Day 137: The Salmon Panic

The case count in Beijing is now up to 106, with 8 more cases exported to a couple of other Chinese cities, all attributed to an outbreak at the Xinfadi wholesale market. Much of the capital has been locked down, and testing of about 300,000 residents and workers from the area is underway. China's chief epidemiologist noted that the cold, wet conditions of the wholesale market may have contributed to the outbreak (not unlike similar incidents in meat packing plants in the US, though he did not make that comparison).

Beijing's salmon panic happened a few days ago now, but it's one of those crazy coronavirus things that deserves a note in the historical record. The salmon in question was imported from Europe, as was the coronavirus strain behind the current Beijing outbreak, and chopped in a now-closed smaller market where a few cases were found along with the smoking SARS on the cutting boards. Yet coronavirus on food remains an unlikely route of contamination; it's far more likely that human beings carried the European strain into the wholesale markets than a boatload of dead fish did it. Nor does that reimportation need to have been recent (PlagueBlog has not seen the genetic analysis that traced it to Europe); the spikier Western strain of coronavirus has been on the rise worldwide for a while now.

Nevertheless, mostly harmless salmon was purged from the shelves of many Beijing grocery stores this weekend. PlagueBlog is not necessarily reassured that the Chinese people can still panic and do irrational things over a handful of cases, just like westerners. It seems innumeracy is still the biggest scourge of the pandemic.

A new paper in Physics of Fluids examines the droplets coming shooting out of and around surgical masks over time and "cough cycles". Ersatz masks are not considered, and N95 masks are mentioned mainly in the context of prior results, including this handy overview of ten other papers:

Previous studies report limited effectiveness in using surgical masks to reduce respiratory illness, and clinical trials report little effect on infection rates with and without surgical masks. In contrast, laboratory studies concerning coughing and infectious subjects showed that surgical masks are effective at reducing the emission of large droplets and minimizing the lateral dispersion of droplets. However, they allowed simultaneous displacement of aerosol emission upward and downward from the mask. Several randomized trials have not found statistical differences in the effectiveness of surgical masks vs N95 filtering face-piece respirators (FFRs) at reducing respiratory diseases for healthcare workers.

While the physics is interesting, the conclusions are somewhat more optimistic than they ought to be, considering that the paper considers the many routes of particles out of masks but not their relative infectiousness compared to unmasked droplets. If the masks were helping as much as the physics says, one would have expected better results on the previous studies that did consider actual respiratory infection.

Massachusetts cases are up a fifth of a percentage point today.