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We currently have two main risk budget options. You are, of course, welcome to choose any risk budget for yourself.
| Title | Risk of getting COVID per year | Weekly microCOVID budget | Suggested for.. |
|---|---|---|---|
| Standard Caution Budget | 1% chance of getting COVID per year = 10,000 microCOVIDs per year |
= 192 microCOVIDs per week | Healthy people NOT in close contact with more vulnerable people |
| High Caution Budget | 0.1% chance of getting COVID per year = 1,000 microCOVIDs per year |
= 19 microCOVIDs per week | Suggested if you or your close contacts are more vulnerable to COVID. |
Vulnerability increases with age. We think age over 60 confers substantial increased vulnerability to severe illness from COVID. Certain underlying medical conditions also confer increased vulnerability:
Risk to you & risk to others
When choosing your budget, you want to factor in two important aspects: risk to you, and risk to others.
Risk to you: If you are in the under-40 age bracket and do not have other risk factors, then a 1% chance of COVID per year puts your risk of disability due to COVID at about the same level as your risk of disability due to driving a car. See below for how we arrived at the 1% annual risk budget.
Risk to others: Even if you are personally comfortable with risk-taking, it’s important to choose a lower risk tolerance in order to protect vulnerable members of your community. Here are a few ways to think about that:
If you get COVID, on average you will pass it to at least one other person, who in turn will pass it to others. Eventually it will reach someone who is at very high risk of death. By avoiding getting infected yourself, you are protecting these vulnerable members of your community.
A useful ethical rule-of-thumb is to behave in a way that would be good if everyone behaved that way. So, we can ask: what risk tolerance would keep the pandemic at manageable levels if everyone followed it? In most places in the US right now, a 1% yearly risk of COVID would keep the pandemic at manageable levels. For public health reasons, we wouldn’t recommend voluntarily choosing a risk tolerance higher than 3%.
Some people (such as essential workers) have no choice but to run a high risk of getting COVID. Containing the pandemic across society only works if those of us who do have a choice choose to take fewer risks.
We think roughly 1 in 10 COVID cases lead to long-term negative health consequences for the infected person ("long COVID"). Data from the UK's Office for National Statistics suggests that 13.7% of COVID-infected people may continue to experience symptoms after 12 weeks and 12% continue to experience symptoms after 18.5 weeks.[1] That would mean that a 1% chance of COVID is the same as up to a 0.12% chance of ongoing negative health consequences from COVID for at least 4 months.
The average American drives 13,476 miles per year. In 2018 there were 2,491,000 car-crash-related injuries (of any severity) spread across 3,240,327,000,000 vehicle miles travelled (see the NHTSA website for more detailed breakdowns). A study of Swedish car crash injuries found that 11.4% of them met the criteria for at least “1% permanent medical impairment” five years later. (“Permanent medical impairment” is a legally-relevant concept that attempts to quantify disability, and the 1% level was the lowest level of disability that was discussed in the Swedish car crash study.) Multiplying this out, we get a 0.11% chance of ongoing negative health consequences from driving for one year.
You may choose to follow a different risk budget. Here is some information that might be helpful in deciding what is right for you:
Also remember that, if you take on personal risk, you also present a risk to others. Take care to isolate from others who do not consent to your risk tolerance.
Yes, definitely! The most important such precaution, in our opinion, is communication around symptoms:
Other important precautions include washing & sanitizing your hands, covering your sneezes with your elbow including when you are at home, using a better-fitting mask, and wearing eye protection (we particularly recommend wearing goggles and a P100 mask if you are looking for extra safety when shopping!)
In the case of a long indoor hangout, we suggest just to cap the total Activity Risk at the household member number (40%). Or if you’re cuddling or being intimate, cap the total Activity Risk at 60% as mentioned above for spouse/partner. These are not particularly principled answers. As we’ve mentioned in footnotes before, the ability to just add microCOVIDs (rather than multiplying probabilities) starts to break down as probabilities get larger. Additionally, the data we’re basing our guesses off is more relevant to the smaller risks that we more commonly see with typical activities. Certainly don’t use an Activity Risk larger than 100% for anything.
No. We’ll explain what this means and why we don’t account for it in the specific case of a single household.
Imagine I live with Alice and Bob. Alice gets sick. There are two things that could happen next that could get me sick:
There’s a 40% chance that I get sick directly from Alice. There’s a 60% * 40% * 40% = 10% chance that the second scenario happens instead. So you might think the total risk to me should be more like 50% than 40%.
