Yesterday I wrote about how indoor contra dancing, with one set of precautions, seems to be safe enough that I would be ok resuming organizing dances. How safe it is, however, depends a lot on which precautions organizers take. I looked over a range of potential options, many of them suggested in the comments, trying to figure out which ones are worth it. I'm going to try to keep this post general, though the specific scenario I have in mind is a social dance.

Summary: requiring vaccination, requiring (and offering) surgical masks or better, using air purifiers, not serving food, and discouraging cheering are worth it; checking vaccine cards and requiring rapid tests are not.

For each option, let's start by estimating the cost in time and money, and the benefit as a percentage reduction in transmission. For the latter, all numbers are from microcovid unless otherwise specified.

  • Vaccination
    • Require participants to be fully vaccinated. Relative to no vaccination, reduces transmission by 0.5x and reception by 0.17, for a total of 0.08x. Relative to no requirement, depends a lot on your community. Since 81% of adults here are full vaccinated, requiring vaccination has a marginal effect of 0.3x.

      The vaccine is free to individuals, and takes two doses at under an hour each, plus, say, a 50% chance of losing a day to side effects after the second dose. This cost is shared over many activities and events, not just a single event. Let's scale the cost down by at least 365x to adjust for that sharing, though I could see arguments for much higher scaling factors. Cost is then 2.3min, for the 19% of unvaccinated possible attendees, or ~30s per person on average.

    • Verify vaccination status, by asking to see vaccine cards, physically or via photo. Whether this is worth it is going to depend a lot on how trustworthy your community is. At very high levels of trust, you can just take people's word. At extremely low levels of trust, and for a high demand event, you start to run into problems with people making fake vaccine cards. I'm going to estimate that about 10% of unvaccinated people (2% of population) would casually lie if asked but not go to the trouble of making a fake card. Effect is then 0.8x. Time is ~30s/person in checking and 2min/person in participant hassle.

      When estimating the harms of covid, at the end of the post, I ignore harm to people who lie about their vaccination status.

  • Masks
    • Require participants to wear a mask. Assuming a mix of 1/3 snug cloth (1/3x on transmission, 2/3x on reception), 1/3 surgical (1/4x on transmission, 1/2x on reception), and 1/3 kn95+ (1/6x on transmission, 1/3x on reception), the overall effect from random pairings is 1/8x (same as assuming all surgical). At this point everyone already has a mask, because so many things require it, so no marginal cost on participants. This does make them event less enjoyable, and some people will not be able to attend due to breathing difficulties. This is much harder to adjust for.

    • Require participants to wear a surgical mask or better. These cost ~$0.08 in bulk, and could easily be provided by the event. This moves the mix to 2/3 surgical and 1/3 kn95+. Relative to the previous scenario, it's 0.8x.

    • Require participants to wear kn95 or better. These cost ~$0.40 in bulk and again could be made available at the event. Relative to the previous scenario, it's 0.6x. Some participants might find these unpleasant.

  • Testing
    • Require participants to take a rapid test right before coming or on entry. Microcovid doesn't have this, but this estimate gets 1/4x. $12 each, plus 10min in hassle and testing. [This looks different outside the US where tests are often cheaper and more widely available.]

    • Require participants to present a recent PCR result. With the large delay between testing and results, I think this one is nearly useless unless combined with pre-event isolation.

  • Air
    • Hold the event outdoors. 1/20x generally, but not for people within 1ft. For contra in particular this comes to ~1/5x. More hassle for organizers, worse surface for dancing, more weather risk, impractical in many places for much of the year.

    • Heavily ventilate the space. 1/4x, if you can get a level of ventilation equivalent to losing a full wall. How practical that says it's going to depend a lot on the location and weather.

    • Air purifiers. Microcovid has 1/4x for HEPA filters handling 5x the room volume hourly. I'm not sure using the room volume is correct here, since all things being equal I would expect doubling the volume of the room while keeping filtration capacity and occupancy constant to be safer, but this would say it's less safe? Microcovid cites Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS-CoV-2. The room was 27x20 with a 12ft ceiling, with 29 people. This is 233 ft^3/person, and their purifier was running at 600 CFM. For extrapolating to very differently shaped rooms (much taller) with very different occupancy (fewer people per sqft) I think it's probably more accurate to figure 20 CFM/person (600 CFM / 29). Posted on the issue.

      For a long-term installation it's probably worth buying a purpose-built filter, but for a shorter event the box fan approach is more promising. A Physics professor at the University of Arkansas recommends four filters per box fan and measured 470 CFM through the filters. At 19 CFM/person, for a 100 person event you'd need four of these. Each MERV-13 filter (they disagree with microcovid on whether HEPA is a good tradeoff) is $11, so this would be $176 in filters, or $1.76/person. My guess is you already have the box fans? Not that clear to me how long these last, but if your event is a few hours you might be able to reuse the same filters about 10 times, so $0.18/person.

  • Activity
    • Excluding food. Many events often have food as a "nice to have", but once people start socializing over and they tend to do a lot of talking close together without masks. You can think of this as the inverse of requiring masks, skilled by the portion of the event for which people will be eating food. For example, if your event would otherwise be on average surgical masks, having 15% be socializing with food increases total risk from the event by 2.2x. For a social dance, where food is not a major component of the event, I think skipping it can make a lot of sense. For other sorts of events, you might come to a different conclusion.

