Mass Transit in the Time of Coronavirus

My commute includes 45 minutes each way on BART, the San Francisco Bay Area’s mass transit system. How can I stay safe from coronavirus when work-from-home ends and I’m called back to the office?

There’s a great new review article in The Lancet by a group of researchers calling themselves COVID-19 SURGE. The paper tells us a lot about staying safe in close proximity to people who are infected with coronavirus. It gathers results from 172 studies of different safety measures from SARS-1, MERS, and SARS-2. Most of the studies took place in healthcare settings, where the baseline risk of infection was much higher than in the general public. That makes it easier to gather data on which interventions matter. None of the studies were randomized, so we’ll have to make do with observational studies. That means this paper could be overestimating the benefits of these tools, if the people using them also used other tools the researchers didn’t track, or underestimating the benefits if people used these tools because they knew they were in particularly risky situations.

The researchers consider four protection measures.

0. Physical distancing: This does you no good on a packed BART car, but it matters elsewhere. Healthcare workers who were able to stay over 1 meter away from infected patients were about 80% less likely to be infected than those who got closer. Staying 2 meters away gave a bit more safety. They tracked which studies also included N95 masks, but I can’t tell if or how they adjusted for that.

  1. Surgical / Cotton masks: People wearing surgical or cotton masks were about 70% less likely to be infected than people with no masks.
  2. N95 masks: People wearing N95 masks were over 90% less likely to be infected than people with no masks. N95 masks should be more effective than surgical masks, but the error bars are wide enough it’s still possible there’s no difference. The authors discuss some evidence that N95 masks are only different for healthcare workers who a) are around a high concentration of aerosolized virus and b) wear them properly (I swear most people leave off one of the straps so it’s easier to breathe, but that completely defeats the purpose!).
3. Eye Protection: This is the part of the study I’m most excited about. I hadn’t previously seen anyone quantify the benefits of eye protection and I was on the fence about wearing ski goggles on BART. In short, they find a huge benefit of eye protection, nearly an 80% risk reduction. That’s so large it strikes me as nearly implausible. The study with the highest weight in the meta-analysis (Pei et al) reports the benefit for each item of PP&E separately, without controlling for people wearing more than one kind. Still, the third highest weighted paper (Yin et al) reports that the benefits of goggles were still large even if people were already wearing a mask and shoe coverings.

Ultimate SARS-CoV-2 Defense

The paper by Yin et al, which studied healthcare workers exposed to SARS-1, considers the ‘dose response’ of wearing many different kinds of PP&E. It’s a funny use of the term. Usually that means taking more of a drug, and benefits always plateau. In this case, though, they find that wearing multiple forms of PP&E gives a larger relative benefit than you’d expect from each kind of PP&E alone. They suggest the ultimate combination that reduces the risk of infection to near 0: wear a mask, a gown, gloves, goggles, foot coverings, and wash your hands. If the coronavirus risk is so bad I feel the need to wear a disposable gown and shoe covers and wash my gloved hands, I think I just won’t be getting on BART. After reading all this, though, I’ll definitely be wearing ski goggles on BART until I get vaccinated.

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