But first, what is herd immunity? Here’s a list of short videos that illustrate the concept (I prefer video #5, above, because who doesn’t love gummy bears?). If you want to start planning for a herd immunity classroom activity with your students in the fall, consider using this good one from the Centers for Disease Control.
In short, the idea behind herd immunity is that when enough people in a community are immune to an infectious disease, the disease can’t spread. If there are very few susceptible people, an infected person is unlikely to come into contact with one, so the virus will spread slowly, or even disappear. The term “herd immunity” is used because even if a small percentage of people remain susceptible, they will still be protected, just as a baby elephant can be protected from a lion attack by the adults in the herd. (Warning: the video is kind of scary. At least to me.)
Just what percentage of the population needs to be immune to provide herd immunity (also known as the herd immunity threshold, or HIT) depends on the virus. Measles and pertussis (whooping cough) are so infectious that the threshold for herd immunity is over 90%. The threshold for Ebola, by contrast, is less than 60%. (Bonus question: why might thresholds be different? Think about how the virus spreads and how contagious it is.) We don’t yet know what the herd immunity threshold is for the virus that causes COVID-19, SARS-CoV-2 — in April, most estimates, based on what was known about the virus’s transmission rate, were around 70%. A New York Times article published on August 17, 2020, and updated on August 31, described interviews with more than a dozen scientists who are modeling coronavirus herd immunity thresholds. The article does not detail each estimate, but suggestst that most estimates were around 60%, with a minority of researchers arguming for an even lower threshold of 10-20%.
Of course, models about the coronavirus must by necessity be based on many assumptions about variables that are far from well-established at this point, which explains why the results of the models were so different. However, it would be very good news if the threshold does turn out to be 50% rather than 70%, although the cost of reaching that threshold without a vaccine is still high.
Some simple math shows the human cost of reaching that percentage of immune people. The U.S. population is about 328 million, and 50% of 328 million is 164 million, so we would need to have 164 million Americans recovered from COVID-19 to approach the herd immunity threshold, assuming one can only contract the disease once. As of September 3, 2020, we’ve had about 6.1 million confirmed cases in the U.S. That case count is probably a huge underestimate, since many, many infected people, especially in the early days of the pandemic, never received tests and so aren’t included in that total count. But even if the number of cases is 10 times higher than the official count—say 60 million Americans—that’s still a long, long way from 164 million. And remember that there have been 186,000 deaths so far, which suggests that there would be over a 500,000 deaths on our way to the herd immunity threshold. This is a dramatic improvement, however, over the likely number of deaths — more than 1,000,000 — on the way to a herd immunity threshold of 70%.
A quicker way to reach the herd immunity threshold — whatever it might be — is through vaccination. But there is no vaccine yet. And even if one becomes available by the end of 2020, it will take quite a while to reach vaccination levels of 50%. So herd immunity through vaccination is not just around the corner.
A final complication is that herd immunity requires that people who become immune—either by contracting the disease or through vaccination—stay immune permanently, or at least for enough time for the virus to burn itself out. It is worrisome that we are so far unsure whether being infected by SARS-CoV-2 provides lasting immunity. There have been anecdotal reports of people being infected more than once within just a few months (although those infected a second time seem to be asymptomatic and possibly not contagious) but just one confirmed case of an individual who tested positive in late March and again in mid-August. On the other hand, there are some indications that people who have experienced mild or asymptomatic infections do not develop protective immunity. Resolving this question will be a major focus of research in the coming months. It appears likely, though, that achieving herd immunity, in the end, will require an effective vaccine, administered to at least 50% of the population.
That’s all pretty sobering. But I’d like to end on a more positive—and empowering—note. Achieving the herd immunity threshold is a very lofty goal. At that level, life really could go back to normal, including open schools with no restrictions, large gatherings both indoors and out, and all the other activities we used to take for granted. But, as we’ve been saying from the beginning, individual behavior can vastly slow the spread of the virus even in a mostly susceptible population. Social distancing, wearing masks, and spending our social contact budget wisely will all slow the transmission of the disease and minimize the number of cases and deaths until a vaccine or a treatment becomes available.