David Fisman Profile picture
25 Oct, 12 tweets, 3 min read
Global R(t) has been remarkably stable since the first wave. Here it is plotted against global doubling time. Will try to unpack this further when I get a chance but it’s a good news/bad news story: many infections ahead, but this isn’t open-ended.
“Moving average” = 7 day moving average for doubling time.
Just to unpack this a bit, there's a direct relationship between R and doubling time, inasmuch as R(t) is a function of growth rate, as is doubling time. Doubling time may just be a bit more intuitive.
We can see in the graph that both R and doubling time have actually changed little since April.

We are currently around 42 million COUNTED cases globally; let's say we're under-estimating infections by a factor of 20...probably about right.

That would mean that the 42 million counted cases represent around 800 million infections. If doubling time remains around 75 days (as a function of this on again, off again, institution of distancing followed by relaxation of measures)...
we're at 1.6 billion by mid-January, and around 3.2 billion by March. That's likely getting to the point where an uncontrolled epidemic would peak due to approaching the critical fraction of susceptibles to sustain exponential growth.
This assumes R of 2-3 with that critical fraction being a bit lower due to the non-random nature of population mixing.
That can still result in a very large future stream of cases due to high prevalence, so the pandemic doesn't screech to a halt.
But the dynamic is likely to change as our R effective is near 1 even without intervention. The longer term future will likely result some kind of endemic equilibrium with "covd season" as we now have flu season, unless we have a potent vaccine that makes elimination viable.
But hopefully it's helpful for folks to understand that this isn't open-ended. And the timeline I describe, while very approximate, would look very much like historical pandemics, like 1918-19.
I know that billions of infections by early 2021 sounds implausible, but that's the wonder of exponential growth. We're currently well above 400,000 counted cases globally each day. At 5-10% case identification fraction, that's 4-8 million infections...

it adds up!
Meanwhile, the United States is approaching 230,000 counted COVID deaths in a 6 month period. In terms of deaths, that's 4 Vietnam Wars, and the Vietnam War lasted 19 years.
Yes, thanks everyone. My bad on this tweet, where the "deaths" I referred to were US military casualties. As many have pointed out there were millions of deaths that resulted from conflict in S.E. Asia over multiple decades.

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More from @DFisman

26 Oct
Asked by a friend to comment on the reasonableness of the IHME forecasts for Canada (30+k deaths by Feb). (covid19.healthdata.org/canada?view=to…). The IHME model is impressive...
And again, based on the 2nd (winter) wave of the pandemic ahead of us, and given that we currently stand around 10k deaths, the projection of 30k deaths by February seems reasonable. Note my earlier tweet about 2:1 ratio of 2nd to 1st wave in 1918/1919.
What's impressive to me in IHME is the forecast that we would/could save 10,000 Canadian lives in the months ahead with a national mask mandate. This, again, seems reasonable, based on best available data. Image
Read 8 tweets
24 Oct
This is amazing. We're headed into the second wave of a once in a century pandemic. Ont and Qc have been backstopped by the feds. Now here come the Cons with a motion to initiate a massive AUDIT of the public health response? Now? October 2020?
Like, THIS is when you're going to tie up PHAC, Procurement, Health Canada, not to mention the companies the govt is working with to procure vaccines and tests, with what looks to be the audit from hell?

Because PHAC, Health Canada, and every other federal office/bureau/department isn't already hanging on by their fingernails trying to build systems, structures and responses to protect Canadians?

Now? Really?
Read 10 tweets
13 Oct
This (as with the Quebec karaoke superspreader event) is extremely difficult to explain without invoking aerosol. High viral load in resp mucus of the index case (asymptomatic) created a superspreader event.

It would be extremely interesting if the index case was the instructor: prolonged shouting/exhorting during a spin class would make sense in terms of aerosol generation...is that info out there?
Events like this are literally showing us the gateway out of this situation. It is distressing to see that institutional public health across Canada is still reluctant to acknowledge the importance of aerosol: costing time, money, lives now.
Read 4 tweets
12 Oct
I had some wonderful back-and-forths with @aatishb about this and it’s fantastic to see the finished product on “math of masks” at

There is a wonderful YouTube video here:

If you want a mathy-er version of same, we have an approach using next generation matrices which you can access here (and which we’ll cover this week in chl5425 at @UofT_dlsph)

Read 4 tweets
10 Oct
Thanks to the friend who reached out to suggest that having my own dedicated troll army suggests that our messaging and information are effective. That's a nice shot in the arm.
With that in mind, I'll use this account to disseminate good info on what's happening with covid...and hopefully drive the trollbots crazy.

Let's start with covid in Canada today:
R(t) by province...note New Brunswick is the place with a qualitative change in status, and is few days into a LTC outbreak after a few months of elimination of local trans.
Read 8 tweets
7 Oct
@LeaferReefer_ @BogochIsaac Getting to herd immunity via disease transmission vs. getting to herd immunity via vaccination are very different.

For disease transmission, you cross the susceptibility threshold for transmission when fraction immune = 1 -(1/R0). Let's say R0 for covid is 2; that means you...
@LeaferReefer_ @BogochIsaac hit that threshold when (1-(1/2) = 50% of the population is immune. In Canada, that's at 20 million infections.

x 0.7% IFR = 140,000 deaths.

And at 50% immune, R = R0 x S = 2 x 0.5 = 1. R hits 1 at the PEAK of the epidemic curve...so you still need to come down the other side
@LeaferReefer_ @BogochIsaac of the epidemic curve. As these curves are somewhat symmetrical, once you hit that threshold, you still have 10's of thousands of deaths to go, and the final epidemic size is much greater than 50%.
Read 9 tweets

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