San Francisco Mayor London Breed sent an important but unintentional message recently when she was caught violating her own mask mandates while partying away, maskless, in a jam-packed jazz club. Her excuse was incoherent; she said she was "feeling the spirit," enjoying the music and so not thinking about a mask.

The more serious problem wasn't her hypocrisy and lame rationalizing so much as the mixed and misleading messages sent by the rules themselves. Americans are in dire need of guidance that's coherent, fair, sustainable, and backed by evidence. And they're not getting it from public health authorities or the rulemakers who rely on them, even as the country slouches toward a confusing new normal with no end to COVID-19 in sight.

"We don't need the fun police to come in and micromanage and tell us what we should or shouldn't be doing," Breed said when questioned. She was making a good point! But as Charles C.W. Cook wrote last week in National Review, she is the person who authorized the mask mandate. She is the fun police.

What we do know from scientific observations is that some environments are more risky than others. Epidemiologists had at one point used the term “The Three C’s” to describe the major risk factors: crowds, close contact, and closed spaces. The San Francisco club where Breed was caught might as well have been flashing all three C’s in neon signs.

Her explanation after the fact made the point that it's kind of absurd to expect people to enjoy live music in clubs while wearing "well-fitting" masks, as required by local rules. Even the most enthusiastic maskers probably wear a mask about 20% of their time in a club, or cocktail party or similar event. Does that really help?

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Not really. There is scientific evidence to suggest that this small amount of mask-wearing is mostly symbolic in such situations.

Early in the pandemic, infectious-disease doctor Muge Cevik collected studies in which researchers employed contact tracing to figure out how and where the virus was actually jumping from person to person. What the studies kept showing was that the virus was transmitted indoors, and the longer people spent indoors together, the more likely transmission would occur.

Duration is extremely important; it's not all about that six-foot distance. The virus travels on small airborne particles, which would be diluted quickly outside but which builds up in indoor air. That means spending three hours mostly unmasked in a club is much worse than spending 10 minutes totally unmasked to grab a soda in a convenience store. Other studies showed that risk skyrocketed when there was singing and the kinds of loud conversations people have in clubs and bars.

The San Francisco club did require patrons to be vaccinated, but new data on the delta variant suggests it's still possible for fully vaccinated people to get a mild or asymptomatic case and possibly to transmit the virus to others. That could lead to more hospitalizations among children, who can't yet be vaccinated, or the immunocompromised, who aren't getting full protection from their vaccines.

At the same time, COVID prevention has to be balanced with human needs. Nobody wants to live in a world where live music is outlawed. Imposing some mask mandates might seem like a reasonable compromise.

But let's not stumble into a future in which mask rules seem arbitrary, stupid or unfair. In San Francisco, runners in the marathon have to wear a mask, though that kind of venue is much less risky than the club where Breed was caught partying. Maybe she just likes music more than marathon running.

The first step toward reshaping policy would be to agree on a goal. A pair of researchers from Harvard and Boston University recently wrote in the New York Times about the need to agree on a purpose for COVID-19 rules and restrictions.

"Sleepwalking into indefinite masking is not in anyone's interests and can increase distrust after an already very difficult year," they wrote.

As risk-communication expert Peter Sandman wisely said early in the pandemic, science can tell us which activities are riskiest, but it can't tell us how much risk to accept. That is, by its nature, a political decision.

Science can help shape coherent policies that would achieve a goal once people decide what they want. If the situation is indeed serious enough that goals can't be achieved without masking marathon runners, then music lovers would have to adapt and enjoy live performances outside or in venues where there's no drinking so people remain masked. Fair is fair.

It may turn out that an additional vaccine shot will be enough to stem the tide of delta cases, but scientists are still split on who should get them and whether the purpose is to keep people from being hospitalized or to cut down on all cases. And the tangle of booster recommendations coming from Washington is hopelessly confusing.

For a healthier new normal, we also need more information to help people navigate a world with less draconian rules. Many younger, healthier people who've been vaccinated are rightly not all that scared about getting severely ill. It can happen, sure, but so can brain cancer or getting wiped out by a drunk driver. It's one of many risks we face, but what makes the virus different is that we don't want to give the disease to people who are more vulnerable than we are.

That means we need to know if we've been somewhere that would warrant staying away from vulnerable people for a while, or getting tested a couple of times. It's important to know that clubs are a worse bet than a walk on the beach.

If Breed really wanted to do damage control, she could have promised to quarantine, or stay home until she'd had a couple of negative tests. Or she could have said that she will rethink her policy to be more reality-based for the long haul.

Faye Flam is a Bloomberg Opinion columnist and host of the podcast, "Follow the Science."