Activity Risk is the chance that an activity will transmit COVID to you, assuming the other person currently has COVID.
In this section we explain how we estimate Activity Risk for an activity involving one other person. If you just want to compare our risk estimates of specific activities (such as grocery shopping for an hour, or eating in a restaurant), and you are not interested in learning how to do those calculations yourself, you could skip this section and instead explore the example scenarios in the calculator.
We start by first estimating the risk of interacting with a single COVID-positive person indoors for 1 hour at a normal socializing distance of 3 feet (1 meter) while having a normal-volume conversation.
Activity Risk of talking to 1 person who has COVID, for 1 hour, indoors, unmasked, at 3 feet (1 meter) = 14%
So if you have a friend over to your house to chat for an hour, and your friend turns out to have COVID, we estimate the chance of you getting COVID from that single interaction as 14%.
How did we come up with this number? It's a rough estimate combining many sources: the Hu et al. train passenger study, the Jimenez Aerosol Transmission Model, Bi et al. which uses contact tracing data, the Chu et al. meta-analysis, and the Cheng et al. prospective study. We have since increased this number by 1.5x based on a study by Davies et al on the increased contagiousness of B.1.1.7., and then again by another 1.5x based on a study of the Delta variant by Allen et al.. For the gory details of how we combine these sources, please see Research Sources.
Think of this as our “reference interaction.” We can now use it as a starting point to estimate the risk of other kinds of interactions.
Not all interactions are exactly an hour, at a distance of 3 feet (1 meter), etc.
To estimate the Activity Risk of a different interaction (an outdoor picnic, or being in a restaurant), we modify our estimate based on how the interaction in question is different from the “reference interaction” above, based on the following factors:
For example, we might consider having lunch with a friend in the park as interacting for 2 hours (duration), outdoors (location), with someone who is sitting about 6 feet (2 meters) away (distance), without masks because you are eating, talking at a normal volume.
Or we might think of a trip to the grocery store as interacting for 30 minutes (duration), indoors (location), with people who are more than 6 feet (2 meters) away most of the time (distance), who are wearing masks, and who are not talking.
Here are our estimates for the change in risk based on these modifiers:
| Modification | Change in COVID risk to me | Citations: Why do we think this? |
|---|---|---|
| I’m wearing a surgical mask[1] | / 2 | |
| Other person is wearing a surgical mask | / 4 | |
| Outdoors | / 20 or more | |
| 6+ feet (2+ meters) distance | / 2 | |
| Each additional 3 feet (1 meter) of distance (up to 12 feet (4 meters)) | / 2 | |
| Loud talking (shouting, talking over music, singing) | ⨉ 5 | |
| Not talking (such as riding the train) | / 5 |
If you’re taking multiple precautions, multiply the COVID risk reductions together. So if you’re wearing a mask and they’re wearing a mask, then your reduction in COVID risk is 2x * 4x = 8x.
What about if the interaction is with more than one other person? We’ll get to this later, in the section on Putting it all together. For now, even though it may seem a little silly, we will still imagine there is just one other person (a picnic with one friend, just one other person in the restaurant, etc).
There are plenty of other precautions you can take to reduce your risk that we don’t describe here. For more on other precautions, see the Q&A.
There is substantial uncertainty in many of these numbers. We’ve taken uncertainty into account when giving our estimates, so that even if we’re off the mark it’s unlikely to expose you to much more risk than you’re comfortable with. We've also used a slightly conservative prevalence estimate, so that our overall estimates are conservative without distorting the relative risk comparisons. See the discussion of Research Sources for details about the data we based these numbers on.
To calculate the Activity Risk of lunch in the park with your friend, start with 14% (the Activity Risk for our “reference interaction”) and apply modifiers as needed: 14% * 2 (hours) / 20 (outdoors) / 2 (distance of 6-9 feet (2-3 meters)) = 0.7%. You have a 0.7% chance of getting COVID from this single activity if the other person has COVID. Note that there is no modifier for masks or volume because the reference interaction is already unmasked and at normal volume.
