How an Immunology Blog Became a Covid-19 Guide to Going Out

Indeed they are. What studies have been the most informative for you as far as assessing the risks of various places and activities?

It’s not so much been any one individual publication, but rather the conversations that scientists are having on Twitter. There’s an epidemiologist at Boston University named Ellie Murray, who put together a feed of the 60 most prominent scientists and communicators in the field. So you can watch in real time this brilliant collective hive mind go to work on every new paper that comes out. That allows consensus to converge fairly quickly.

But you did take a closer look at a number of known super-spreading events. What was the common thread that emerged from that analysis?

The commonality with all of them—a call center, a restaurant, a sporting event—they all involved lots of people in enclosed spaces for prolonged periods of time. More recently we’re seeing clusters in places like churches, a bus tour in Singapore, nightclubs in South Korea. It’s all the same thing. The longer you’re with lots of other people indoors, the higher the risk. The evidence on this is almost unquestionable by now.

So what’s the epidemiological principle at play behind that observation?

It all comes down to the idea that for an infection to establish itself in your body, you need to get an infectious dose of virus. That’s going to be different for each person based on their genetics and how healthy they are. But on average, the science predicts that exposure to as few as 1,000 SARS-CoV-2 particles is enough to establish an infection.

And as more science has been done to establish the various routes of transmission with this new coronavirus, we know that can happen in different ways. It can come all in one blast—a sneeze, a cough, loud talking, or singing. But you can also get that dose of the pathogen over an extended period of time, just being around people who are breathing or talking normally. So it’s not just one interaction that can lead to an infection, it can be a longer period of exposure that can make you sick. It’s a well-known principle that dose and time are the two most important factors for risk of infection.

I work with agricultural animals, mostly with managing infectious disease in fish farms. And when a pathogen gets into one of the larger operations, it can move through that population incredibly quickly. We’re not talking three months to hit peak infections; we’re talking as little as a week. And literally millions of animals can die over that time span. So a lot of my research is focused on identifying viral and bacterial threats and mitigating the risks of disease spread, either through vaccines or managing the environment of those animals in a way to make it harder for the pathogen to move through the population. But the same basic principles apply to humans and coronavirus.

Are there any rules of thumb you use to determine what sorts of things you and your family will do?

Yeah, it’s really no different than crossing the road. We have rules for that. We know it’s safe if we look and there aren’t any cars. It’s the same with this virus. If you’re in the outdoors and there is good spacing—more than six feet between people—you probably don’t have to be on your guard. Large groups of people not respecting social distance out of doors is riskier, especially if they’re hanging out for long periods of time. The riskiest scenarios involve being indoors with other people who are talking for longer than five minutes. Sustained contact is the important part. That’s where you need to really be cautious.

To go back to Ellie Murray, she built a beautiful graphic with a sliding scale of risk that goes to the heart of what’s important. Getting outside, even with groups, as long as you respect physical distance, is a low-risk activity that you can do. We need to be able to tune our anxieties to where the real risks are.

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