When states had strict door-to-door orders and locks, many decisions about the risk of getting the coronavirus were easy. People didn’t have to think if dining at a restaurant was safe when the restaurant was closed.
As states open up – perhaps prematurely, and with varying degrees of precaution and compliance, individuals will have to weigh certain risks themselves.
It is not easy; information about what is and is not safe can be contradictory and confusing. A state can allow restaurants to reopen and resume concerts: but should you really go? Is it safer if people can only dine outside?
The hunger for guidance is clear: on 6 May, infectious disease expert Erin Bromage posted a blog post with a summary evidence of transmission risks of the coronavirus, and 17 million people have since read it, he says.
As Bromage points out, the scientific understanding of how the virus transmits in public is improving. Contact tracking studies around the world have taken a magnifying glass to the “super-spreading” events, where one person eventually infects dozens of others. These studies shed light on the main risk factors that create dangerous situations.
One thing is clear from these studies: The main way people get sick from SARS-CoV-2 is from respiratory droplets spread between people up close. The risk of contracting the coronavirus, simply put, “breathes everyone’s breath,” said Charles Haas, an environmental engineer at Drexel University. Droplets fly out of people’s mouths and noses when they breathe, talk or sneeze. Other people can breathe them in. That’s the biggest risk and that’s why face masks are an essential precaution (they help prevent the droplets from spitting far from someone’s mouth or nose).
The Centers for Disease Control and Prevention (CDC) recently updated their guidelines, focusing on the risk of close contact through other forms of transmission. “The virus doesn’t spread easily in other ways,” the CDC said writes. It is still possible for someone to catch it by touching a contaminated surface (more on this below). But it’s “not the primary way the virus spreads,” the CDC says.
As Bromage said in his piece, “We know that most people get infected in their own homes,” from roommates or family members who have contracted the virus in the community.
So how can we estimate the risk of going outside?
The story is one little more complicated than the simple guidelines “stay 6 feet away”. The risk of coronavirus is simply not one-dimensional. We need to think about risks in four dimensions: distance from other people, environment, activity and time spent together.
Let’s walk through it.
A simple suggestion: imagine people smoking or farting badly, and try not to breathe it in
It’s easy to get into the weeds by talking about the risk of catching and spreading the coronavirus when people enter the common areas of society. We can talk about the number of virus-laden droplets expelled by a single breath (a lot ofperhaps 100 or more), by a talking person (10 times more than breathing), about how far sneezing can propel those drops (much further than 6 feethow long those viral drops linger in the air (about eight to 14 minutes, at least in a controlled laboratory environment).
But what all this really means is that Covid-19’s biggest risk is breathing, laughing, coughing, sneezing, talking, people.
However, the risk is still difficult to imagine because the respiratory droplets are not visible to our eyes.
Maybe useful: Imagine everyone smoking, like Ed Yong reported in the Atlantic, and you want to avoid breathing in as much smoke as possible. In a tight interior space, that smoke will quickly become dense and heavy. When the windows are open, some of that smoke blows away. If there are fewer people in the room, less smoke will build up and it may not come to you if you are far enough away. But spend a lot of time in an enclosed space with those people, and the smoke gets denser.
An alternative image for thinking about this risk: “With my kids, I’m kidding that if you can smell their farts, you have to go further apart,” says Bromage. So if you don’t smoke, imagine everyone farting. Keep this in mind and you will realize that outdoor activities are better than indoor activities. “This tells you the degree of risk,” says Bromage. “The closer you get together, the more it will smell, the more dangerous it is.”
At a barbecue, you can still imagine being close enough to people to smell their farts. So even in outdoor areas we have to limit our contacts.
A crowded covered place, with poor ventilation, filled with people talking, screaming or singing for hours is the most risky scenario. A sparsely populated interior with open windows is less risky (but not completely safe). Running quickly past another jogger on the other side of the spectrum: minimal risk.
There are many scenarios in between. “In general, outdoors is a lower risk,” says Muge Cevik, a physician virology expert at the University of St. Andrews, says. But “if you have a meeting or barbecue outside and you spend all day with your friends, your risk is still higher.”
What recent contact tracing studies can teach us about risks
Scientists pointed to a few recent contact detection studies that nicely illustrate the dimensions of the Covid-19 risk.
In China, 8,437 customers and supermarket employees were tracked in late January after one of the employees was positively confirmed with Covid-19 while working in the store.
The risk of infection was much higher for the workers than for the shoppers. About 9 percent of supermarket employees (11 out of 120 employees) became ill as a result. But only 0.02 percent of shoppers (2 out of 8,224 shoppers) fell ill.
What does this show?