We currently don’t model this effect, and we think that’s pretty safe if (and only if) you and your household members are conscientious about monitoring for symptoms and isolating from one another at the first sign that someone in the house is feeling unwell. Such isolation should include some reduction in contact between the household members that still feel fine, since one or more of them might have become infected by the unwell person before their symptoms appeared.[3]
Here’s why isolating helps reduce this source of error:
As a result of these considerations, we don’t think we’re making a huge error by using the unadjusted 40% number for the Activity Risk of having a house hold member.
Of note here: while full isolation is the gold standard for reducing infection risk, it’s useful to also have some lighter-weight tools that you can deploy if you’re uneasy about infection risk for some reason but not uneasy enough to find hard isolation to be worth the (admittedly considerable) social and practical costs. Extrapolating from the MOD factors, we think wearing a mask around the house for several days probably reduces your risk of infecting your housemates by about 4x, which is a lot of protection to get from a relatively simple intervention. In the authors’ household we deploy this one if we’re feeling at all unusual, even if the symptoms (such as a scratchy throat) aren’t suggestive of COVID; or if we’ve recently done something moderately higher-risk than usual.
So long as the people in your “bubble” have some risk of getting infected from anywhere, the risk they pose to you contributes to your total microCOVIDs.
In short, we think “closed quarantine bubbles” are a good strategy for very-low-risk groups who don’t venture outside the house often, but make less sense for larger bubbles with more exposure to the outside world.
To spell it out a bit more: If nobody in your bubble has any exposure to other people or public spaces outside the bubble, then in fact you can count their Person Risk as very minimal.[6] This isn’t a magical property of the bubble being "closed" per se; it’s a result of everyone only socializing with other people who also have very minimal Person Risk. Seeing a friend who lives completely alone and literally never leaves the house would also keep you at this same low risk level. The “closed bubble” framework is a way to coordinate around maintaining a very low risk level together.
But if people in the bubble are doing things that expose them to others somewhat—going to work, grocery shopping, taking trips—then even if they aren’t meeting up for coffee, dates, or events with anyone outside the bubble, they still have some nonzero Person Risk, and you should multiply this with the Activity Risk of seeing them.
This means that, if the people “in your bubble” have some exposure to the outside world, then reducing your total amount of contact is likely more important than making sure you are only seeing people in your bubble.
To summarize, reducing your total amount of contact (and choosing to socialize only with other people who are also reducing their total amount of contact) is (we think, for most people) the most important strategy for staying safe.
Right! To illustrate, compare the following two scenarios:
The two houses merge into one large “bubble”. Everyone gets together for a full group dinner between the two houses.
No specific bubble arrangement, but nobody else in your house socializes with anyone else in the other house. Just your housemate sees just their partner, in the partner’s room with the door closed.
Which strategy is less risky depends on which factor dominates: the partner’s other socializing, or every bubble member’s combined external exposure. This is why we think closed bubbles are the safer choice for people who are generally not working outside home or going out at all, and reducing total contact is the safer choice for people who have a moderate amount of external exposure. We haven’t done the math on this yet, and we would be interested to see an analysis of this.
Yes! Cloth masks and surgical masks protect others more than they protect you (see Research Sources), but they still reduce your risk of catching COVID by half, so they’re well worth it even from a selfish perspective.
These masks provide more protection than a cloth or surgical mask:
Masks that claim to have N95-like filter media (e.x. KN95, Vogmask), can be considered equivalent to KN95's - designed to filter, but not rigorously quality controlled or tightly fitted. These receive a 6x protection factor.
For a well-fitted, well-sealed N95, we roughly estimate an 8x reduction in risk for the wearer (versus 2x for a surgical mask or high-quality cloth mask).
A P100 is even better. We estimate that P100's provide a 20x reduction in risk.
We absolutely love the GVS Elipse. They're available on eBay for $60-90.
Each of these modifiers changes the total number of particles that might reach you, so we do think it’s reasonable to keep piling them on somehow. How exactly they combine is not precisely clear. But if you consult the Research Sources section, the masks and distance modifiers were estimated from data including healthcare settings where they were sometimes combined.
The “outdoors” modifier is the one we have the least confidence of. At larger distances, we speculate that outdoors might provide more than 20x protection, because particles have more time to diffuse upwards into the vast empty space above everyone’s heads, whereas indoors small particles could hang in the enclosed room air. On the other hand, at smaller distances (especially at close range: cuddling, tango dancing, etc.) we don’t have any reported data that we feel sheds light on how much of a protection factor there might be, so we suggest not to use the 20x modifier for outdoor interactions with unusually close range or with other factors associated with superspreader events (yelling, dense crowds, etc.).
