Thread by @DrRobDavidson

As a doctor who cares for #COVID19 patients the idea #HerdImmunity through infection is alarming. It's been rejected by the head of the NIH as "fringe," but is being embraced by the White House and @realDonaldTrump. Just what is herd immunity? 1/XX nytimes.com/live/2020/10/1…

Herd Immunity is an epidemiological phenomenon achieved when enough of a healthy population is immune through vaccination so that an infectious organism can no longer spread in a population. 2/XX

So what is enough of a population? For COVID19 most experts think around 70% of the population would need to be immune to achieve herd immunity. The range may be 40% to 90%, but it is likely somewhere in between. Currently 10-20% of the US has likely been infected. 3/XX

But with #COVID19, we currently have no FDA approved vaccine, therefore herd immunity would need to be achieved through infection with the virus. This could be a very deadly prospect with a virus with an infection fatality rate (IFR) 5-10x higher than seasonal flu (0.1%). 4/XX

First we must make a few assumptions:

Assumption #1- Infection=immunity. Currently this is unknown. We think this is likely, however there are reports of reinfection. 5/XX thelancet.com/journals/lanin…

Assumption #2- Immunity is lasting. Again, this is currently unknown. Serologic studies show a decay in neutralizing antibodies like the ones in convalescent plasma and the ones in the Regeneron monoclonal antibody cocktail that @realDonaldTrump. 6/XX nejm.org/doi/full/10.10…

Assumption #3- We can protect more vulnerable populations. COVID19 has a higher death rate for older and sicker people. Any policy of herd immunity would presuppose the ability to protect such populations. But @realDonaldTrump is 74 and obese, and he was infected. 7/XX

So if the above assumptions are accepted as true, and that COVID19 has an IFR of 0.5-1%, the question is: How many deaths would the US have to endure to achieve herd immunity? We cannot determine an absolute number, but the range is quite alarming. 8/XX

On the very low end, if 40% (or an additional 20%) would need to be infected, and 0.5% die, the simple math is: 330 million x 0.2=66 million x 0.005=330k. That is an additional 330,000 dead Americans to achieve herd immunity. 9/XX

On the high end, if 90% (or and additional 70%) need to be infected and 1% die, the math is: 330 million x 0.7=231 million x 0.01=2.31 million. That is an additional 2,310,000 dead Americans to achieve herd immunity. 10/XX

At the very low end, is 330,000 more dead Americans acceptable? What about over 2 million? With hospitals stretched beyond capacity, how many more will die from non-COVID related illness? How many nurses will die? How will our society survive such tragedy? 11/XX

So when you hear @realDonaldTrump, or @SWAtlasHoover mention a policy of #HerdImmunity to get through this crisis, consider the cost in human pain and suffering, and please just #WearAMask and socially distance, and we can get through this together. /END