A few weeks ago, before organizing an outdoor contra dance I used microcovid to get an estimate of how risky it was, concluding that it is likely very safe. It's now too cold for outdoor dancing here in Boston, and I'm thinking about how risky indoor dancing would be, with masks and vaccination required. Breaking up the risk the same way as last time, but assuming a longer event and two lines:
[EDIT: this post originally had the scenario described as having a "talking volume" of "silent", but I've switched it to "normal" after discussion in the comments. While there isn't that much talking, the respiratory effect of moderate exercise is probably more similar to "talking" than "silence" or "shouting". I've updated the post below.]
- ~16 from your partner. While your partner is not the only person your head gets close to, you're this close to at most one person at a time, so for simplicity assume its your current partner.
- ~32 from your neighbors and next/previous neighbors.
- ~48 from your next/previous hands fours, and the corresponding hands four in the other line.
- ~24 from the hands fours one farther away, and the next/previous neighbors of the corresponding hands four in the other line.
This comes to ~120 microcovids, or ~60% of a cautious risk budget of 200 microcovids/week (1% risk of covid/year). It's about 5x safer than going to a restaurant. Outdoor dancing is still safer than indoor, but only by about 5x.  High-capacity air filters or high-turnover ventilation would help, if practical in the venue, lowering the risk to ~24 microcovids. 
How risky is this event compared to what people are generally doing? Bars are open again, and have been for months, as is indoor dining. This sort of event is now fully legal here.
If we don't think it is currently ok but will be at some point, what would have to change? If the answer is that people who don't want to get vaccinated would have to get vaccinated, I don't think we should be waiting for them.
What is the current goal in limiting transmission? Essentially, everyone either needs to get vaccinated or get covid: there aren't really other options. So it seems to me that the main reasons to be cautious are making sure that we do not overwhelm the medical system, and allowing people who are at much higher than average risk to still meet their basic needs safely. For the latter group of people, it's not clear to me whether a slower or faster pace of infections is better: slower means more activities are possible, while faster means this ends sooner.
I now think it would be ok to resume indoor dancing, with masks and vaccination required. Am I missing anything? Other important considerations?
 This is actually a little surprising: in general microcovid models outdoor activity as 20x safer. What's happening here is that most of the risk for their model of the outdoor event came from the person you were currently interacting with 1:1, and they ignore ventilation in estimating how risky that is:
We are ignoring environment choice in this calculation. We are not confident that good ventilation or being outdoors substantially reduces the risk when people are this close to each other.
 In general, my experience has been that the numbers from microcovid are on the cautious side: they're well sourced, but if you plug in people's actual behavior and compare to actual infection rates, people seem to be getting infected less often than it would predict. Additionally, vaccinated people are less likely to be hospitalized, much less likely to die, and less likely to pass covid on to others.