"Why in the world did he ever get tested?” asked a couple of relatives at a family reunion I attended recently in Orlando. They thought it was unnecessary, knowing I was fully vaccinated and had tested negative three days before getting on the plane. Other family members appreciated that once I realized I couldn’t smell the freshly delivered pizza, I slipped away to a clinic. There, a rapid antigen test and then a slower PCR confirmed that, yes, I had a breakthrough case of covid-19.

The symptom — I use the singular, since loss of smell is all I’ve experienced so far — is mild. I’ve had nary a sneeze. In the hotel room where I immediately self-isolated, I did jumping jacks and push-ups every morning, caught up on assignments, stared restlessly at the walls. Covid hasn’t even affected my sense of taste, except to expose some modern tricks with aromatics: a canned “peach” cocktail that I drank tasted only of sugar water, the “fruit” flavoring a deception to which I was now immune. In contrast, the pear I ate was tart and delicious.

But what surprised me most was the range of reactions. I don’t mean just among those at the reunion, most of whom were grateful that I got tested. I mean from the Centers for Disease Control and Prevention, the airlines and others supposedly watching out for public health, who offered confusing and often conflicting advice that reminded me how little we still know about this virus and the best way to keep everyone safe.

“Can I fly out on Saturday?” I asked at the clinic in Florida where I tested positive.

“Well, it’s better not to,” the technician said. “But if you do, double-mask and stay away from people.” She also suggested getting tested again just before the flight, because she’d seen vaccinated people quickly test negative again with mild cases like mine.

“Absolutely not!” said a representative at Florida’s Department of Health. “Ten days self-isolation from the start of symptoms.” She promised a follow-up call from contact tracers, which never happened. She also said the airlines had a list of people who had tested positive, and they wouldn’t let me on board. “What’s your name and birth date again?” she asked, with the strong implication that I was being put on the list right then — which was not exactly reassuring, since the clinic where I got tested was seemingly unaware of the existence of any such list.

An airline representative I reached after a nearly three-hour wait on the phone told me the 10 days were counted from the positive test, not from the start of symptoms. A funny thing then occurred: Another attendee at the reunion who had just dropped off my things at the hotel and happened to be standing there — both of us masked, distant, outdoors — leaped at the opportunity to ask a question about their own reservation without their own multi-hour wait. I handed over the phone, at which point the airline representative promptly canceled their flight, because they’d had contact with someone who had tested positive.

That was a good sign from a public health standpoint, but I instantly regretted having involved my helper, and there was some heated back and forth between us about the CDC website, which at the time advised vaccinated people, “If you’ve been around someone who has COVID-19, you do not need to stay away from others or get tested.” (It has been updated to recommend that vaccinated people get tested three to five days after exposure, and wear masks indoors in higher-risk areas like Orlando.)

The CDC website is in a confused, and confusing, state these days. It says that only “a small percentage” of the vaccinated will get breakthrough infections, but there are signs that it the proportion may be higher than previously thought. Los Angeles County reported Wednesday that more than 25 percent of the 13,598 covid cases detected in the first two weeks of July were among the vaccinated. And though the CDC states that vaccinated people are “less likely to spread the virus to others, even if they do get COVID-19,” its recommendation to self-isolate is identical for the vaccinated and unvaccinated.

In other words, the science is evolving; any and all of the above might change. It already has changed, between last week and this week.

Such revisions are normal for science, but as seen throughout this pandemic, they are not good for policy. In the absence of clear, definitive, fact-based guidance, people make up their own rules, and the sense of risk, fairness and responsibility — key social rudders that guide behavior among those of us who care about one another’s welfare — gets badly out of whack. Scott Gottlieb, a former head of the Food and Drug Administration, told CNBC’s “Squawk Box” that we’re “vastly underestimating the level of delta spread” because the vaccinated “by and large are not going out and getting tested.”

I certainly sympathized with the reunion attendees who were exhausted at having to think about covid-19 and masks and self-isolation again. In some ways, it would have been nicer not to know I was positive, to keep believing that the vaccine had gotten us past all that. Being able to board a plane would have spared me the ordeal of driving 1,300 miles in a rented car, eating drive-through and peeing behind bushes, to get home to Cambridge, Mass., where I isolated from my kids for several days after testing positive.

What I keep thinking about in the hours when sleep won’t come — praying that I did not, ultimately, infect anyone else — is Disney’s Animal Kingdom. The day before my sense of smell vanished, I joined thousands of people in lines that seemed designed to simulate an anaconda’s digestive system: Approaching the rides, we were packed into tighter and tighter coils, in deeper interior spaces. I felt perfectly healthy, even virtuous: vaccinated and recently tested negative. But I kept looking around. Virtually none of us wore masks. There were hundreds of kids. In a state whose governor campaigns on “Don’t Fauci my Florida” koozies, someone was bound to be a risk, I thought. The grizzled man in the enormous “Deplorables” shirt that bore the likenesses of the Founding Fathers, for example — what were the odds he’d followed public health guidelines?

The problem turned out to be me, careful as I thought I’d been.

Disney, to its credit, is following the science, too: As of Friday, its parks are requiring masks indoors. But the lesson is clear: Get vaccinated the moment you can. On this point, the science is not confused at all. The vaccine won’t necessarily prevent a breakthrough, but it will by and large prevent the breakdowns — personal and societal — that the coronavirus brought last year and continues to threaten. As my sister said when I called to complain about my boring hotel room: “I’m glad you’re healthy enough to be grumpy.”