    • Asking people not to cheer. A typical contra dance has some cheering, in addition to applauding. Figure a quarter of people cheer, average duration is 15 seconds, and they do it 15 times in a three hour period. This is 0.5% of person-time spent cheering. If cheering is 20x risky on its own, then allowing cheering increases total risk by 1.1x (10%). This does lose a bit of the energy of the event, but clapping is still pretty good?

    Summary, relative to a baseline indoor dance event in a cold place (so no outdoors or high-ventilation options) with masks and vaccination required, counting time at $25/hr. Sorted by cost-benefit, best first:

    • Air purifiers: 1/4x, $0.18
    • Require vaccination: 1/3x, $0.18
    • Surgical mask or better: 0.8x, $0.08
    • KN95 or better: 0.5x, $0.40
    • Don't allow cheering: 0.9x, minimal cost
    • Don't allow food: 0.5x, moderate cost
    • Check vaccine cards: 0.8x, $1
    • Rapid testing: 1/4x, $16

    Which of these are worth it? Let's say were willing to spend up to $10M to avert a covid death, and figure 0.2% of unvaccinated people and 0.001% of vaccinated people who get covid die. A microcovid (1:1,000,000) then is $0.02 if you're unvaccinated and $0.001 if you're vaccinated. Long covid is also a thing, as is missing work or other events while sick, So let's say that people are willing to pay $5,000 to avoid non-death aspects of covid. This doesn't affect the unvaccinated number appreciably, but increases the vaccinated number to $0.006.

    If we modify the calculation in yesterday's post to allow people not to be vaccinated, it goes from 120 microcovids to 400. At $0.02/microcovid for unvaccinated people and $0.006 for vaccinated, that's $3.50/person, compared to $0.72/person if we require vaccination. Since we're counting vaccination at $0.18/attendee, this clears the cost-benefit bar easily.

    For the other options, relative to the scenario in yesterday's post, we get a per-person net benefit (benefit minus cost) of:

    • Air purifiers: $0.36
    • Surgical mask or better: $0.06
    • KN95 or better: 0.5x, -$0.04
    • Check vaccine cards: -$0.86
    • Rapid testing: 1/4x, -$15.46

    Air purifiers and surgical masks are worth it, everything more expensive wouldn't be.

    On the other hand, perhaps you think my estimate from last time was too low. What if it is really 20x more dangerous, at 2400 microcovids? The risk is then $14, and we have:

    • Air purifiers: $10.32
    • KN95 or better: 0.5x, $6.60
    • Surgical mask or better: $2.72
    • Check vaccine cards: $1.80
    • Rapid testing: 1/4x, -$5.50

    Requiring vaccination, setting up air purifiers, and requiring/providing decent masks, all seem pretty good. Requiring rapid tests still does not. Implementing these would give you a total effect of 0.1x, bringing it down from 2400 microcovids to 240.

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Really solid analysis. Regarding rapid tests:

A pretty important downside in many cases is that they're logistically complicated at a large event. The tests need to lie flat on a table or other surface for 15 minutes. Are you gonna have a giant table covered in tests? Do people come in to test and then go back out? Do they take their test out and perform it in their cars? These are solvable problems, but they can add a lot of complexity and crowding to the checkin area, which is already a problem spot at many events.

With that said, I'm a huge fan of tests for smaller events. Rapid tests let you get four times as much socializing for the same level of risk—depending on your risk budget, your financial situation, and your social ambitions, tests might (or might not) be a game changer for you.

They do get you four times as much socializing for the same level of risk, but so do some other options like air purifiers. If you're already doing the others, and you are doing something risky enough or are sufficiently cautious, then rapid tests do make sense.

EDIT: this is also different by country; I'm thinking of the US where rapid tests are somewhat hard to find in stock, and cost ~$12/each

I'm not sure using the room volume is correct here, since all things being equal I would expect doubling the volume of the room while keeping filtration capacity and occupancy constant to be safer, but this would say it's less safe?

If I imagine a large gymnasium, with a woefully underpowered air filter on one side and a handful of people on the other, then the CFM/person model breaks down. But I don't think the microcovid model is based on consideration of corner cases like this.

The microcovid model isn't CFM/person, it is CFM/CF. That is, it says you need to have sufficient purification capacity to filter the volume of the room five times per hour. See above for details on the measurements in the study (27x20 room with a 12ft ceiling).

10min in hassle and testing.

Seems wrong for rapids. I took a rapid with less than 2 minutes of attention this morning. People can socialize while they wait for results. Larger events require more staff time for this, though.

Are you counting test acquisition? I don't see anywhere to buy them in bulk as an event organizer, and stopping at a store is easily 10min.

I’m not. If you’re counting that, perhaps mark that as US-specific, since it’s beyond easy to get tests in eg the UK.

Thanks for the detailed analysis! I massively disagree with your conclusions though. Your claim:

Relative to no vaccination, reduces transmission by 0.5x and reception by 0.17, for a total of 0.08x.

