Not all gatherings are equally risky. The risk of infection is determined by several factors that are related to how COVID-19 is spread from person-to-person. Transmission occurs through the air when someone breathes in droplets or aerosols produced by an infected person through sneezing, coughing, or in enclosed spaces through breathing, or when they touch a contaminated surface and then transfer the pathogen to their eyes, nose or mouth. The risk of infection goes up to the extent that the susceptible person is:
- In frequent contact with an infected person
- Spends a lot of time in the presence of an infected person
- Is in close proximity to and/or physically interacts with an infected person
- Is exposed to many infected people.
Thought of this way, it makes sense that different venues will pose different kinds and degrees of risks of infections. The table below gives an indication how different venues rate on these dimensions. Households are very high on the frequency, duration and intensity dimensions—greatly increasing the probability of infection among household members if one member is infected. But because there are generally few people in any given household, overall exposure is quite low, which is why shelter-in-place strategies work so well.
Close social circles, friends and extended family you may spend considerable time with, can be high on the first three dimensions, meaning the risk of transmission among its members can be quite high. This can be particularly dangerous for older vulnerable populations if grandparents are in the network.
More public venues present a more mixed bag. We’ll leave it to you to consider all the dimensions for different venues, but a couple of comparisons bear mention. Because grocery stores bring together people from all walks of life, they can be quite high on the exposure dimension. But because traffic is spread out throughout the day, we generally don’t spend a lot of time in any one visit, and physical distancing is possible in most stores, with care there are many ways to reduce risk. Maintaining appropriate distances, scheduling your shopping for low traffic times of day, washing your hands, and wearing masks are all effective strategies.
Compare this with going to church, for instance, in which many congregants come together one or more times a week and sit close together in the same room for an hour or two. They are higher risk especially on the duration and intensity dimensions. The same is true of other venues, like bars and restaurants, where large groups of people spend time together in close proximity. Sports arenas, with their huge crowds, are especially dangerous—risk of transmission grows geometrically with group size.
Finally, hospitals are particularly high risk environments as they are very high on every dimension. Health care professionals are in frequent and intense contact with many infected people all day, which is why they should have first priority for scarce personal protective equipment like N95 masks, face shields, and protective clothing.