Abstract
Many of the articles published in this issue are related to the theme of antibodies and immune memory, which is particularly relevant at present as coronavirus disease 2019 (COVID-19) vaccines continue to roll out in many parts of the world. Since before these vaccines were developed or tested, the topic of COVID-19 herd immunity has been a matter of intense interest. Herd immunity is exactly what the term implies, the level of immunity present in the community against a specific pathogen, and such as many things COVID-19-related, this topic is currently being discussed by as many nonscientists as scientists.
When it first became clear that COVID-19 vaccines were going to arrive faster than we all hoped or expected, the question of herd immunity arose rapidly. Unfortunately, there are numerous interpretations of what the term herd immunity actually means. Some believe that herd immunity is a state that is reached when a sufficient portion of the population achieve immunity against a pathogen. By this definition, which is shared by the World Health Organization, both immunity acquired due to natural infection and vaccine-induced immunity comprise herd immunity. Others believe that herd immunity should only refer to vaccine-induced immunity, but this could in part reflect a push to combat the notion that we should let viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) freely infect the population to achieve herd immunity naturally. A thorough environmental scan of the various responses to COVID-19 would reveal a very broad spectrum of measures, or even a lack thereof, employed by different countries to deal with the spread of SARS-CoV-2 and its variants of concern. Irrespective of the preferred definition of herd immunity, we can all agree that we want to get there, but there lies the real question, what is the target for herd immunity? How will we know when we are there? What determines the level of herd immunity we need? Will 75%, 80%, and 85% vaccination of eligible and desiring individuals be enough to establish an effective herd immunity? Or can we even put a number on herd immunity? Does percentage vaccination actually reflect the extent of herd immunity?
The answers to all of these questions will soon be apparent, but until then, we can only consider what we know, and unfortunately, what we know further complicates the hypotheses around herd immunity. Unfortunately, very few factors relevant to this discussion are straightforward yes or no issues. For example, we know there is a spectrum of immune response to SARS-CoV-2 infection depending on multiple factors, including age and underlying health issues. We have seen infected individuals having no symptoms, but we also have observed individuals progress rapidly to death. We are seeing a range of responses to the various vaccines, with some people appearing to be protected from actual infection even after a single dose, whereas others have experienced vaccine failure despite receiving two doses. We have to assume that any immunomodulatory therapy can potentially impact the response to the vaccines. Individuals who recently survived cancer might also respond poorly to COVID-19 vaccines. Then we must remember that higher levels of herd immunity are required for viruses with higher R0 values, and we know with certainty that some of the variants of concern have higher R0 values than the original virus. It is clear that not all vaccines are created equally, but it is also apparent that not all responses to vaccines will be generated equally. Therefore, in the context of herd immunity, we have to consider that there will be a broad spectrum of immunity among individuals receiving vaccines, which will ultimately impact how efficient our herd immunity can be.
With all of these questions still unanswered or at least waiting to be understood, it is very difficult to know what to aim for regarding herd immunity. The virus will tell when we get there because case counts should finally go down and not come back up. The relevant question at that point will be, what about the unvaccinated population? Even in parts of the world where vaccines will be readily available, many might not choose to accept it. So the question then will be, will the herd immunity be high enough to prevent everyone from infection, or will we have to settle for a level that will adequately prevent major outbreaks so that we do not see health care overload?
In conclusion, this remains to be a particularly relevant moment in time for the field of viral immunology. All of the aforementioned questions will be answered, and important lessons will be learned that will undoubtedly help us to be better prepared for the next pandemic.