The results from the first blood surveys that test for evidence of antibodies to the new coronavirus have begun rolling in. They’ve been confirming earlier hints that in hard-hit places a significant share of people — 21.2% in the New York City survey — may have been infected with the virus, and that in most other places the percentages are still in the low single digits. They’re also making it ever clearer that the disease caused by the virus, Covid-19, is much, much deadlier than influenza.
“It’s just the flu, bro” has been a sort of know-nothing rallying cry for those skeptical of government lockdowns and social-distancing efforts this spring. But it has also been the base case of a few prominent experts, such as John Ioannidis of the Stanford University School of Medicine and former Swedish state epidemiologist Johan Giesecke.
One important thing to get straight here is exactly how dangerous seasonal influenza is. The fatality rate that has been cited most often (including by me) is 0.1%, which happens to be about what you get if you divide the Centers for Disease Control and Prevention’s estimates of US influenza deaths over the past nine years by its estimates of symptomatic cases.
But just as with the coronavirus, testing has shown that many people infected with influenza viruses develop no flu symptoms. In a Twitter thread from February that a reader pointed out to me this week, University of Oxford infectious disease epidemiologist Christophe Fraser estimated that the actual infection fatality rate (which I will refer to from now on as IFR) of seasonal influenza is 0.04%.
Fraser also speculated that influenza expert Lone Simonsen “may have more accurate numbers.” So I emailed Simonsen, a professor of population health sciences at Roskilde University in Denmark who has worked at the CDC and National Institutes of Health in the US, to ask. Her answer: Fraser’s estimate is spot on. Simonsen believes that the IFR for the coronavirus will eventually turn out to be on the low end of current estimates, possibly as low as 0.2% or 0.3%, but emphasized that this is “still far greater than … for seasonal influenza.”
What are the new coronavirus serology surveys saying about the infection fatality rate? Before I run through the numbers, some caveats: Because of the difficulties in getting a truly representative sample of the population and possible flaws with the tests, none of these estimates of the virus’s prevalence is necessarily right. None has been peer reviewed, either, and only one has even been presented in the form of a formal scientific paper. One bit of guidance is that surveys that find a large percentage of infected people are likely to be more reliable in calculating IFRs because tests that deliver false positives skew their results less — if 20% of the population is infected and 1% of the tests are false positives, that just changes the result from 20% to 21%; if only 1% is infected, the false positives double the apparent rate from 1% to 2%.
In New York, preliminary results of an ongoing state survey of people approached outside of grocery and big-box stores that were announced by Governor Andrew Cuomo found that 13.9% of those tested statewide had coronavirus antibodies, and that 21.2% of those tested in New York City did. The official estimate of Covid-19 deaths statewide is 15,740, which makes for an infection fatality rate of 0.58%.
In the German town of Gangelt, preliminary results of an ongoing University Hospital Bonn survey of a “representative set of households” found that 14% of the people tested had the antibodies for the coronavirus and an additional 1% were still infected with it, which according to the study’s authors came out to an IFR of 0.37%.
In the Netherlands, tests on donations to the blood-bank organization Sanquin in the first half of April (the testing continues) found that about 3% had coronavirus antibodies, which with the Dutch death total of 4,177 works out to an IFR of 0.81%.
Bloomberg
