Thursday, March 19, 2020

Why is COVID-19 proving so much worse than SARS?

So, here's the next in my short occasional series of Asking the Hard Questions About COVID-19 (or at least trying to plug some of the more glaring holes in my understanding of it).
COVID-19 is, in the scheme of things, a similar disease to the SARS outbreak of 2003, and supposedly we are better prepared for it. And yet here we all are in lockdown for weeks, possibly months, to come. Borders are closed, flights cancelled, social distancing is the norm, all live entertainment and many stores have been closed down indefinitely, and governments are throwing trillions of dollars at supporting the world's teetering economy. SARS, on the other hand, lasted about three months in total (or at least its main period did), and, while we all took some precautions, hardly anything closed, and the world's economy did not crumble. Panic and existential gloom did not pervade the world.
As of March 19th, COVID-19 has already infected at least 220,000 worldwide in 137 countries (double the number of less than two weeks ago), and nearly 9,000 have already died, with no end or let-up in sight, while SARS infected a mere 8,000 in 26 countries, with 774 deaths in total. And remember how bad we thought SARS was at the time!
So, what makes COVID-19 so much worse?
My first thought was the R0 or "basic reproduction number", i.e. the number of new cases an infected person will cause during their infectious period, a measure of the transmissibility and intensity of a disease. Many people (even some "experts") seem to believe that COVID-19 is much more transmissible than SARS. My pretty well-informed wife, certainly, was under the impression that COVID-19 is much more infectious and virulent than SARS. But it turns out that best estimates for the R0 for COVID-19 is somewhere between 1.5 and 3.5 (other ranges I have seen are anywhere from 1.4 to 3.9 - this is a contentious issue). The SARS pandemic had a R0 of 2.75 in its early stages, falling to less than 1.0 just a month or two later (ranges for this are between 2 and 5 - like I say, contentious). Most people (like this Nature article, for example) seem to be going with a R0 of 2 - 2.5 for COVID-19 and 2 - 4 for SARS. Either way, COVID-19 is clearly not significantly more virulent than SARS, and probably less so.
As for the deaths rates, SARS resulted in a horrific 10% death rate, while COVID-19 appears to be settling around 4%, probably less if all cases are included - bad enough, but not even close to SARS (the MERS coronavirus outbreak of 2012 had a death rate of 34%!).
Incubation period, then? The median incubation period for COVID-19 is about 5 days, with a range of anywhere from 2 to 15 days, not dissimilar to that of SARS.
So, what else? An article in The Lancet offers a few theories (although it too assumes a higher transmissibility rate):
  • the epicentre of the COVID-19 infection, Wuhan, China, is a denser, and more open, trade-orientated  city than the epicentre of the SARS outbreak (Guangdong province) -personally I find this particular factor inconvincing: SARS actually started in the huge, bustling city of Guangzhou; 
  • the Chinese people in general are much more mobile than they were 17 years ago, as are the populations of other countries that were instrumental in spreading it around the world (particularly Iran and Italy);  
  • in fact, the whole world is much more interconnected than it was 17 years ago;; 
  • at the time of the initial COVID-19 outbreak, many people were already on the move because of Chinese New Year and Spring Festival; 
  • probably most importantly, mild and asymptomatic cases are much more common with COVID-19 than SARS, and transmission appears to peak during the early stages when it is much  more difficult to spot, whereas for SARS the "peak viral shedding" period was after patients were already quite ill and therefore easily identifiable; 
  • for the same reason, there are many more unknown contacts, and therefore much more community transmission, with COVID-19, whereas transmission of SARS was largely within a (relatively manageable) hospital environment.
Sounds convincing to me. Depressing, but convincing.

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