We do not think that, as a blanket rule, every person who works outside the home is riskier than every person who works from home. Activites outside of work play a major role in a person's chances of contracting COVID.
For instance, a Healthcare Worker who wears a sealed N95 mask while seeing one unmasked patient at a time for 40 hours per week, but does not see any other people in a typical week, would be calculated as follows:
average_risk * 40 hours * 14%/hr * 1/8 (N95) = 0.7 * average_risk
Therefore, this worker could be less risky than the average person in the area (although the final risk will depend on what else they are doing in terms of chores and socializing).
We recommend using the Advanced Method to estimate the risk of all your close contacts, rather than using blanket risk categories. The Risk Tracker is an excellent tool for keeping a tally of one's personal risk. The microCOVID team includes a Primary Care Provider who tracks and reports her risk using this tool, which suggests that her weekly risk is on the order of 100microCOVIDs per week.
We originally proposed an "Intermediate Method", in which people who work outside the home are guestimated at 2x or 3x the population average. This was based on the following two studies:
However, we have retracted this method for the following reasons:
*Summary: Vaccinated people receive a multiplier on all incoming microCOVIDs from their activities. The multiplier depends on which vaccine they've gotten and is contingent on having waited long enough for their immune system to respond.
People who have been vaccinated are less likely to catch COVID and transmit it to others, which results in them getting fewer microCOVIDs from activities. However, they are not totally immune to COVID, and it is quite easy for a vaccinated person to counteract the decreased risk per activity by doing lots of risky activities (for instance, the Moderna vaccine confers a 1/10x multiplier for incoming microCOVIDs, so a vaccinated person who does 10x more risky behavior after being vaccinated would be just as many microCOVIDs as before).
There are two questions that are important for understanding vaccines:
Below is our best estimates for the vaccine efficacies for various vaccines. These are based on a weighted average of the efficacy of each vaccine vs. symptomatic and never-symptomatic COVID-19 infections. See Research Sources for the full derivations.
| microCOVID multiplier 14 days after 1st dose | microCOVID multiplier 14 days after 2nd dose | microCOVID multiplier 14 days after 3rd dose | |
|---|---|---|---|
| AstraZeneca | 1 | 1 | 0.3 |
| Moderna | 1 | 0.8 | 0.25 |
| Pfizer | 1 | 0.8 | 0.25 |
| Johnson & Johnson | 1 | 1 | 0.95 |
For perspective, the first dose of either vaccine confers equivalent protection as wearing a good quality cloth mask all day, every day on top of your actual mask and even when you normally couldn't wear a mask. Completing the 2nd dose of AstraZeneca’s or a single dose of Johnson & Johnson's vaccine confers more protection than wearing a surgical mask 24/7, and Pfizer or Moderna’s vaccine is as protective as wearing a KN95 mask all the time.
While the vaccines are nowhere near making you invulnerable to COVID, remember that these vaccines are absolutely game-changing — universal vaccination would reduce everyone’s chance of catching COVID by 50% or more. This will (likely) reduce the rate of community spread to the point that every week has fewer cases than the last (R<1). The end of the pandemic is in sight.
The Israeli government released data that suggests that citizens with Pfizer's mRNA vaccine were getting infected at 36% the rate of unvaccinated citizens, but hospitalized at only 7% the rate. This suggests the vaccine confers a 5x reduction in serious cases of COVID in addition to the reduces chances of getting COVID at all.
We have not done a thorough analysis of this effect in other vaccines.
We have not seen data that describes the effect of vaccines on long COVID in mild/asymptomatic cases. The 1% annual risk budget is based on risks of long term effects of COVID (as opposed to hospitalization or death). Reports of prevalence in long COVID in unvaccinated people varies widely:
As a result, getting a vaccination yourself confers a reduction in incoming microCOVIDs based on how the vaccine affects your chances of infecting others (as outlined above). You may additionally increase your budget based on how the vaccine reduces your own chances of negative outcomes, but this is a personal choice dependent on your own view of how averse to the various possible negative outcomes you are.
