Immunity is a person’s ability to fight off pathogen infection after exposure. Whether from prior disease or a vaccine, immunity prevents a person from being infected.
Herd immunity is a different concept.
Misconception: Herd immunity is just the turning point of an epidemic.
Reality: The concept of herd immunity is primarily about protection from a new exposure.
The idea is related to the number of people in a population who are immune. Pathogen transmission will be lower in a population when some people are immune and others are not compared to a population where everyone is susceptible. That is, some people in the population are indirectly protected by the immunity of others. Most interestingly, there is a threshold level of immunity in a population that, once achieved, will prevent successful pathogen invasion.
By analogy with the concept of a person’s individual immunity, we say the population has achieved herd immunity when this threshold is reached, because it prevents the population from being infected.
With Covid-19, many people have used the phrase “herd immunity” to refer to something else, namely some important point in the epidemic. But, if herd immunity is really about a population’s resistance to invasion (when a pathogen is rare in the population), why has the phrase been commonly used to describe Covid-19?
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The answer is that, in many situations, an epidemic will reverse course once the herd immunity threshold has been achieved. The reason a pathogen can’t get a toehold in a population with herd immunity is that enough people are immune that when the occasional infection does arise, there are not enough potentially infectious contacts to keep transmission going. On average, every case gives rise to fewer than one secondary case. So also when the immune threshold is reached in the middle of an epidemic. If, on average, every case gives rise to fewer than one secondary case, the epidemic must begin its decline.
Misconception: Herd immunity marks the end of the pandemic.
Reality: Herd immunity is only the beginning of the end.
Herd immunity can protect a population if it is achieved before pathogen transmission begins. If herd immunity is achieved through a combination of natural infection and vaccination — like in the case with Covid-19 — the path to eradication could still be quite long. The length of a pandemic’s decline depends, among other things, on how many cases there were at its peak. Covid-19 will burn out only when both adequate population immunity and a low level of transmission have been achieved.
Misconception: Reaching herd immunity means a population can return to normal.
Some of the biggest questions people in America have been asking is whether herd immunity has been achieved, and, if yes, does that mean we can return to normal, pre-Covid-19 activities.
This is a multi-part question and it has a multi-part answer. At one level, yes, we have achieved the herd immunity threshold and we know this because cases have been going down consistently. But this answer is actually too simple. Cases are going down because of both herd immunity and because much of the population continues to practice protective measures, like physical distancing and wearing a mask in public. The threshold level of immunity in a population taking protective measures will be different than in a population that is not.
It might make sense, then, to talk about two different thresholds for herd immunity. The first threshold to be reached is the amount of immunity required for the epidemic to turn around in the presence of protective behaviors. It is reasonable to conclude that, for summertime (when transmissibility of respiratory infections is generally lowest), we’ve already reached that threshold in the US. A second threshold is reached when there is enough immunity in the population that the epidemic can’t keep sustaining itself even without protective behaviors. It’s not yet clear whether or not we’ve reached this threshold. But, this is the threshold that has to be achieved to truly return to pre-Covid-19 behaviors without risking a resurgence.
Finally, a few words of caution. There are several things that complicate herd immunity beyond what I’ve described here. An important one is clustering among people who are not immune, for instance, because they have not been vaccinated. Transmission can still be expected in such pockets where the reproductive number is locally above one.
Second, there are seasonal differences in transmission. We can expect transmission to tick up next winter.
Finally, several different variants of SARS-CoV-2, the virus that causes Covid-19, are now circulating. It is to be expected that immunity to one variant will not necessarily confer full immunity to the others (although partial immunity is likely). Just as the rise of variant strains can compromise our individual immunity, so also variants will compromise our herd immunity. It follows that even after our population has achieved both low transmission and herd immunity, we will need to remain vigilant against new variants and develop new vaccines when they arise.