At risk

We often hear groups of people described as ‘high risk’ in the context of COVID.  The problem is that this means three different things, and they often aren’t distinguished clearly

• People who have a relatively high exposure to coronavirus, so they are more likely to catch it: nurses, doctors, supermarket staff, police (high probability)
• People who are more likely to get seriously sick if they do become infected: elderly, immunocompromised, people with chronic lung disease (high consequence)
• People who are more likely to spread the infection if they get it. Some overlap with the first group, but also migrant workers, prisoners, and at least in the US, meat processing workers. (high transfer)

The second group are very different from the others. Suppose you were doing intensive testing to try to see if there was undetected community transmission of COVID.  You’d definitely want to test the first group, because that’s where you’re most likely to find the virus, and you might want to test the last group, because missing it there would be serious (as it was in Singapore).  You might well not go after the second group, because the safest thing for them is isolation — having a bunch of health workers barge in and stick swabs up their noses is unpleasant and possibly risky.  You’d absolutely want to test the second group if there was any indication of symptoms or exposure, but not just in the ordinary course of business.  The three groups are different.

Even Cory Doctorow confused the first and second groups a bit, in his rant about the risks of contact-notification apps

The proximity sensing they do is going to miss out on people who don’t have smartphones and/or don’t have the technological savvy to install them. That overlaps broadly with the most at-risk groups: elderly people and poor people.

Epidemiology is a team sport and the most vulnerable people are the MVPs on the team. “Our app will tell you if you came in contact with an infected person (but not if that person is from the most likely group of infected people)” is a fundamentally broken premise.

Elderly people are a ‘high consequence’ group — infection is serious for them. They aren’t a ‘high probability’ group — there’s no special reason why elderly people in the community would be more likely to get infected (or in residential facilities, if good care is taken)