Swedish update, and which places need to fear second waves?

Your recent question intrigued me. Do you have any new info/opinions on what’s happening in Sweden? Despite no mask wearing, continued indoor dining (at least judging from recent photos on instagram), their case AND death daily counts are plummeting (looks like an inverse exponential). This would also explain excess deaths returning to normal throughout US. Bizarrely, my cursory reading of Swedish newpapers online did not result in any recent articles discussing the dramatic decline in cases there!

One theory circulating is they achieved herd immunity on the math: 10x true seroprevalence (from CDC tests in US) * 2x true immunity (from Tcell things not measured by antibody tests that I don’t fully understand) * 0.75% reported case penetration * 2x for relatively low tests per capita rate = 30% true immunity (likely much higher in densest areas where spread would be much faster resulting in maybe >70% immunity in Stockholm). This puts them r0 < 1.

The nice thing about this hypothesis is that it’s easily falsifiable. If true immunity rates are 20x reported case load (dropping last 2x factor since test rate higher in US), then Florida should have just gotten to the 1.4% necessary to trigger similar immunity in dense cities and from now on, cases per day should follow an inverse exponential.

This would also explain why NYC has not seen a resurgence despite very similar reopening as SF and LA – they achieved dense herd immunity in May and thus the subsequent decline in reported cases was driven by herd immunity rather than more strict closures or mask compliance, reversing either of those factors now doesn’t reverse immunity. To be clear, I’m not disputing that distancing or mask wearing works – they do. But so does infecting everyone quickly. No value judgements on what’s the better policy decision here, just trying to make a predictive statement.

At least, one can hope!

That is my email from Mayank Gupta.  In my view, some version of this view is looking more true with each passing day.  We also are not seeing second waves in hard-hit northern Italy.  Still, many surprises remain and we should not leap to premature conclusions.

To be clear, I was and still am pro-lockdown (without regrets), pro-mask, pro-testing, and I believe Denmark followed a better path than did Sweden.  Long-term damage (rather than death) still may be a significant risk, and furthermore many parts of the world may be far more vulnerable than the United States.  Still, you need to put all of the moralizing and partisanship aside and ask what we are learning from the new data, and I think Mayank Gupta has put it (probabilistically) very well.  And see this related Atlantic piece, though I would have some quibbles with it.  And here is a bit more commentary on the new T-cell results.

In any case, always be prepared to revise!  I believe that within a month we will have a much better sense of these questions.

Addendum: You will note these hypotheses also significantly raise the probability of much earlier animal-to-human transmission, especially in Southeast Asia.  A very good baseline principle for reasoning is simply “Origins usually go back longer and earlier than what you first might think!”

Second addendum: If you go back to March, leading epidemiologist Michael Osterhalm argued: “We conservatively estimate that this could require 48 million hospitalizations, 96 million cases actually occurring, over 480,000 deaths that can occur over the next four to seven months with this situation.”  Covid-19 has been terrible, and the performance of the executive branch (and many governors) absymal, but do those look like good predictions right now?  (Hospitalizations for instance have yet to hit 250k.)  If not, why not?  How hard have you thought about this question?  (Added note: one correspondent suggests that Osterhalm misspoke and in fact meant 4.8 million hospitalizations — note that still would be off by quite a large margin, almost a factor of twenty.)


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