Some ways you may want to change your budget in after you and your close contacts are vaccinated:
(200 / (1 + 0.4*N)). After everyone is vaccinated with three doses, this increases to (200 / (1 + 0.4*0.25*N)).Example A:
You live in a pod of 6. Prior to vaccination, all 6 housemates had a personal budget of 200 / (1 + .4 * 5) = 67 microCOVID/week. All 6 of you are now vaccinated with three doses.
200 / (1 + .4 * .25 * 5) = 133 microCOVID/week.67 / 0.25 = 268 microCOVID/week.Example B:
You live with or care for someone with a condition that reduces the efficacy of a vaccine (consult their doctor).
Not necessarily. A person's risk is the product of their precautions and activities. It is true that a vaccinated person who does the same activities as a unvaccinated person will be less risky to be around. However, if the vaccinated person starts doing more risky behaviors, they could end up being more risky than before they got vaccinated! The only way to accurately understand a person's risk is to model each of their activities and multiply that total by the modifier from the table above. The Risk Tracker makes it easier to do this.
We dug into claims that the effectiveness of vaccines in Israel is 64% but had concerns about the methodology. These reports control for “age group..., sex, and calendar week” (Haas et al) but not individual behavior. Since Israel’s policy allows vaccinated individuals to participate in many activities with high risk of exposure (restaurants, movie theaters, etc. without masks), we hypothesize that the 64% effective number captures a combination of reduced efficacy of the vaccine vs the Delta variant AND increased opportunity for exposures.
Therefore, we used data from research in the UK that compared # of cases of the Delta variant vs the Alpha variant among vaccinated and unvaccinated individuals, which attempts to assess the vaccines’ efficacy in isolation (Bernal et al.), (Stowe et al.). These studies found 88% vaccine efficacy vs symptomatic COVID, which we adjusted to 84% to account for asymptomatic cases.
We believe that if the person is indoors for <5 mins and wears a mask the whole time, then the risk will be negligible for all but the most cautious risk budgets. This is especially true if you can open a window to ventilate the bathroom space. See this article for more tips about this.
Based on the Jimenez Aerosol Transmission Model, we could contemplate up to a 5x reduction in risk for silence, and a 5x penalty for singing, chanting, yelling, or speaking loudly; plus an additional 5x penalty for heavy exercise.
Yes! Based on the Jimenez Aerosol Transmission Model and estimates of the total area of rooms of different sizes, we think you might consider the following multipliers: ×4 higher risk for a ‘tiny room’ (~10ft across) ×2 for a ‘small room’ (~15ft across) ×1 for a ‘normal room’ (~20ft across) ×0.5 for a ‘large room’ (~30ft across)
These assume you are already sufficiently distanced (at least 6 feet/2 meters) that the size of the room is relevant to the amount of aerosol you’ll breathe. In close quarters, we think the size of the room has less of an impact.
We personally use an additional multiplier of at least 2x for cuddling, Our original take on this was based on the assumption that our data for infection risk under “no particular distance” (which were largely collected in hospitals) reflect a distance closer to 3 feet (1 meter) than 0 feet. If each additional 3 feet (1 meter) adds a 2x improvement in safety, then being right on top of each other (0ft apart) might involve a 2x reduction in safety. This looks consistent with recently released data from train passengers sitting directly adjacent to one another versus merely in the same row (Hu et al.). If you’re breathing right into each other’s faces, more so than train passengers would, maybe use a higher number.
We think it doesn’t make sense to take the full “outdoor” bonus when cuddling, because your faces are very close together. Finally, we think a brief hug probably doesn’t meaningfully change your risk, but we don’t have any evidence for that. Basically, we really don’t know and don’t have any evidence here, so take this with more grain of salt than the rest of our more-research-backed numbers.
A one-time hangout combined with the kissing multiplier is an Activity Risk of 5 ⨉ 14% per hour = 70% per hour, which can be capped off to the live-in partner multiplier of 60%. As a side note, it seems to us that masked sex might be a lot safer from a COVID standpoint than making out without masks... provided you actually have the willpower to keep your masks on!
We don’t feel we have enough data to fully model this risk. But if we assume you aren’t changing how far apart you are, then we think that hand-to-hand touching is more risky than other kinds of touch. Any type of touch is safer if you wash or thoroughly sanitize your hands both before and after touching.
If you’re within hand-holding distance of someone, and you reach out and touch their hand, it does not change your chance of inhaling a respiratory droplet, because your faces didn’t really move. What does change is the chance that any virus that’s on their hands (from rubbing their nose or coughing) could get from your hands into your eyes, nose, or mouth (when you touch your face). Touching hand-to-hand seems likely to be riskier for this route than other types of touch (such as clasping forearms, giving a shoulder massage, or a brief hug with faces averted).
