Question: I’ve seen in the news that if the US had responded to COVID-19 just two weeks more quickly, as many as 84% of deaths could have been prevented. Can you explain why that is?

Answer: I think the study you’re referring to, which made waves a few months ago, is this one from three researchers with Columbia University, “Differential Effects of Intervention Timing on COVID-19 Spread in the United States.” I assume that it’s making waves again given the revelations revealed yesterday that the President knew of the severe danger COVID-19 posed at least as early as Feb. 7 when he stated in a call with Bob Woodward that “You just breathe the air and that’s how it’s passed, and so that’s a very tricky one. That’s a very delicate one. It’s also more deadly than even your strenuous flus… This is more deadly. This is 5 percent versus 1 percent and less than 1 percent. This is deadly stuff.” Given that backdrop, I’ll share an overview of the study and discuss how actions (and lack of action) can impact the trajectory of the pandemic.

• Study Overview. This study, published in late-May on a pre-print server, has not been peer-reviewed. The authors used an SEIR (susceptible, exposed, infectious, recovered) model to estimate the impact of the non-pharmaceutical interventions (NPIs, e.g. social distancing; limited population movement) implemented between March 15-May 3rd on rates of transmission, new cases, and deaths. The authors then re-ran the model to examine the impacts on new cases and deaths of earlier applications of NPIs — 1 week earlier and 2 weeks earlier. In doing so, they estimate that had the US adopted NPIs 2 weeks earlier, more than 1 million cases and 58,000 deaths would have been averted. At that time, this would have been equivalent to an 89% reduction in deaths (95% confidence interval: 84%-94%) Note: for overview of models see Q&A of 4/5 and Q&A of 5/4 #Models.
• Similar Findings Elsewhere. Referring to the UK’s response, Prof. Neil Ferguson of the Imperial College of London testified during a House of Commons committee meeting in June, “Had we introduced lockdown a week earlier we’d have reduced the final death toll by at least half.” This comment, to my knowledge, stems from analyses using the Imperial College’s COVID-19 model. I highlight it here to show that researchers around the world have come to similar conclusions about the timing and efficacy of NPIs for curbing the spread of COVID-19 and saving lives. A couple of writers from Harvard wrote an opinion piece in June published in STATNews that used the University of Oxford’s stringency index to estimate how many lives would have been saved had the US taken a faster and more holistic approach responding to the COVID threat, finding that had the US acted as effectively as Germany, at least 70% of US deaths may have been averted.
• Why? Exponential growth is a difficult thing to wrap your mind around. In just a short time, you can go from having 15 cases to having >100,000 cases and beyond. Because of the speed at which SARS-CoV-2 travels through a population (plus its ability to travel undetected for some time among asymptomatic and pre-symptomatic people), any delayed action gives the virus time and space to grow. In just 30 days between end-Feb and end-March, we went from ~40 cases/day to 25,000 cases/day! Because NPIs are so effective at curbing the virus’s spread, widespread implementation of them has sweeping impacts on the virus’s reproductive rate. This has sweeping impacts on the number of people who become infected, the ability of our health system to respond, the ability of our residential facilities (like nursing homes) to keep their residents safe, and the number of people who become seriously ill and who ultimately die.