How should we update our priors on COVID-10?

I want to think about how two pieces of news should change my thinking about COVID-19.  (Warning:  I have no expertise in medicine or public health, and you have no reason to take my thoughts seriously – but you knew that already.)

A new serological study in Santa Clara county (discussed by Kevin Drum here) suggests that far more people have been infected with COVID-19 than researchers had previously believed.  This is only one study and full of uncertainties, but it suggests that the infection fatality rate of COVID-19 (the chance that you die if you get the virus) may be much lower than was previously believed.  The authors suggest an IFR of .12% to .2%.  The Imperial College study, in comparison, used a rate of .9%.

It is tempting to say that a lower infection fatality rate is a good thing:  COVID-19 is less deadly than we thought.  I’m not sure this is right.  The fact that many more people have had the disease than we previously believed will not only affect our estimate of the infection fatality rate, it will also lead us to revise our estimates of other critical parameters.  For example, it suggests that COVID-19 is much more contagious than we previously thought and that the rate of asymptomatic infections is higher than we believed.

Higher rates of contagion and asymptomatic infection are both troubling.  Think about it this way.  Your chance of dying from COVID-19 depends on the probability that you die if you get infected times the probability that you get infected.  The new study suggests that the first number is lower than we thought, but the second number is higher.  I don’t know enough about the dynamics of the SIR model to know if this is a good tradeoff, but it’s not clear that it is:  Ebola had a very high fatality rate but a very low transmission rate, and we would clearly be better off with an easily contained Ebola epidemic than with COVID-19.  Higher asymptomatic transmission will make it much more difficult to control the spread of the disease without prolonged social distancing or massive testing.

Second, a piece by Matt Stieb in New York Magazine reminds us that death is not the only way COVID-19 harms us.  Serious long-term health impacts from COVID-19 infection – kidney and liver disease, heart and lung disease – may turn out to be relatively common among those who develop serious illness.  In addition to the life long suffering this will cause, it is conceivable that the long-term loss of life-years among people who survive initial infection with COVID-19 will exceed the loss of life-years due to immediate fatalities.  There may also turn out to be negative health consequences for people with less severe illness.  And the immediate suffering caused by the disease is often severe as well.

I don’t have a lot of confidence in this analysis, but overall, I’d say the news of the day raises my estimate of the likely health impact of the epidemic.  Of course, your mileage could easily vary.  What this means for policy is a separate question.

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