The CDC says that contact transmission is not a main driver, but we haven’t yet seen research on this topic we feel we can trust. For our house, we’ve learned to treat our hands as “contaminated” any time we’re outside the house, and to wash our hands as soon as we get home. As such we don’t currently add an extra budget factor for touching or not. You could perhaps assume that touching bare hands adds another 2x factor (which is as much as we use for cuddling) if you expect you won’t manage to wash your hands before touching your face, but we’re entirely making that up and we don’t use it in our estimates. We don’t have a better suggestion we can stand behind right now.
Touch is a psychologically powerful way to show affection and appreciation. In our experience, outdoor masked hand-holding has a huge positive impact on our mental health and feeling of connection, as compared to assiduously not touching one another. Feeling “touch starved” is a real thing with real psychological effects, and for many people we think touch is a good use of your risk budget.
We don’t currently count microCOVIDs from packages, because of our understanding that fomite transmission is not a primary driver of the spread; most of our risk comes from time we spend indoors with others. People maintaining much lower annual risk levels than us might prefer to sanitize packages to maintain a stricter risk tolerance.
Health officials are often quoted as saying the risk from packages is “very low” and “unlikely,” but until they quantify that in microCOVIDs we’re just going to proceed with assuming it’s less than 1 microCOVID per package. We do know there’s no known evidence of transmission from food packages.
You’re indoors, about six feet (two meters) away from one other person (the driver), and both of you are hopefully wearing masks. It’s an unusually tiny space with poor air circulation by default.
We did some informal tests with a CO2 meter in a Lyft[7], and found that if you crack the windows open by an inch or so, this makes it “just” as well ventilated as a normal indoor space, rather than much stuffier than usual. So we suggest to at least crack the windows, then count it like an indoor space. If they’re chatting or talking on the phone, you might consider politely asking them not to.
If you keep the windows all the way open, the air circulation according to our CO2 meter is just about as good as being outdoors, so you’ll probably reap some of the benefits of being outdoors. We wouldn’t recommend taking the full 10x outdoor modifier for a windows-open Lyft, since it’s unclear how much of the safety of outdoor interactions is due to factors like UV light that aren’t present inside a car. But 2x or so might be reasonable.
You can very conservatively treat transit as an indoor hangout with however many other people are in the bus or train car with you, continuing to take a decrease of 2x per additional 3 feet (1 meter) away that the people are. You’re also probably up to 5x safer in the likely event that nobody is talking (but if anyone starts yelling, which happens on public transit sometimes, you could be 5x less safe). Overall, very few transmission clusters have been linked to public transit.
Unlike transit, airplanes have pretty good air filtration systems: all the air is replaced with new air from outside every 4–5 minutes, and all the air passes through a HEPA filter that eliminates 99.97% of particles (the same as a P100 respirator) every 2–3 minutes (source). That means your risk will primarily be from people near you.
The exact numbers for the Activity Risk will depend on how full your flight is; we’ve used 20 people at 6 feet (2 meters) of distance as a reasonable estimate for a moderately full flight. The worst case of a middle seat on a totally packed flight might be twice as bad as that:
For the Person Risk, we suggest you use the maximum prevalence of your source or destination region.
What about the airport rather than the plane? We still suggest you wear a high-quality mask. However we also note that airports are huge spaces and at the time of writing we’re still at only about 20% of the passenger volume per day compared to what we saw last year—with decreases in demand, the flights get fewer but the airports don’t get smaller. It’s also easier to distance in the airport. Our intuition is that most of your risk comes from the plane.
Overall, there are very few documented cases of transmission on airplanes, which is surprising given that 500,000 or more Americans are flying every day at the time of writing (which, if we naively assume current US-wide prevalence rates, would mean ~100 COVID-positive passengers per day). We are aware of one flight in China in January 2020 that infected 12 people, one case in February probably acquired on a flight, and one flight in March from London to Vietnam that infected 13 people. Erin Bromage has written more on flying in the age of COVID-19.
For grocery stores, we count the average number of people near us: in our neighborhood this is usually about 5 people, keeping 6 feet (2 meters) away, everyone wearing masks. For drug stores, pharmacy pickup, or medical buildings, you might want to add an extra boost to Person Risk to account for the fact that the people you’re encountering are more likely than average to be sick.
