As some places in the United States and other countries are opening back up (some very prematurely), immunologist and biologist Dr. Erin Bromage has written a practical guide to the known Covid-19 risks and how to avoid them that’s based on recent scientific research. He begins:
It seems many people are breathing some relief, and I’m not sure why. An epidemic curve has a relatively predictable upslope and once the peak is reached, the back slope can also be predicted. We have robust data from the outbreaks in China and Italy, that shows the backside of the mortality curve declines slowly, with deaths persisting for months. Assuming we have just crested in deaths at 70k, it is possible that we lose another 70,000 people over the next 6 weeks as we come off that peak. That’s what’s going to happen with a lockdown.
As states reopen, and we give the virus more fuel, all bets are off. I understand the reasons for reopening the economy, but I’ve said before, if you don’t solve the biology, the economy won’t recover.
But since things are opening up anyway (whether epidemiologists like it or not), Bromage goes through a number of scenarios you might potentially find yourself in over the next few months and what the associated risks might be. His guiding principle is that infection is caused by exposure to the virus over time — increase the time or the exposure and your risk goes up. For example, public bathrooms might give you a ton of exposure to the virus over a relatively short period of time:
Bathrooms have a lot of high touch surfaces, door handles, faucets, stall doors. So fomite transfer risk in this environment can be high. We still do not know whether a person releases infectious material in feces or just fragmented virus, but we do know that toilet flushing does aerosolize many droplets. Treat public bathrooms with extra caution (surface and air), until we know more about the risk.
But being in the same room with another person simply breathing may not carry a large risk if you limit the time.
A single breath releases 50-5000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released through nose-breathing. Importantly, due to the lack of exhalation force with a breath, viral particles from the lower respiratory areas are not expelled.
But that time would drop sharply if the person is speaking:
Speaking increases the release of respiratory droplets about 10 fold; ~200 copies of virus per minute. Again, assuming every virus is inhaled, it would take ~5 minutes of speaking face-to-face to receive the required dose.
Again, this is all indoors. Being in enclosed spaces with other humans, particularly if they are poorly ventilated, is going to hold higher risks for the foreseeable future.
The reason to highlight these different outbreaks is to show you the commonality of outbreaks of COVID-19. All these infection events were indoors, with people closely-spaced, with lots of talking, singing, or yelling. The main sources for infection are home, workplace, public transport, social gatherings, and restaurants. This accounts for 90% of all transmission events. In contrast, outbreaks spread from shopping appear to be responsible for a small percentage of traced infections. (Ref)
Importantly, of the countries performing contact tracing properly, only a single outbreak has been reported from an outdoor environment (less than 0.3% of traced infections). (ref)
The Michael Pollan version of advice for socializing during the pandemic might be: Spend time with people, not too much, mostly masked and outdoors.
Tags: COVID-19 Erin Bromage medicine science