What about indoor dining with that friend? You’re there for 2 hours, indoors, at a distance of about 3 feet (1 meter), without masks. Start the same way with 14% and apply modifiers: 14% * 2 (hours) = 28%. Since the reference interaction is already indoors, at a distance of about 3 feet (1 meter), without masks, and at normal volume, you don’t need further modifiers. You have a 28% chance of getting COVID from your indoor lunch if your friend has COVID.
Compare the risk of getting sick from these interactions: 28% for indoor dining vs. 0.7% for lunch in the park. That’s a big difference! Remember, your friend won’t necessarily know that they have COVID, and it’s very common to be infectious before you show symptoms. And that’s with only one person!
Hopefully this puts the risk of indoor gatherings into perspective. To compare the Activity Risk of other activities, such as going grocery shopping or attending a large outdoor party, try our calculator or check out the Q&A for tips and tricks. In the next section, we will look at calculating the risk of activities with multiple people.
Indoor unmasked interactions are quite risky, but being outdoors and wearing masks both make a huge difference.
Masks: We estimate that masks reduce your risk by 8x. This is if both people are wearing masks. Your mask decreases the risk to you by about 2x. And their mask decreases risk to you by 4x. This assumes a reasonably well-fitting surgical mask. For more protection, there are other types of masks you can wear (addressed in the Q&A). Bandanas, buffs, or other single-layer coverings provide significantly less protection than we estimate here (see Research Sources), so for simplicity we treat them as "no mask", even though we do believe they provide some benefit.
Outdoors: We estimate that being outside reduces your risk by 20x or more because the outdoors is well-ventilated, so small respiratory droplets are less likely to accumulate. With any wind, the risk is even less. We think being outdoors is by far the most valuable thing you can do for your safety, although even that is not a guarantee.
Distance seems to be not quite as beneficial as wearing a mask or being outdoors. We estimate keeping 6 feet (2 meters) apart reduces your risk by 2x, and another 2x for each additional 3 feet (1 meter), up to 12 feet (4 meters). This gives a total risk reduction of:
If you’re outdoors and 20 feet (7 meters) away from a person, you can probably ignore the risk from them.
Volume of conversation also matters. If the other person isn't talking, we estimate that reduces the risk to you by about 5x, because they are not expelling as many respiratory droplets. This actually makes some activities where people don't talk much, such as taking public transportation, safer than they would otherwise be. On the other hand, we estimate that loud talking, shouting, or singing increases the risk by about 5x (as compared to a conversation at normal volume) because more respiratory droplets are exhaled and expelled when you are speaking or breathing forcefully.
Curious how we got these numbers? Again, check out the Research Sources!
We do the calculation differently for household members and spouses/partners because you’re likely to interact with them on a recurring basis. We estimate a single, fixed Activity Risk for one week of living in the same household as someone who is COVID-positive.
Activity Risk of living with 1 household member, who has COVID, for 1 week = 40%
Activity Risk of living with 1 spouse/partner, who has COVID, for 1 week = 60%
These numbers clearly show that if your housemate gets COVID, it is not inevitable that you will get COVID too! Even if your spouse or partner (who you are likely to share a bed with) gets COVID, your chance of getting it is still only about 60%.
You might ask: why isn’t the risk of getting COVID from someone in your house higher? Why is the estimate of household member risk (40%) only as bad as about 3 hours of hanging out indoors with a friend (at 14% per hour)? We don't really know why this is, but we do know that over the last year and a half, studies continued to find transmission rates within a household well below 100%. Some hypotheses include:
The original household member estimate comes directly from Curmei et al. meta-analysis. The partner estimate is very speculative, based on adjusting Curmei et al. using a datapoint from Li et al.. We increased the household transmission rate for delta based on Allen et al.. We scaled the partner transmission rate based on the same study. See Research Sources for slightly more detail.
To learn about the scientific research supporting our estimates, please see the Research Sources section.
Now we understand Activity Risk, or how your chance of getting the virus changes based on the activity you’re doing. But Activity Risk assumes the other person is COVID-positive. What are the actual chances that whoever you're interacting with has COVID? Let’s look at Person Risk to understand that.
The Calculator and Risk Tracker include a more detailed breakdown of different mask types with different multipliers. Our masks research section explains the detailed sources and reasoning for these different types. ↩︎