How many people have been exposed to Covid-19 ?

John Quiggin44 Comments

In the last few days, there have been quite a few reports of studies suggesting that the number of people who have been exposed to Covid-19 is far larger than previously thought. These studies have been based on testing for antibodies against coronavirus (it is unclear whether they are specific to Covid-19, or might reflect exposure to other coronaviruses).

I’m finding it difficult to square these estimates with inferences from direct testing, which (as I understand it) tests whether people currently have the disease. This is a point on which I would really like to see a clear explanation from an epidemiologist, but I haven’t seen one, so I am going to set out my own thoughts.

Alert: Unlike my discussion of the exponential growth rate R, where I was confident in the analysis and rapidly proved correct, this is an amateur effort and I could easily be missing something crucial

Suppose that

  • the virus has been around for some number of days D
  • infected people will give a positive response to a direct test for d days
  • the proportion of positive test results in a random sample of the population is p

Then, as a first approximation, the proportion of people ever exposed is (D/d)p. To estimate D, I’ll assume that very few* people outside China were infected before 1 February, which gives D = 84 as of April 25. I’m less clear about d, but 14 days appears to be the standard estimate for asymptomatic cases. That gives D/d = 6.

The big problem is p. Most places are only testing people who are at high risk because of symptoms or known contacts. Iceland gave 1800 tests to randomly selected volunteers in March, and got a positive rate of 1 per cent, suggesting that the proportion ever exposed would be 6 per cent. That’s a lot of people, but nowhere near enough to make herd immunity a relevant possibility.

Any ideas?

  • Some were, it’s clear, but if the numbers had been large, we would have seen many more deaths.

Are the IMF’s forecasts too pessimistic?

John Quiggin8 Comments

That’s the headline for my recent piece in Inside Story, responding to statements from the IMF that “the world is about to experience the worst recession since the Great Depression”. My complaint isn’t so much about the numerical estimates made by the IMF (which are actually optimistic) but about the framing in terms of the Great Depression. It’s almost certain that we will see a larger decline in global income and output in 2020 than we did in the GFC year of 2008. However, if things are managed well, we should see a substantial recovery in 2021, unlike the years of austerity that followed the GFC in many countries. The comparison is also distorted by the fact that growth rates in China and India have been slowing for some time for reasons independent of the pandemic.

Pedestrians and pandemics

John Quiggin24 Comments

A couple of days ago, Adam Creighton had a piece in the Oz, downplaying the risks of the coronavirus pandemic, under the headline “Under 60, in good health? Crossing the road is more risky” Authors don’t choose headlines, but in this case, it’s an exact quote from the article.

There was no supporting analysis, so I decided to do the numbers myself. I looked at Sweden (quoted as a success by Creighton) which seems reasonably comparable to Australia, except that they haven’t gone for a lockdown. I started with some rough estimates on Twitter, and corrected them in response to comments. What follows is the final version.

In 2019, Australia had 172 pedestrian deaths a figure that has been stable for some years. As with the virus, over-60s are more at risk, accounting for 40 per cent of fatalities, compared to 15 per cent of the population. So, just over 100 pedestrians under 60 died last year in Australia. That’s about 2 per week.

Looking at Sweden, there have been 86 virus-related deaths of people under 60 so far, virtually all of them in the last three weeks. That’s about 28 per week, or 14 times the Australian rate

Now we need to adjust for the population difference. Sweden has about 10 million people and Australia about 25 million (ideally, we’d look at the under 60 population, but I’ll leave that adjustment for later). So, as of now, the virus risk to Swedes under 60 is approximately 35 times as great as the risk to Australian pedestrians under 60.

We don’t know how things will change in Sweden. Their strategy is one of “flattening the curve” while allowing the virus to spread slowly. Unless (as some have claimed) there are a huge number of undetected case, it will take a long time to reach herd immunity. Perhaps the current restrictions are sufficient to keep R below 1, but that seems unlikely to me. Even if it turns out that way, thousands more will die before the infection fizzles out.

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Noises off

John Quiggin30 Comments

A couple of weeks ago, I recorded a video presentation about the likely employment effects in Australia, as part of my university’s response to the pandemic. The sound quality wasn’t great, what with reliance on my computer microphone, a spotty Internet connection and my accent, which is too strong even for some Aussies.

The communications people at the Uni got back to me and said it might have to have subtitles, but they could improve things by lowering the volume of the background music. My immediate reaction was unprintable, and while I managed to calm down, I wrote back to say that under no circumstances would I accept any kind of musical accompaniment. They cut out the music and managed to get it done with closed captions (the kind that are turned off my default).

But, obviously, I’m in an aging and shrinking minority here. David Attenborough’s documentaries, which I used to love, are now unwatchable (or rather unlistenable), with lush orchestral music crashing over his narration. If it’s not that, it’s an annoying metronomic repetition of the same five notes over and over. When people complain, the answer is “this isn’t a lecture”. But that’s exactly what I want from a documentary – a lecture with high-quality video combining to convey more information than either alone. Music, by contrast, conveys no information at all (except, I guess, “this is bad music”). If I wanted a content-free audiovisual experience, I’d far prefer a live band at the pub, with smoke and strobe lights, to someone’s musical interpretation of animal behavior overlaid on some barely audible talk.

Thinking about this brings up the more general issue of background music in films. It’s such an established convention you barely notice it most of the time. But I’ve quite often had the experience of hearing vaguely dissonant music as a character enters a room, and not knowing if this is part of the film, supposed to be audible to the character, or just part of the soundtrack. It’s just as artificial in its way as the characters in a musical bursting into song at the drop of a hat, and yet it’s a standard part of what is supposed to be realistic drama.

That’s it from my Grumpy Old Guy persona. Does anyone share my grumpiness, or want to persuade me out of it.