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Do we have enough ‘herd immunity’ to avoid our own ‘second wave’ of Covid-19? (Photo: theconversation.com / Wikipedia)
There is much talk in South Africa about a “second wave” of Covid-19. Spotlight puts what we do and what we don’t know about herd immunity and potential second waves into perspective.
At the time of writing this article (mid-October 2020), there is much talk in South Africa about a “second wave” of Covid-19. In Europe, it is much more than talk, with Covid-19 diagnoses, hospital admissions and deaths, not to mention the “restrictions” all on the rise.
Studies looking for antibodies in community surveys suggest that in most parts of Europe and North America a much lower proportion of people have anti-Covid antibodies than we see in developing world settings like India and Africa, including South Africa. This implies that there is more immunity here today and people have been wondering aloud if we have enough “herd immunity” to prevent our own “second wave”, and for how long.
There is a lot of jargon and technical discourse on such issues, framed in the language of mathematics and “dynamic modeling.” This discourse involves analyzing how things work in “model worlds” where the rules are fully known and analyzable, because we invent / declare them ourselves. Hopefully, we will do this in a way that allows us to draw useful parallels between our model worlds and the real world. In order to understand how to prepare for that possible second wave of Covid-19, this speech is dark, technically inaccessible to most people, and largely irrelevant.
However, we can break down the key concepts in accessible ways that hopefully people will find helpful.
Herd v individual
First, what is non-herd immunity, for example “individual” immunity?
To say that I am immune to measles, or Covid-19, is to say that I am not at risk of becoming infected, even if I come into contact with sources of infection. Can I claim immunity by carefully dodging really infectious exposures, or do I need to be able to avoid infection even if they literally throw me at a crowd of people who deliberately babble on me for hours on end? Precisely what I am immune to and under what circumstances can be difficult to define.
Let’s say I get a dose of a vaccine and then avoid infection for two years, after which I catch it. Did I “have” this thing called “immunity” that I later “lost”? Was I just lucky enough to avoid infectious exposures for two years, even though the vaccine did nothing for me, or did I have partial immunity all the time, but less and less over time, and eventually my numbers increased?
The flip side of immunity is susceptibility, and being susceptible does not mean that one is guaranteed to be infected with minimal exposure. The idea that someone may or may not have immunity is not absurd, but it is slippery.
Physicists and engineers talk about conductors and resistors. High resistance means low conductance, and vice versa, but every part of a simple electrical circuit has both a resistance and a conductance (one implies the other) and we even have a way to assign numerical values to these properties, just like a bar. it has a length, a width and a mass. Humans are on a continuum from highly susceptible to highly immune, but it’s not obvious that we can summarize this with a single number.
Herd immunity
Now, is there a sensible way to speak of “herd immunity” or “herd susceptibility”? In fact, there are many ways we could try to define such a thing.
We could simply try to recycle some definition of individual immunity and ask how many or what proportion of a population have it. This would not help us much, because it tells us nothing about who has immunity, and what various people in the population do when interacting with each other, and how this contributes to the growth or recession of the outbreak.
We can try to define herd susceptibility in a more “emergent” than “microscopic” way, so that it directly captures something about the tendency of infections to spread. Let’s try. The “herd susceptibility” of a population, at some stage of an epidemic, is the average number of new infections that any currently infected individual is expected to produce. This sounds potentially useful, and it even comes directly with a numeric value that is easy to interpret. If the collective susceptibility is greater than one, we will see an increase in cases. If it is less than one, we will see a decrease in cases. But wait, this is old. This is what we’ve been reading for months in the context of the Covid-19 pandemic – the “R number” – short for “reproductive number.”
Public health agencies, ministerial advisory committees, and the like have been discussing this reproductive number for months, trying to estimate it from the data we have available, and trying to figure out how to bring it “below 1.0” by encouraging mask use. , decreased social contact and hand washing, among others. Hopefully, a safe and effective vaccine will also arrive.
No one has yet seriously suggested that the reproductive number can be affixed to a country, a city, or even a neighborhood like a harvest or crop label can be affixed to a wine bottle.
Herd susceptibility and its reverse herd immunity are constantly changing. In colder climates, people spend more time indoors. Under lockdown, they meet few people outside of their families. Depending on the rules and regulations, people adopt various levels of mask use and hand sanitization and more importantly, as the number of cases increases and people recover, many people gain individual immunity, at least for a while. Everything is fluid. Daily case counts jump like stock prices, but generally they go up and down slowly as we go through the waves of the epidemic.
Group immunity
So what is herd immunity? It is essentially the state of affairs when the “collective susceptibility”, also known as the reproductive number, is less than one. In a population that currently has some infections, saying that there is herd immunity is just a fancy way of saying that infections are decreasing. In a population where there are currently no infections, saying that there is herd immunity expresses that any new outbreak would be expected to disappear.
But would you replace your speedometer with an indicator that only tells you if you are over or under the speed limit, or would you like to know the actual speed? Would you replace a thermometer with a device that simply said if it was “hot” or “cold” or would you like to know the temperature?
When we have plausible estimates of reproductive number, or herd susceptibility / immunity, that vary smoothly over a range of values, we make no progress by forgetting the actual number and simply remembering whether it is greater or less than one. It matters a lot if it is 1.2 or 5; and what we are currently doing to manage collective susceptibility and what cards we have left to play is very important.
Bottom line: The Covid-19 pandemic comes and goes in waves and waves, and perhaps there are even features that look like tides. We can all try to be reasonably aware of whether and how quickly things are getting better or worse, and we can all try to be reasonable in the care we take to reduce the spread of infections while still moving on with our lives and means. of life. The authorities must also be vigilant and reasonable. However, “herd immunity” not only makes little sense or comfort; in fact, it has very little meaning beyond taking a grosser look at what we should all be watching more closely in any case.
How then to face the imminent future of the Covid-19 pandemic? A higher proportion of people with detectable antibodies must mean higher levels of herd immunity, everything else being equal. But in what sense are “all other things” “equal? In fact, to what extent are all other things defined and known?
While it is not bad to hope that we can breathe a bit between waves, we cannot predict much and we should expect more waves. Unfortunately, the end of the pandemic requires the interaction of tolerable / sustainable precautionary habits and a sufficiently extended individual immunity. Unfortunately, we do not know how long individual immunity really is, either after infection or the use of a vaccine.
Thus, we are stuck for the foreseeable future working on our herd immunity primarily through tolerable infection control habits. An eye on estimates of reproductive numbers gives us as much solid information as we can hope for about what we could possibly mean by herd immunity. DM / MC
Alex Welte is Research Professor and Former Director of the South African Department of Science and Innovation (National Government) – Center of Excellence of the National Research Foundation (DSI-NRF) for Epidemiological Analysis and Modeling (also known as SACEMA), at the University of Stellenbosch.
This article was first published in Highlight – Public interest health journalism. Sign up for our newsletter.
