CG Mini-Lectures: Herd Immunity
Professor Sunetra Gupta of Oxford University explains herd immunity, highlighting critical details about both the concept and its relevance to the COVID-19 pandemic that are often overlooked in public discussion
The development of immunity through natural infection is a common feature of many pathogens, and we now know that COVID-19 does not have any tricks up its sleeve to prevent this from happening. If it did, it would have posed a serious problem for the development of a vaccine.
That being said, COVID-19 belongs to a family of viruses that do not typically confer lifelong immunity against infection. Most of us have never heard of the other four ‘seasonal’ coronaviruses that are currently circulating in our communities. And yet, surveys indicate that at least 3% of the population is infected by any one of these corona cousins during the winter months each year. These viruses can – and do – cause deaths in high-risk groups or require them to receive ICU care or ventilator support. Hence, it is not necessarily true that they are intrinsically milder than the novel COVID-19 virus. And like the COVID-19 virus, the other coronas are much less virulent in the healthy elderly and younger people than influenza.
One important reason why these corona cousins do not kill large numbers of people is that, even though we lose immunity and can be reinfected, there is always a sufficient proportion of immune people within the population to keep the risk of infection low for those who might die upon contracting it. Also, all of the coronaviruses in circulation — including COVID-19 — have some features in common, which means that getting one coronavirus will probably offer some protection against the others. This is becoming increasingly clear from work in many labs, including my lab in Oxford. It is against the background of acquired immunity to COVID-19 itself, as well as its close relations, that the new virus has to operate.
It is misleading to speak of “reaching” herd immunity. Herd immunity is a continuous variable that increases as people become immune and decreases as they lose immunity or die. There is a threshold of herd immunity at which the rate of new infections begins to decrease. We do not yet have a clear idea of what this threshold is for COVID-19 as the transmission landscape includes people who are susceptible to it, people who have built up immunity to it, and people who have immunity to other coronaviruses.
Unfortunately, we do not have a good way of telling how many people have been exposed to the new virus, nor how many people were resistant to begin with. We can test for antibodies but, as with other coronaviruses, COVID-19 antibody levels decline after recovery, and some people do not make them at all. Thus, antibody levels will not answer this question. More and more evidence is accumulating that other arms of immunity, like T cells, play an important role.
Indications of the herd immunity threshold having been reached in a given location are visible in the time signatures of epidemics where death and infection curves tend to either “bend” in the absence of intervention or to stay down when interventions are relaxed (in comparison with other locations where the opposite happened). Unfortunately, we do not know how far (or close) we are to that threshold in most parts of the world. This means that we need to make public health decisions based only on limited information and do so in a constantly changing environment.
Focused Protection was initially proposed as a solution for how we could proceed in the face of such uncertainty and it remains relevant now. It suggests that we exploit the fact that COVID-19 does not cause much harm to the large majority of the population and allow those individuals to resume their normal lives, while shielding those who are vulnerable to severe disease and death. We have good information about who falls into these groups and the availability of vaccines, which offer excellent protection for vulnerable populations and guard against hospitalisable illness, provide us with the ideal setting in which to implement such a plan.
Sunetra Gupta is Professor of Theoretical Epidemiology in the Department of Zoology, University of Oxford and a member of Collateral Global’s Scientific Advisory Board.