americans have now received at least one dose of a Covid-19 vaccine, and with children ages 12 to 15 now eligible for immunizations, the United States appears to be getting the coronavirus pandemic under control. But despite the tremendous progress, it’s still not clear that the nation will ever truly reach the herd immunity threshold — the point at which enough people in a population are immune to a pathogen to limit its spread.
More important, we may not need to achieve that goal in order to escape the pandemic.
Reaching the herd immunity threshold doesn’t guarantee that people cannot be infected by a pathogen. But the closer a community gets, the more transmission slows down, which benefits everyone. The current rate of new Covid-19 cases in the United States right now shows this phenomenon in action. Forty percent of the U.S. population is fully vaccinated, and the number of new cases of Covid-19 reported each day is now one-tenth of what it was at the pandemic’s apex in January.
Last year many scientists suggested the herd immunity threshold would be reached when 60 to 70 percent of the population was immune, either because of vaccination or exposure to the virus. Scientists have now revised this number upward, to at least 80 percent. But there is no single, universal herd immunity threshold. The number depends on the transmissibility of a disease, its variants and the characteristics of the population it’s invading.
Transmissibility is not fixed: It can vary based on a population’s behavior, demographics and health. We’ve seen this in action with Covid-19, which has spread far more quickly in some populations, as a result of differences in disease-mitigation efforts, housing density, age, occupations and health conditions. New coronavirus variants such as B.1.1.7, which is now the predominant strain in the United States, are more transmissible. And that increases the herd immunity threshold.
Still, countries with high rates of vaccination like the United States may be able to move past many of the restrictions of pandemic life before reaching that goal. The seasonal flu, a virus that has a lower fatality rate but is also highly transmissible, offers a good example of how this works: Most years the flu does not become a disastrous epidemic, even though vaccination rates aren’t sufficient to get to the threshold and even though the efficacy of flu vaccines varies from year to year.
With Covid-19, some communities will likely reach the herd immunity threshold, even if the entire United States does not. Universities that require their students and staff members to be vaccinated when they return to campus, for example, will achieve strong protections against the coronavirus even if it doesn’t disappear from their surrounding area.
Of course, no one will be fully safe from the coronavirus until everyone in the world is protected. Right now, the United States is among a few privileged countries with an adequate supply of vaccines. Declaring the pandemic over once the risk subsides for Americans would be premature and a great moral failure. More infectious variants will continue to emerge in populations where vaccination rates are low and the virus can find new hosts in which to evolve.
Even in communities with high levels of immunity, new variants can infect people who are not vaccinated, whose immunity is waning or whose immune systems did not respond adequately to the vaccine. The approved vaccines appear to protect against the main variants of concern, but future adaptations of the virus could evade this immunity. That’s why it is so important to vaccinate as much of the world’s population as possible, as quickly as possible.
Even with adequate vaccine supply, the United States as a whole may never reach the herd immunity threshold — because of vaccine hesitancy, inequities in health care access and coronavirus variants. But rather than fretting about this, Americans should stay focused on the bigger picture: Every person vaccinated is one who is very unlikely to get infected and spread the virus to friends and family.
Erin A. Mordecai (@morde) is an assistant professor of biology at Stanford University. Mallory J. Harris (@malar0ne) is a Ph.D. candidate at Stanford University, where she studies infectious disease. Marc Lipsitch (@mlipsitch) is a professor of epidemiology at the Harvard T.H. Chan School of Public Health.
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