A narrative rocketed around social media earlier today: An Imperial College study said that COVID-19 could kill 500,000 Brits, but in recent testimony, Neil Ferguson, the head of the group behind the study, put the number below 20,000. Clearly the lying alarmist was walking back his ridiculous predictions!
Well, no. The paper actually offered simulations of numerous scenarios. The one resulting in 500,000 deaths was one where Great Britain just carried on life as before. Other scenarios, where the country locked down whenever it was necessary to stop the disease’s spread, put death totals below 20,000. (See the rightmost death columns of Tables 4 and 5.)
Since the paper came out, Great Britain has adopted a strategy of aggressively containing the virus and expanded its intensive-care capacity, so a prediction of a much lower death toll and less stress on ICUs hardly seems surprising.
Models like this will always turn out to be wrong in some way or other, because they rely on very strong assumptions about aspects of the disease we haven’t thoroughly studied yet. If nothing else, the original Imperial model will be obsolete soon, because it didn’t predict what could happen with extensive testing and contact tracing, which is likely the next step once the spread is contained and we have enough tests to go around. But it hasn’t been walked back just yet.
Update: Ferguson himself explains all this in a Twitter thread:
1/4 – I think it would be helpful if I cleared up some confusion that has emerged in recent days. Some have interpreted my evidence to a UK parliamentary committee as indicating we have substantially revised our assessments of the potential mortality impact of COVID-19.
— neil_ferguson (@neil_ferguson) March 26, 2020
Update 2: A variation on this argument people keep throwing at me is that if the death toll hasn’t been walked back, the original study’s claim that we need to quarantine until a vaccine is developed a year or more from now has been. This doesn’t really work either.
The model says that you can’t lock things down for a while and then just lift the lockdown, because the virus will come back. You need some kind of continuing containment efforts. The original study modeled an idea where lockdowns are lifted and go back into effect repeatedly — that’s where the sub-20,000 death estimates come from — and the text of the report explicitly noted the possibility that strategies could change over time to include test and trace. (As I say above, though, not actually modeling this type of effort is a big limit of the study, because that’s where things seem headed.) In his testimony Ferguson did not advocate a brief lockdown that was later simply lifted; he said the lockdown would be effective until new methods could come online. Which is completely consistent with the original report.