Thread by @statesdj

Good discussion of the COVID19 epidemic and the imporance of hygiene and social distancing. Actually may be even more important than they suggest. Let me explain: 1/
thelancet.com/pb-assets/Lanc…

2/ The spread of virus stops when enough people have acquired immunity. If R0 is fixed, this happens when the fraction of the population infected is 1-1/R0. In the case of SARSCOV2, R0 is ~2.5 which means about 60% of the population gets infected before viral spread ends

3/ 60% of the population is REALLY BAD NEWS. If that happens, we are talking about overwhelmed ICUs and hospitals and millions of deaths, especially in the elderly (OK Boomers and older, this is you we're talking about)

4/ But R0 isn't really fixed. It depends on the number of contacts and how well hygiene is practiced. Some estimates suggest that the initial R0 on the Diamond Princess may have been as high as 14.8 academic.oup.com/jtm/advance-ar…

5/ Conversely, Hong Kong and Singapore are cities with long hard earned respect for public health. Both reacted early to the COVID19 epidemic and both have seen lower rates of spread compared other countries outside of mainland China

6/ R0 is a statistical measure, on average how many new people are infected by a case. If everyone limits social contact and practices disciplined hygiene, infection is less likely and R0 falls. R0 is a function of society and environment as well as the virus

7/ The Lancet article estimates that 70% of contacts need to be tracked to stop spread if R0 is ~2.5. If societal behavior pushes R0 down to say 1.5 or 2, fewer cases need to be tracked and stopping spread happens at a lower tracking rate

8/ The total disease impact also falls. If R0 is ~2.5, 60% of the population will be infected before spread stops. If R0 is 2 or 1.5, spread stops at 50% or 40%. Reducing attack rate by 10% means sparing millions of people and saving many lives

9/ Reducing R0 also reduces the risk of disease reactivation. If we stop spread early, fewer people are infected and this is less risk that one of them will seed a new cluster as restrictions are relaxed

To put this is stark terms, China instituted draconian measures, but they seem to have stopped the spread after ~100,000 people were infected in Hubei, a province of 40 million. 100k is bad, but it's only 0.25% of the population.

11/ 100,00 people infected is bad, but if China had allowed the epidemic to run its course, it would have been 60% of 40 million. That's 25 million cases. Hospitals would have been even more overwhelmed and the CFR would have been even higher. Millions would likely have died

12/ So the trillion dollar question for the US is how far are we going to let this go before we get serious about slowing the spread? You think I'm exaggerating? 60% of 300 million is 180 million cases of a really bad flu and millions of deaths, particularly among the elderly

13/ It probably won't get that bad. The case count is now in the hundreds, and people are paying attention. At a thousand with hundreds of deaths, we will probably get serious about social distancing and hygiene. The sooner we do it, the fewer the number of people who need to die

/fin