I'm seeing transmission x0.35 ( and x0.11 for reception (, for a total of x0.0385. 

Combined with the protection against serious illness, this pretty much makes COVID a non-issue for vaccinated people, especially the young and healthy. Yes, some risks remain, but car accidents and the consequences of processed food, sugary drinks, a lack of exercise all seem much, much more relevant for a majority of the population. 

You do count the 'costs' of getting a vaccine, but not the costs of getting a mask, because "everybody already has one". The costs of always having a mask with you (and the costs of getting rejected because you forgot your mask), and especially the costs of wearing a mask (for long periods of time, while exercising and socializing), seem much higher than the costs of 'being vaccinated'. 

I live in the Netherlands, and we have an easy, reliable app on our smartphones with a QR code related to our vaccination status. This makes "check vaccine cards" pretty painless. 

"Not allowing food" seems very costly as well. You could skip it for 1 - 1.5 hours of dancing, but if it's a longer event and people are socializing, food seems quite valuable. 

For COVID, I would solely focus on vaccines. Get everybody vaccinated, done, allow normal life to resume. Removing food and wearing facemasks all the time seems way too costly. 

Air purifiers / ventilation / a focus on air quality do seem worth it, but not just for COVID. It helps prevent the spread of other diseases as well, and good air is pleasant and healthy. 

transmission x0.35 ... x0.11 for reception ... total of x0.0385

I'm using's numbers here. If you think they're giving out the wrong numbers you could file an issue at, but they're probably going to want citations in papers and not in news articles.

The costs of always having a mask with you (and the costs of getting rejected because you forgot your mask)

I proposed organizers have surgical masks available, which at $0.08/each is very reasonable. On the other hand you probably already need a mask to get to the event: both public transit and taxis require masks here.

The costs of always having a mask with you and especially the costs of wearing a mask, seem much higher than the costs of 'being vaccinated'.

I agree. The cost of being vaccinated (allocated to a single event) is super low, because it divides across so many activities/events.

"Not allowing food" seems very costly as well.

It depends a lot on the event. Most contradances here already don't offer food; the one I organize is unusual in that it (in normal times) did put out some snacks (usually popsicles) at the midpoint.

I also think that ignoring the loss of people who can't reasonably dance in masks is a mistake, but that's strictly my opinion rather than a shortcoming of your writeup.

It isn't now, which means the inclusion of those people isn't counted a loss. When you revisit the analysis in six months or two years, will you remember to put them in then?

People sweat while dancing. Some people sweat a lot. I have heard that wet, even damp, cloth masks are essentially useless. Does this hold true for surgical type masks as well? If so, many folks will be effectively unmasked for much of the evening, even if they do something reasonable like get a fresh mask every 2-3 dances.

On an entirely separate note, my community has re-started some of its dances with mask and vacc required, no exceptions. This has excluded several families with children who were active participants pre-pandemic. What is your plan regards to families with children? The "I" in BIDA perhaps makes it extra un-pallatable to exclude them.

On damp masks, this wasn't something I noticed when we hosted an outdoor contra. People who've held dances like this indoors: is that something you've seen?

With children, Pfizer received the EUA for 5-11 last week, and the CDC will likely sign off this week.

Speaking for myself and not for BIDA, while I think intergenerationality is important, I don't think it is so important that it should preclude having dances before vaccination is available for all, if vaccination is necessary for safety.

I think it depends mainly on the ambient humidity. If the temperature and humidity are both on the high side, then ambient humidity plus sweating/exhaling dancers sometimes crosses a threshold that leaves everyone drenched in sweat. It only happens in summer and in spaces without A/C.

While most dancing children will be covered by the recent EUA, many younger ones (and therefore likely one or both parents) will not. I understand that you yourself have a <1 year old for whom the vaccine availability timeline is unclear. She would not be welcome at the dance events hosted in my area.

All the children here who would be capable enough to dance at our regular dances are old enough for the EUA.

In the before times, people would bring infants and wear them in carriers, but I wouldn't bring one currently even if a dance allowed it.

I'm finding this exchange strangely frustrating. I was trying to ask whether you planned to explicitly exclude children under 2 and/or 5, since my understanding is the local laws would permit an event with them and so your plan in this regard wasn't clear to me from your writeup. I expected there to be either a small additional risk in your analysis from including them or an included cost to not, since then presumably people are paying a sitter (or staying home). I don't have a strong opinion either way on the correct approach. But "I would not personally bring my small child" is not an answer to the question.

I will say that I, personally, have not returned to the dances ~one hour away because they will not permit my <1 year old to enter the building. It's not a principled disagreement, it's just that the cascading impacts mean that it is not worth it to me personally to attend. If my kid were to attend, I wouldn't do a carrier. Various people would trade off sitting out. One venue managed to accommodate this by having a space in a different room (not the one used for dancing) for the kid and the kid-minder, but the main venue that has resumed dancing does not allow entry to the kid under any circumstances. And that's fine! But it means it is not worth it to me to attend. 

I do bring my kid to other indoor places (e.g., shopping, buses) which I understand is not the cultural norm in the Boston area currently.