Over the last year, the number of immune individuals necessary to achieve herd immunity has risen from 70% to 90%. Is it even possible to achieve herd immunity? Every other disease has herd immunity; why not COVID-19? And another problem, why is COVID-19 different from its siblings SARS or MERS?
Herd immunity is when the virus or infection is contained at levels that do not allow it to reproduce exponentially - it is the equipoise between our immunity and its infectivity based on a huge number of variables. It is another way to describe the current equilibrium of fitness between the two species.
It is not a specific number, and it is certainly not accurately predictable. We can, as we always do, make predictions based upon experience, it is hardwired into our circuitry. But here is an interesting truth, we have never lived through a pandemic with the same measurement tools, understanding, communication, and miscommunication. Pandemics share some commonalities, such as the development of herd immunity, but they all differ in significant ways.
In our human-centric way, the concept of herd immunity is applied primarily to the herd of most importance, us. But it could apply equally well to say bison. In 1800 there were 50 to 60 million of them; they had reached an equilibrium with their environment and fitness. But due to acute lead poisoning (from bullets) brought along by a more invasive species, that would be us, and their numbers were reduced to 1,000. They had no herd immunity to us, about 1% remain due to our decision to let them live.
So, for us, the good news is that when it comes to microbial disease, we have herd immunity to all of them, to varying degrees. But without herd immunity, just as with the bison, the herd, and now I am talking about us, would be gone.
SARS and MERS
Severe Acute Respiratory Syndrome and Middle East Respiratory Syndrome are both due to a coronavirus. But COVID-19 has a superpower. It is infectious, meaning transmissible, for 1 to 2 days before an individual becomes symptomatic if they become symptomatic at all. SARS and MERS become infectious once they cause fevers, coughs, and other respiratory distress.
In the last year, much has been written about our innate immune response, all those B cells, and antibodies designed to protect us. But there is a less frequently mentioned component to our innate response, hidden in our behavior, disgust.
“…an adaptive system that evolved to motivate disease-avoidance behaviour. It arose in our animal ancestors to facilitate the recognition of objects and situations associated with risk of infection and to drive hygienic behaviour, thus reducing micro - and macro-parasite contact.”
Now we all have varying disgust responses; some people can’t stand the sight of blood, as a surgeon, that would be debilitating. But can we all agree that we tend to shy away from things we find disgusting?
SARS and MERS become transmissible once you are symptomatic and the others around you can innately shy away – think of it as our evolutionary social distancing. COVID-19 doesn’t allow for that unconscious response, which makes it more easy to transmit – we are exposed long before we even know we are at risk.
Masks, social distancing, and hindsight
One of the important, and quickly discarded insights about COVID-19, is the humbling knowledge of how little we did and do know. We are not masters of the universe. We knew that COVID-19 was a respiratory virus but knew little about the actual means of transmission. Is it breathing, or coughing, or sneezing, or droplets, or aerosols, or toilets, or surfaces?
One of the ways that COVID-19 was more fit than us was in its transmissibility while we were asymptomatic. It was also more fit because it didn’t kill many of us and left us to walk around and spread the genome. Our fitness comes from the many ways in which COVID-19 doesn’t get a foothold, either because we lack the proper spike attachments or mount a robust immune response. Herd immunity is the balance of COVID-19’s and our fitness measures.
To save the greatest number of lives, many public health officials sought to emulate our disgust response, to separate from the infected – they called it social distancing and the use of masks. We can fight forever over the correct distance and the proper mask, but can we really argue with our innate response, developed over millennia?
Some argue that we shouldn’t have locked down or that social distancing and masks are ineffective. They can point to this country or that. Hindsight is always 20-20 vision because it is written after the fact when you can craft a narrative. At least for me, the problem is that the herds being protected differ, often quite a bit. Do we believe that mitigation measures like social distancing and masks play a role in nursing home populations? Are they as effective in schools? We do not have all the answers, but from the point of view of a public health official, moving away from the disease is both reasonable and, dare I say, innate.
One more point about disgust to consider. That innate response to move away from the other impacts our political behavior. Some even believe it is a way to determine our political beliefs. Disgust is innate, learned, and modifiable. Disgust is one of those evolutionary adaptations that I think works well for avoiding disease but may not work as well, if at all when we let it thrive in social media.