Haircuts can be modeled by counting each person in the hair salon. We definitely recommend wearing the best mask you have that won’t interfere with the haircut, and checking in advance that your mask style will be fine. If your stylist is usually chatty, we suggest for an extra safety margin to ask the hair stylist to chat less! We found an anecdote on Twitter about 140 clients who were in close contact with two infected hair stylists, indoors, with both client and stylist wearing masks. 45 clients were tested – all negative – and we haven’t heard of any cases among the others. Last we checked this was statistically consistent with the Activity Risk and modifier suggestions we use in this writeup.
Warning: This answer hasn't been updated for the Delta variant.
The risk of attending a protest depends on how close you get to others, whether they are yelling/chanting, whether they are wearing masks, and other factors we have not modeled in this writeup (for example, we have heard anecdotally that it is lower risk to be in a group where people are moving constantly, rather than staying near the same people for a long time). If you are in a shoulder-to-shoulder group of yelling people without masks, you might be near 10 people within 6 feet (2 meters): Activity Risk = 6% per hour ⨉ 10 people ⨉ (1/10 outdoors) ⨉ 5x yelling (see earlier in Q&A) = 30% per hour. Alternately, if you are in a group of cyclists protesting by biking down the road banging drums and gongs, that might be more like Activity Risk = 6% per hour ⨉ 10 people ⨉ (1/10 outdoors) ⨉ (1/5 silent) ⨉ (1/2 at least 6ft away) = 0.6% per hour. This is a 50 times less risky activity. There are many ways to protest. We encourage you to keep all the same heuristics in mind: if possible, avoid very dense crowds, yelling, and enclosed spaces; and wear masks to protect yourself and others.
The most important thing to remember in attending a protest is that you may not have control over what happens to you. Police might pull your mask off. If pepper spray is used, people are likely to cough uncontrollably. You might get kettled into a small area, even if you were planning to keep your distance from others. One thing you can do to help stay safer is to make a plan in advance about what you will and won’t do: for example, you might decide in advance that if you see any gas used near the protest, you will leave. By thinking about what you are and are not up for, and making choices that take into account the risks, we hope you can confidently and proudly participate in protests that make sense for your risk tolerance.
"Initial dose" — sometimes referred to as “initial viral load” — refers to how big a “dose” of virus you get when you first get exposed to the virus. If you’re exposed to more viral particles, evidence suggests that you’re more likely to develop a severe infection. This means it’s important to try to reduce the strength of your initial exposure, not just in order to avoid getting sick.
For the same total number of microCOVIDs, our guess is that getting them from many smaller independent sources (many mask walks, numerous brief grocery runs, etc.) is more likely to involve a low initial viral dose than getting them from fewer riskier contacts (household members who don’t take many precautions; cuddling a random person whose recent activities you don’t know).
Here are some quantitative analyses we like:
In terms of qualitative scales, we like the following:
The data from the study are truncated at 130 days (~18.5 weeks), at which point 12% of the study participants infected with COVID continued to experience symptoms (Figure 3 from Prevalence of ongoing symptoms following coronavirus (COVID-19) infection in the UK: 1 April 2021). ↩︎
More concerning symptoms include cough, chest tightness/discomfort, obvious sore throat, body aches, malaise, loss of taste/smell, nausea/vomiting, loss of appetite, diarrhea, any subjective “feverish feeling” or elevated temperature, fainting, or thermometer reading of >100.4. ↩︎
You probably can't do this anywhere near as reliably with someone you don't live with, which is why the discussion here is specific to housemates. ↩︎
Perhaps a lot less likely that I get it from Alice if Alice isolates promptly: Li et al. found that isolating an infected household member from the rest of the household as soon as they showed symptoms was effective in all the 105 cases they studied at preventing anyone else in the household from getting sick. ↩︎
This is due to the fact that much less transmission occurs more than 3 days before the appearance of symptoms (He et al, figure 1c middle graph) or fewer than 2 days after infection (Ferretti et al, figure 1 “generation time”). In order for Carol to infect Bob before Alice shows symptoms, Carol’s generation time would have to be greater than the delay between Alice’s infectiousness and Alice’s symptoms, which is unlikely. ↩︎
When we do these calculations, the lowest Person Risk we allow ourselves to assign anyone is 1/100th the Intermediate Person Risk, just as a safety margin. ↩︎
Jimenez’s aerosol transmission tool lends some support to the idea of using CO2 as a proxy for density of exhalations. ↩︎