Written 11/7/20
The Herd: Thundering Toward a Tipping Point
I want to explain a concept that is vitally important to understanding our current pandemic and where it is headed. Herd immunity is a term used in epidemiology, and you will hear much about it in the coming months. It is not just some abstract idea; it is a real issue that impacts all of us. Whatever you think of the COVID-19 pandemic and whether you personally worry about the infection or not, herd immunity will affect your life over the next year.
Herd immunity will influence the shape of the curves that we so obsessively follow. It will impact the success of any vaccine. It will factor into the total number of dead. It may determine whether you personally get infected with COVID-19. Regardless, it will certainly help decide when normal life resumes.
To Build a Fire
To understand herd immunity, think in terms of fire and water. If you want to start a campfire, how dry must the kindling be? Imagine that you have a pile of one hundred well-seasoned, dry pine sticks. If you drop a glowing coal in the middle, the whole stack will practically burst into flames.
What if you add just a little water? Suppose you take the same bundle of sticks, and as an experiment, you thoroughly wet just one of the hundred. If you repeat the step of adding the coal, the result is the same – an almost instant inferno, fueled by the ninety-nine flammable sticks and barely hindered by the single wet stick that resists burning.
But if you repeat the experiment by progressively soaking one more stick out of the one hundred each time before you introduce the coal, then at some point, maybe when half the sticks are soaked, you would hit a threshold at which the fire would not light – not even the dry sticks, if they were distributed among the wet.
How does this relate to human beings and the COVID pandemic? Think of people who are at risk for getting an infection as flammable, dry wood. And think of those who are immune to an infection (either from previous exposure or vaccination) as wet sticks. Just as wet sticks prevent dry sticks from igniting when they are bundled together, so individual people who are resistant to an infectious disease can prevent an infection from blazing through a whole group, even among those who are still susceptible.
This is what we mean by herd immunity. The threshold effect is important too. Just as preventing a fire from igniting can be achieved by reaching a certain threshold of wet sticks, so when a particular percentage of individuals within a population become immune to a disease, this can grant protection to the whole population. This threshold, known at the Herd Immunity Threshold (HIT) is based on a mathematical formula that factors in how contagious a disease is, using a number called the R0, or reproductive number. Think of the reproductive number as sort of an estimate of the hotness of the coal that we place on the campfire.
To put the COVID pandemic in perspective, the R0 for SARS-CoV-2 is 2 to 3, meaning that one person with the disease will on average pass it on to 2 or 3 others. That is pretty high. But compare this to the appalling R0 for measles (an extremely contagious illness), which is somewhere in the 12 to 18 range.
A population of people who have never been exposed to a new virus is like a forest of dry kindling, waiting just a tiny spark to touch off an inferno. Indeed, the current pandemic virus started in China and rapidly swept to every corner of the globe. So how much water do we need to dowse the flames of this current pandemic? For many infections the threshold to reach herd immunity is well above 50%, and that was the assumption initially for COVID. And having started at zero – no people who are immune, no wet wood to slow the wildfire – we appear to have a long way to go to extinguish the flames.
Tip of an Iceberg?
Yet things may not be as grim as that. In our current COVID-19 pandemic, the sticks may not be as dry as we thought at first. For instance, maybe more people have already had the virus than we suspect. We have some idea of the number of COVID-19 cases out there, but that only counts the ones with symptoms and the ones who undergo testing. Is it possible there are cases we don’t know about? Maybe some among us have had COVID-19, but never knew it, because we were never tested and never exhibited symptoms.
In fact we don’t yet know what proportion of people infected with SARS-CoV-2 actually develop symptoms. We simply have not had enough time or done enough testing to ascertain this (and many important things) about this novel pathogen. But asymptomatic infection is increasingly being recognized. The latest data suggest that perhaps forty percent (40%) of cases may be asymptomatic.
Usually the media report this as something ominous. And indeed, if there is a lurking, submerged portion of cases, who could unknowingly spread the virus to the most vulnerable, that is disturbing.
On the other hand, if the goal is for us as a population to achieve herd immunity by a sufficient number of people being infected or receiving a vaccine, it would be great to wake up one morning and find out that a sizeable percentage of us have already been infected without even knowing it, and the pandemic is nearly over!
