That’s more or less the question posed by a new Oxford study making the rounds, especially thanks to a Financial Times write-up. The study posits a scenario in which only 1 percent or even 0.1 percent of people are susceptible to falling severely ill from the virus. If there are 99 or 999 mild cases for every serious one, of course, a bigger share of the population has already had the virus than we previously thought. In the U.K., 40 or even 68 percent of the population might already be immune, per the study’s modeling.
But crucially, this is a possibility the paper sketches out, not an actual finding inferred from the data. As the study itself puts it, “Our overall approach rests on the assumption that only a very small proportion of the population is at risk of hospitalisable illness.” (Emphasis mine.)
Now, the paper does show that this assumption can be made to fit the observed death data. But as noted by Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, other, less rosy assumptions fit just fine as well:
The authors suggest that multiple scenarios could be consistent with the number of deaths reported, including ones in which there have been a lot of infections. Their figure below shows some possibilities. But remember, this is just focusing on the death data… 3/ pic.twitter.com/aLoWiAYdgT
— Adam Kucharski (@AdamJKucharski) March 24, 2020
Kucharski goes on to note that other sources of data on asymptomatic infections suggest far lower rates than the study posits. But to know for sure how many folks have had COVID-19 without knowing it, we need to randomly select lots of people and give them special “serology” tests that detect antibodies. The paper’s authors, in collaboration with other scientists, are planning to start doing that, and we’ll know the results soon.
I’d advise against getting carried away with this study in the meantime, but do keep your fingers crossed.