Science Seen Physicist and Time One author Colin Gillespie helps you understand your world.
Herd immunity—friend or enemy?
We can find a way out of the COVID-19 isolation trap.
Herd immunity. What is it? What can it do for us? Or to us?
It can be our best friend and our worst enemy.
The starting point to understanding herd immunity is: It is not about me; it is about us. It is math, pure and relatively simple to understand. Understanding what it is may prove helpful—and even reassuring—in these uncertain times.
Scientists who study a virus determine a number that reflects how easy it is to catch. Let’s call that number R0 (that’s one of the names scientists use). If I catch a virus, I may in turn infect many people . . . or a few . . . or maybe none. So, let’s track some people who have it and see how many other people catch it from them.
Then we divide the total number of people who caught the virus from them by the total number of people who gave it to them. That number—the average number of people infected by a typical infectious person—is the R0 (also known as the basic reproduction number) of that virus.
Experts are working
This kind of tracking is easier said than done. All over the world, experts are working night and day to pin down the new coronavirus’s R0 amid many practical difficulties. The best overview I’ve found is in a March 30 report from a team of 61 epidemiologists in England. Collecting data from eleven European countries they find R0 numbers that are “all over the map” — figuratively speaking.
Their best estimate of R0 for the new coronavirus is about three. Higher R0 means more infectious: For example, measles is horribly infectious with an R0 of about eighteen. Lower R0 means less infectious and a virus with R0 of one or less is not infectious enough to cause a pandemic.
Next key thing to understand is: COVID-19’s high R0 is our enemy; but it can lead us to a friend. What matters is the actual value of R (also known as reproduction number) where we live at any given time. This value is called Rt .
Why does it matter?
The reason that this matters is: Wherever we live, our local Rt can be made lower than the bug’s R0 , depending on what we do, like physical distancing or lockdown.
The last key thing to understand is: When we reduce COVID-19’s Rt to a number less than one, it cannot maintain a local epidemic (let alone a pandemic—a new worldwide epidemic) even if it has a high R0 . It’s simple: If everyone who gets the bug infects less than one other person, the epidemic is dying out instead of increasing and will soon be gone.
That’s what all this staying home and physical distancing is about: getting our Rt below one. Once we do this, the epidemic is on its way to ending—but only if we keep on staying home and distancing.
Another way to reduce COVID-19’s Rt
However, there is another way to reduce COVID-19’s Rt to a number less than one; and it’s a way that does not depend on staying home or distancing: If two-thirds of any given population is immune, its Rt is one.
Here is the explanation: If on average each person who gets the bug “infects” three people—but two of those three are immune—only one of those three gets the bug and only that one is then able to infect others.
This is called herd immunity. It is not a property of “me”; it is a state of “us” (the herd) and that infection.
In general, we have herd immunity to any infectious disease when all but a fraction of the population (one divided by R0 ) has immunity to it. Right now, for us this is a BIG DEAL: This new virus has a not-too-large R0 so we need only some (i.e., about two-thirds) of us to become immune, a relatively achievable target, to stop the wave of infection. And keeping it stopped does not require us to keep on distancing.
Of course, herd immunity does not descend upon us suddenly when that mathematical average hits exactly one point zero. The virus becomes less infectious as Rt becomes closer to one.
So how can Rt be made to decrease? In just two ways. One way is natural: Those who get (and survive) the virus will gain a degree of immunity. The other way is artificial: Vaccination.
We will achieve herd immunity
Thing is, either way we will achieve herd immunity regardless of what we do. The key questions are: How long will we take to achieve it and what will it cost us in lives and economic damage along the way? Both the delay and the damage will depend on what we do.
And there’s the rub (thankyou Shakespeare): Developing, testing and producing a vaccine plus vaccinating billions of people would take years. In practical terms, this is unlikely to happen. One way or another, we will have moved on, just as we did after SARS.
This is where we should be careful what we wish for. Right now, we just want to slow the spread (reduce Rt ) so that critical care in hospitals is not overwhelmed. We are enduring draconian measures to achieve this: Anything to stop the virus! But, once success is in view people will want to relax those draconian measures. What then?
It appears that for the early and for-now-suppressed outbreak in China, only a modest fraction of its population was infected. And in Europe this fraction may be less than ten percent to date. So only modest progress is being made toward herd immunity. As each country moves to restart its economy, its epidemic may come back—perhaps more overwhelmingly.
Is a likely-losing gamble our only alternative to economic ruin while we wait for a vaccine that will arrive too late, if ever?
Here’s the deal, for healthy volunteers only:
- Get infected (a small controlled dose; small dose means lower risk).
- Get monitored, with free access to health care.
- Get treated when a treatment becomes available, if needed.
- Get better, mostly (but maybe not all) with mild if any symptoms.
- Get tested for immunity (a different test, more below).
- Get an International Vaccination Certificate and photo “Certified Immune” ID.
- Get $1,000 for your time and trouble (far cheaper than current crisis programs).
- Get back to work.
It will be controversial but this “thread-the-needle” faster approach toward herd immunity could get the economy going at optimal speed while ensuring local health care systems are not threatened. If done well, fewer should die than would otherwise.
It will need a new test (one that is simpler, faster and cheaper than the PCR test that sees if you have the virus). A serological test, now under urgent development, basically will see if you had the virus and so have acquired immunity.
Expect new tests soon, as well as approved treatments and a growing conversation about our way forward.
In sum, we are coming to a three-way fork in our road:
- We can stay locked down waiting for a vaccine that will not arrive in time;
- We can go back to work with some precautions praying the bug won’t come back; or
- We can offer controlled infection to volunteers who almost all recover and go back to work.
I’m in line for the third way.
Kiera Campbell, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79523883
Vaccines: Their Risk, Safety, and Effectiveness; https://vaccine-risks.com/natural-herd-immunity/