Employees are at much greater risk because of the time spent in the store. Both employees and shoppers were in the same physical space, but their risk was not the same. (The study did not notice whether shoppers and customers wore masks in the store.) Employees may have had more contact with colleagues, but were also more likely to inhale the virus.
What we need to learn from this: If we need to spend time with people indoors, try to make it fast!
Another recent research from China investigated an outbreak that started during a Buddhist temple event.
Two buses brought people to the position. In one of the buses, there was a person who later tested positive for the coronavirus who did not yet feel any symptoms. The other bus was free from infected people.
Both buses brought people to the same temple, where they mixed and mingled. But who was most at risk of getting sick? Those who rode on the bus with the infected person. Twenty-four of the 67 people on that bus fell ill. Nobody on the other bus did that. The event was attended by a further 172 people arriving by other transportation. Only seven of these people fell ill.
The lesson? The boundaries of a bus are a much, much more risky environment for viral spread than a much larger space, such as at the temple. The risk in the temple was not zero. But it was much smaller compared to the limits of the bus. So if you have to choose between a large open interior space and a smaller one, choose the larger one, where people can spread out.
“If you look at public transportation, workplaces, restaurants – places where we just try to put a lot of people in a small confined space – respiratory viruses like those spaces,” says Cevik. It is ‘just common sense’.
There is no set time to be safe in these places. “We generally say 15 minutes for drip transmission,” says Cevik. “So if you’re face to face with someone for 15 minutes, you’re in close contact [and at high risk], but that doesn’t mean that if you spend 14 minutes, your risk is zero. “
And it’s not just the location or the time spent together: the activity that people are also engaged in.
In Washington State, a person with the virus attended a choir practice, and more than half of the other singers subsequently fell ill. This was labeled a “super spreading” event, as one infection led to 32 others. Why was this so risky?
“The super-spreading event is about the person’s behavior,” says Cevik. There are many reasons why someone could become a ‘super spreader’: some people lost more of the virus than others, and it seems that people lose the most when they are just starting to feel symptoms. But what made this event so risky was the convergence of many risk factors: the singing activity (where the infected person released viral particles into the air), the time spent together (the exercise was 2.5 hours), and the interaction between the choir members in a confined space (not only did they all practice together, they also split into smaller groups and shared biscuits and tea).
How about touching something with infected drops? Is that still a risk?
In the updated guidelines, the CDC says that the coronavirus does not spread easily from people who touch surfaces. That is, if someone with Covid-19 touches a handrail, does that handrail make other people dangerous to touch? The CDC now says that such events do not pose a major risk for the transmission of Covid-19.
But there is a caveat: it is still the case that surface transmission is possible. Scientists to believe the virus can remain viable for about three days on a hard, non-porous surface such as plastic or steel, and a rough surface such as cardboard for about one day.
Bromage warns that it is simply very difficult to study surface transmission. In contact tracking studies, it’s much easier to ask people they’ve been in contact with than to have them remember every surface they touched.
“I agree with this [CDC] explanation, “says Cevik, who agrees that surfaces are not the main transmission mode.” But this doesn’t mean it doesn’t happen. ” Cevik points me to a contact search study that suggests (with much uncertainty) that some people contracted the infection in a shopping center through the toilet. “The bottom line,” she says, “it’s still important to maintain personal hygiene and wash hands.”
Also consider how scientists recently found Covid-19 virus in the stool. So good bathroom hygiene is still as important as ever.
There are no magic numbers to eliminate risks
It would be great if there are very specific numbers and guidelines that we can follow to minimize the risk of coronavirus.
But there is none. Although it is a meter or two away from another person, it is not the case that the virus immediately decides to drop dead. Therefore, we must consider risks in terms of many dimensions: so that we can each think critically and not fall back on rules that are simplified.
“When I first said restaurants were a risk, people interpreted it as ‘every restaurant is a risk,'” says Bromage. “Each restaurant has its own unique environment, its own unique challenges that must be worked out. If you have a large open seating area and you can open the windows and doors … the risk is much lower than a five-table boutique restaurant that creates a really intimate atmosphere. ”
When we go out into the world, we have to remember that we can reduce but never eliminate risks.
“Wearing a mask will not completely reduce your risk, washing hands will not completely reduce your risk, and keeping away from people in a confined space will not reduce your risk completely,” Haas, the Drexel professor says. “But using all those strategies at the same time will hopefully reduce your risk to a lower level. We can never get to zero. There is no such thing as zero risk. “
And we still need more data.
Contact tracing studies have taught us a lot so far. But as of now, most of this work has been done in Asian countries, which may have different expectations when it comes to wearing masks, among other things.
“Contact tracking, testing, isolation – these are the building blocks of understanding where the transfer is taking place,” said Cevik. And the more we learn, the more powerful we become to stop the spread of this pandemic.