A friend recently wrote that they wouldn't be attending a mask-optional contra dance weekend I'm playing at because it doesn't require vaccination. As an organizer of unrelated dance events, which also don't require vaccination, here's how I see the situation.

When the covid vaccines first came out they were a huge improvement over the status quo. Getting the vaccine reduced your risk of severe illness or death, reduced your chance of catching it, and reduced your chance of giving it to others after you were sick. Our events initially required vaccination, which I think was the right call.

At this point, however, there are a few different things you might mean if you say your event requires vaccination. Usually it's either:

• A complete primary series counts, even if it's one shot of J&J from early 2021.

• The most recent ("updated") booster is required.

The CDC used to call the first category "fully vaccinated", but no longer talks about it prominently. They've switched to focusing on the second one, which they call "up to date".

This change makes sense: from a perspective of avoiding getting infected and passing it on, only a recent booster does very much. A few months ago I wrote about the results in Menegale et. al (2023) where they saw that as an infection control measure vaccines wane quickly: effectiveness halves about three times a year.

Additionally, lots of people got sick this winter, which acts similarly to an a vaccination. Given how quickly the vaccine wears off, I'd be less concerned about risk from someone unvaccinated who'd had covid over Christmas than someone who got their booster in the early Fall and dodged the winter wave.

What does this mean for a dance event?

• If you want a vaccination requirement to be doing anything useful, you need to require people be up-to-date with their vaccine. Risk from someone last boosted in Fall 2022 or before is not appreciably different from someone who was never vaccinated.

• Requiring up-to-date vaccination rules out 80% of people 18+, though if they're excited enough about your event they could go get the shot.

• Unless your event is in the fall or early winter, even requiring up-to-date vaccination doesn't help much: if someone got their "updated" booster in the fall they're down to ~35% of peak efficacy by the time of an April event.

If an event does want to reduce risk of transmission, I'd recommend considering some combination of ventilation, filters, tests, and masking.