Coronavirus: How herd instincts can help to limit the damage
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Tom Whipple
, Science Editor
Friday March 13 2020, 5.00pm, The Times
We don’t think of ourselves as a herd. The idea of the needs of the collective subsuming those of the individual is antithetical to much of western culture. According to the government’s chief scientific adviser, we might have to reacquaint ourselves with the concept.
Because to control coronavirus, says Patrick Vallance, will require something called “herd immunity”. It will also require controlling who in the herd it is who gains that immunity.
Herd immunity is not achieved when everyone in Britain’s “herd” has been infected. It happens when a proportion of them have. That proportion is the second most important number in epidemic modelling. To see how it is calculated — and perhaps even changed — requires, however, understanding the most important number in epidemic modelling: R0.
R0 is a measure of how infectious a disease is. If R0 is 2, say, then it means that every person infected goes on, on average, to infect two others. If it is 20, as is the case for measles, then each person infects 20 others.
R0 is important because so long as it is more than 1 — even if it is 1.000001 — a disease proliferates exponentially. If it is less than 1 — even if it is 0.9999999 — the disease is doomed.
Infection rates, though, are not fixed for eternity. Imagine a disease with an R0 of 2, which spreads through a single sneeze. At the start of an outbreak of this disease, you sneeze on two people and infect them. In the middle of an outbreak, it is different. Infected people still sneeze the same amount, but some of those downwind of the sneeze will have had the disease already and be immune.
Until the point comes where 50 per cent of the population have been infected. Then if you sneeze on two people on average only one will be susceptible. The infection rate has now dropped from 2 to 1 — and the disease dies out. Herd immunity has been achieved.
For coronavirus, R0 is 2.5. That means, in this simplified example, that for every 2.5 people you sneeze on you want 1.5 to already be immune. Herd immunity for coronavirus, then, is 1.5 divided by 2.5, or 60 per cent.
If indeed it is the government’s belief that the coronavirus cannot be stopped until we gain herd immunity, then that seems at least superficially to be an astonishing admission: an acceptance that more than 40 million people will get the disease. With even a 1 per cent fatality rate, that is 400,000 deaths.
There are good reasons to think it will be nowhere near that bad, and there is one good reason to think it could be worse.
One reason it will not be so bad is that the 60 per cent who get it need not be a random sample of the population. Instead, with sensible measures and “cocooning” of those most at risk, it may be possible to protect the elderly and sick even as the virus sweeps through the healthy.
The second reason is that the number R0 is just that — a number.
People are not equations and cannot be reduced to a single figure. It may well be the case that in 2019 Britain coronavirus had an R0 of 2.5. But in 2020 Britain, a Britain that has reacquainted itself with handwashing and deacquainted itself with air kissing, R0 could well be very different.
So what is the reason it could be worse? The entire calculation rests on the idea that people maintain their immunity, that once infected they cannot be reinfected for a long period of time. The problem is, as with so much about the virus, we don’t know enough to be absolutely certain that that is true.
What we do know though, is that individualism only goes so far. Coronavirus has reminded us we are indeed a herd — bonded by common obligations. For all the vagaries of disease modelling and complexities of viral mutation, scientists are unanimous on one piece of advice, among the most well-validated in modern medicine. To protect everyone, wash your hands.
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