But if we need to get above 50% immunity, the numbers suggest we are not close. Even if we consider the heaviest hit populations, places like New York or London, and when we perform surveys for antibodies to SARS-CoV-2 in the general population, only about one in five people have these antibodies (23% in New York; 18% in London). And of course in less affected areas, the numbers are much lower. It appears we have a long way to go before we reach herd immunity. There simply are not yet enough wet sticks out there to extinguish this pandemic.
However, based on the patterns of spread we are now seeing with COVID, some researchers have suggested that the threshold to reach herd immunity may be much lower than initially assumed. Some scientists in Scotland recently recalculated the threshold number for COVID and arrived at a much more manageable number of just 10 to 20%. If that is accurate, then we may be well on our way in some places to nearing herd immunity. It is too early to say for sure.
And in my opinion this encouraging possibility does not justify attempts just to let the virus “runs its course” freely in the population, much less encourage its spread to aid in population immunity, the way “chicken pox parties” did in my youth. In some ways that would be the equivalent of burning some forest to slow the spread of a wildfire. It may make sense for certain small populations, but not our population as a whole.
Herd immunity is an increasingly politicized term – what word or concept isn’t politicized these days? When you hear negative remarks (as I have recently) coming from the scientific community about herd immunity, they typically are not dismissing the critical role that herd immunity plays in quelling human epidemics. Rather, they are reacting against the notion that we will defeat the COVID-19 pandemic by allowing the infection to go unchecked and thereby speed its resolution. This is the approach taken to a limited extent by Sweden, an approach that has been variably criticized and commended by differing groups and at differing times. This pandemic is difficult to understand fully and is constantly changing, so most pat answers don’t stay pat for very long. Hence my earlier admonitions to give each other some grace!
In my opinion the recent dismissive comments about herd immunity coming from the scientific media and the press are a type of politicized overreaction. In fact, the success of a vaccine strategy is based on the concept of herd immunity, so anyone who dismisses the herd immunity principle and says they are relying instead on a vaccine may be throwing out the baby with the bath water.
Most likely, controlling the pandemic through herd immunity will involve some combination of natural immunity (for those who have already been infected) and vaccine-induced immunity to put us as a whole population over the threshold.
Good News From The Past About The Future
And there is other encouraging news. We have been counting immunity to COVID by measuring antibody levels. Maybe that is not the full story. It is worth mentioning here the encouraging, though early, evidence that some people may be resistant to COVID infection, even though they have not been previously exposed to the novel coronavirus, and they have no measurable antibodies to it. How could this be?
Immunity against an infection can be conferred not only by antibodies from the B cells (“humoral immunity”), but also by T cells (“cell-mediated immunity”). Preliminary research shows that some people’s T cells react vigorously when exposed to SARS-CoV-2 in a test tube. Somehow these high ranking T cell immune soldiers know – and are provoked by – the novel coronavirus, “recognizing” it, although they have never encountered it before.
Several studies in various countries have shown some degree of T cell reactivity in up to 20 to 50% of people thought to have no pre-existing immunity to the virus. In one study there was some degree of T cell reaction even in blood donor specimens from 2015-2018, long before the current pandemic and as far as we know, before the existence of SARS-CoV-2.
This may be because of the other coronaviruses out there besides the one that causes COVID-19. Some coronaviruses are not especially dangerous. You may well have had an infection with one and suffered basically just a common cold. Other, rarer coronaviruses, though, are extremely virulent, like the ones that cause SARS (which thankfully has not afflicted anyone since 2004) and MERS. MERS is particularly deadly, killing four in ten of those afflicted! But you are very unlikely ever to be exposed to MERS unless you spend time around camels. Even if you ride a camel at a zoo or are tapped to play a wise man in a fancy Christmas pageant, don’t lose any sleep over it – unless you plan to dwell long-term with desert nomads.
SARS-CoV-2, the cause of COVID-19, may “look” to the immune systems of certain people enough like some of the other, previously encountered coronaviruses as to cause cross-reactivity upon first contact with the newer virus. It is like a savvy soldier recognizing and detaining the twin brother of a previous enemy combatant as he sneaks across the border. Perhaps this sibling resemblance among different strains of coronaviruses will end the COVID-19 pandemic sooner than some fear.