The idea that a community either has herd immunity and is protected from a disease or does not and is thus susceptible to outbreaks is so attractive that even epidemiologists who know better (including me) tend to talk about it this way. But herd immunity — or community immunity, as I prefer to call it — is really a much broader concept. It refers to any reduction in the efficiency of disease transmission due to the presence of immune individuals in the community. In other words, it’s the help that population immunity gives us in disease control.
To understand how this works, it is useful to think about how immunity impacts transmission. Let’s suppose that if everyone that an average person who’s infected with covid-19 meets is susceptible to infection, on average, they infect three people over the course of their illness (by this point, many may recognize this value as the basic reproductive number, R0). Now suppose that two out of those three people they would have infected are already immune; then that average person with the virus would only infect one new person. If community immunity is any greater than that, then most people won’t pass on the disease at all, and covid-19 will start to die out.
When people talk about herd immunity, this threshold is usually what they mean. It is important because when it is reached, communities are protected from new introductions of covid-19 from the outside, as new cases will, on average, fail to cause further infections. The community, in a sense, thereby becomes “immune” to covid-19, even if some people are still susceptible, and everyone can resume their pre-pandemic lifestyles.
But we need not achieve that magical threshold where the virus dies out locally to benefit from community immunity — there are lower levels of immunity that can also make life better for everyone.
Basically, all of the control measures we have been using to combat the pandemic, from masks to restaurant closures, have the same goal: reduce contacts that result in disease transmission. Specifically, we want to reduce these infectious contacts to the point where the average number that a person with the virus has is less than one. Suppose that to get there, at the start of the pandemic, a community had to mandate universal masking, close schools and end in-person dining at restaurants. But now imagine that one-third of the population is immune due to prior infection or vaccination. Because of this community immunity, we have to stop 33 percent fewer infectious contacts through social-distancing measures — and perhaps we can now open schools or allow dining at restaurants (though we still probably can’t do both).
These incremental benefits of slowly accumulating community immunity mean that as the pandemic goes on, control will get easier. Suites of measures that may have been ineffective in halting the epidemic when the pandemic began might work better when 20 or 30 percent of the population is immune. Places such as New York City that suffered horribly in the spring will likely have an easier time of it in future pandemic waves, even if they are not completely protected from outbreaks.
Waning immunity has been a constant concern when considering the possibility of herd protection to covid-19. Other coronaviruses do not appear to confer permanent immunity, and reinfection with covid-19 has been recently documented. Even if we aren’t permanently protected against covid-19 after infection, though, immunity will not go away all at once for everyone, and we will likely be at least partially protected for years. So when the disease returns, there will be some level of community immunity to help combat it, and less extreme control measures will be needed. Over time, this process should lead to less severe covid-19 epidemics. Combined with changes in the age distribution of infection, that may even make the disease disappear as a major threat to human health.
But whether community immunity is enough to bring down infections depends on context. If the situation changes, due to seasonal effects or changing behavior, levels of community immunity may no longer be adequate to prevent epidemic growth, and covid-19 can still come roaring back.
This means we need to be careful about declaring the epidemic over in a given location simply because cases have started to decline. What may be adequate immunity to stem the virus in summer may not be enough to stop epidemic spread in the winter — a phenomena seen in each of the four modern influenza pandemics, where an initial summer wave quickly receded only to be followed by a large epidemic in the fall or winter. Likewise, as control measures are lifted or people just become less cautious, a community where the virus was on its way out might become fertile ground for a resurgent epidemic.
Even if we are at a point where herd protection has been “achieved,” this does not mean there are no benefits to additional immunity. Let’s say community immunity is around 70 percent, and the average number of infections a case causes is slightly less than one. That number is just an average, and sometimes, during so-called superspreading events, 10 or more people might be infected by a single person with the virus — enough to spark a local outbreak (we see this frequently with measles, and have seen it already in the coronavirus pandemic). If community immunity could be increased to 90 percent, this same event would only cause three infections, and we would be far less likely to see any large outbreak.
The long fight against covid-19 is far from over, and we will likely be living with the virus in some form for years to come. But each round we fight with this virus will get easier due to the accumulating effects of community immunity, even if we never reach a promised land where herd protection makes epidemics a thing of the past. Hopefully, we will soon have a vaccine that accelerates this process, but even when one arrives, it will take time to distribute to large portions of the population. As we wait, every day and every person who becomes immune makes the fight easier. This growing protection will allow us to slowly roll back control measures and return to a more normal life. But we should remember that community immunity is a journey not a destination, and we have to proceed carefully on our voyage back to the way things were before the virus spread around the globe.