Herd immunity is not a number (it's a function)
By Scott Sumner
There’s a lot of talk about what sort of percentage of the population must be infected before you have “herd immunity”. For instance, Tyler Cowen has a post discussing the fact that new Covid-19 infection rates are down sharply in places that have previously been hit hard, such as Sweden, Lombardy and New York. I do agree with Tyler’s claim that it is plausible that this might reflect, in part, the role of herd immunity.
At the same time, I fear that the implications of this might be misunderstood. Herd immunity is not a specific number, like 20% or 70%; it’s a function of “R0”, the rate at which the virus spreads. And that reflects human behavior. Sweden certainly does not have enough herd immunity to go back to life as normal, but they may have enough for a partial return to life as normal. Thus they don’t want to go back to behavior that would normally lead to a R0 of 3 in an unexposed population, but 1.4 might be good enough, given those precautions.
The US is still a long way from having to avoid taking precautions. In New Jersey, the death rate is 1768 per million. And that rate occurred despite New Jersey residents taking significant precautions. Even worse, lots of people still die each day in New Jersey, so the 1768 figure will likely reach close to 2000.
For the US as a whole, the fatality rate is 424 per million (
a tad somewhat below Sweden), which is less than 1/4th the New Jersey rate. If the virus is allowed to spread uncontrolled in the US then we can eventually expect to be hit as hard as New Jersey, and probably much harder. The US has already had 140,000 deaths—I don’t think anyone wants 560,000 deaths, or more. We clearly need to avoid ending up like New Jersey, and that means we cannot rely solely on herd immunity. Almost everyone favors at least some precautions, such as for nursing homes.
Now for the good news. We’ve gotten better at preventing Covid-19, despite all our missteps. So the percentage of the population necessary for herd immunity—when combined with widespread mask wearing and testing—is much lower than when New Jersey was first impacted in March and April. That doesn’t mean we can let down our guard, but it does mean that herd immunity combined with precautions could help the US as a whole to avoid being hit as hard as New Jersey.
There are a lot of moving parts with Covid-19, and it’s important to focus on them all. Not just one in isolation.