The answer is, it’s not that simple. And just as important, it may not matter that much.
Herd immunity is not a clear line. The virus will not be eradicated the moment we administer the shot that gets us to herd immunity. The term describes the inflection point at which each infection results in less than one additional infection and outbreaks sputter out. You can think of it like a wildfire surrounded by firebreaks, where the blaze ultimately burns out without additional interventions.
It’s not hard to see how it came to be viewed as the pandemic finish line, but that line has shifted. Estimates of herd immunity have been adjusted upward from the 60 percent to 70 percent that we expected last year, to 80 percent more recently, largely because of new variants that are more contagious. The threshold is determined by factors beyond vaccination, including immunity due to prior infections, seasonal effects such as humidity and time spent indoors, who is immune and who isn’t, and broader behavioral factors such as whether people are engaging in any public health measures such as mask-wearing and social distancing.
Real-world evidence from Israel and the United Kingdom suggests that even without hitting the herd immunity threshold, vaccination can drive infections way down. Why? Because immunity in a population is not like an on-off switch. As populations begin to build up immunity, infection spread begins to slow. If people practice even modest levels of public health measures such as mask-wearing indoors or avoiding large crowds, it may be enough to drive infection numbers down substantially. To stretch the fire metaphor, even if you don’t have the flames surrounded on all sides, a little bit of a drizzle combined with some firebreaks may be enough to keep it from burning out of control.
So why are some observers worried that herd immunity may be out of reach? Rates of vaccination are declining in the country. Americans eager to reap the benefits of vaccination have all largely gotten their first shot, and the 45 percent or so of adults who remain unvaccinated may have less easy access or more questions about whether they should be getting vaccinated. Unsurprisingly, access and vaccine confidence are not evenly distributed across the country. Several states in New England have surpassed 70 percent of eligible adults vaccinated and look quite capable of reaching a very high degree of population immunity. Other states, particularly in the South, lag well behind, having vaccinated just over 40 percent of adults. The state and federal governments have a lot of work to do to help close the gap.
One way to increase the number of people who are inoculated is to authorize vaccines for use in 12- to 15-year-olds, as the Food and Drug Administration seems set to do with the Pfizer-BioNTech vaccine. This age group makes up about 5 percent of the population, and vaccinating a large majority of this group will help. Over time, we will be able to vaccinate other children, which will help build up population immunity further. And as time goes along, some regions of the nation may see something close to herd immunity with very few infections, rare outbreaks and only very modest measures needed to keep infections under control. In states with lower rates of vaccination, we are likely to see large outbreaks that run for longer before they are contained by public health interventions. But overall, over time, we should see declining numbers, small case counts and few concerns about overwhelming the health-care system.
Finally, even if we vaccinate enough people at home to reach herd immunity, we will still not have seen the end of the coronavirus. There will be communities within the country with low vaccination rates where the infection will become endemic. And every once in a while, an infection or two that originates in these communities will spill over, causing small outbreaks in other states and localities. Of course, because this is a global pandemic and much of the world remains unvaccinated, we will see infections from other nations landing on our shores, often new variants, including even ones that challenge the efficacy of our vaccines. Ultimately, however, we will find ourselves in a situation in which the virus is never gone but some communities are affected more than others. Modest public health measures such as wearing masks during large indoor gatherings such as sporting events and concerts, better ventilation in indoor spaces, and testing in high-risk situations become the norm. We may not have herd immunity, but life will settle into a new normal.
The coronavirus pandemic marks the clearest dividing line in most of our lives. But while the pandemic had a clear beginning, the ending will be much more gradual. As vaccination rates slow, we will require a resource-intensive ground game to reach more and more unvaccinated people and push us toward herd immunity. It is indeed possible that we may not reach that elusive threshold, or we might get there for a period only to have waning immunity, new variants or changes in behavior drop us below that threshold. But with infection numbers low and modest mitigation efforts in place, we will see small outbreaks that will affect the unvaccinated and burn out quickly. The terrifying surges of the past year will be behind us. And the things we value most in our lives — time with family and friends, social gatherings with colleagues, entertainment and sports — things we have missed so much, will be possible and safe.
This pandemic will end when the risk it poses, and the strategies necessary to mitigate that risk, fade into the background and become part of normal life. To get there, we should focus less on the herd immunity threshold, vaccinate more people and get on with our lives. As the old saying goes, pandemics end with a whimper, not with a bang. This one, too, will end. With a whimper.