Rebecca O’Neal didn’t believe she qualified for a Covid-19 vaccine. She had not realized that her turn had come. Last week, when she scrolled through the eligibility requirements for the state of New York, she noticed body mass index on the list.

Body mass index, or B.M.I., is technically a measure of obesity. The quantifier was drawn up in the 1930s by the Metropolitan Life Insurance Company to assess risk. Since a B.M.I. is a formula that does not consider several important factors like where the body fat is or if any vital organs are surrounded by fat, experts say to take the indicator with a grain of salt. But even so, a B.M.I. that indicates obesity has been a source of agitation for people who believe their doctors have used it to discriminate against them because of their weight.

Ms. O’Neal, a 34-year-old comedian and writer in Brooklyn, didn’t worry about that in the moment. She calculated her B.M.I. (it’s essentially your weight compared to your height), found that she met this technical threshold for obesity, and booked a vaccine appointment for the same day. She received the first dose later that afternoon.

“I didn’t know that my B.M.I. was 30,” Ms. O’Neal said in a phone interview. “I cracked a lot of jokes about it on Twitter, but it was a relief that I was eligible at all.”

Ms. O’Neal is one of millions of Americans, in states like New York, Utah and Texas, who are qualified to be vaccinated based on their B.M.I. While obesity has been linked to more severe cases of Covid-19, of the 500,000 Americans who have died from the coronavirus, 17,770 were overweight and had obesity listed as a contributing factor in their death, according to the Centers for Disease Control and Prevention.

Relying on a B.M.I. to judge one’s risk of serious health conditions is complicated. Many healthy people still fall in the “overweight” category based on their body proportions, with no distinction made between bone density, muscle mass and body fat.

That has a lot to do with the fact that the original calculus was developed by and for white men.

For many, using their misleading high B.M.I. to get inoculated is a fraught decision.

As Emma Specter put it in Vogue, writing about her decision to get a vaccine based on the B.M.I. qualification: “A metric of health that has long been called into question by fat activists and medical experts alike could stand to actively benefit fat people for the first time.”

Many other people are making the same decision — and posting about it online.

Some wrestled with whether it was ethical to receive a vaccine based on a metric that could have little bearing on their risk of serious illness.

“Taking care of the sick and the elderly and the health care workers, I understand all that — but at some point they should have open it to whoever can grab,” Raffaele Rispo, 38, a barber from Saratoga Springs, N.Y. who received a vaccine recently because of his B.M.I., said in an interview. “I understood that the older, more sick should get it first — but when they changed it, I was happy.”

Mr. Rispo has not seen his parents, who live two-and-a-half hours away from him, or his 15-year-old son who also lives a few hours away, in a year. He was ready to return to “some normalcy,” even though he understood B.M.I.’s are unreliable, he said.

While unreliable, a B.M.I. can serve a purpose; it can be used to screen for weight categories that may lead to health problems, according to the C.D.C., but, as a single measure, it is not diagnostic of the person’s body fat or health.

“B.M.I. by itself is not a great measure,” said Dr. Fatima Cody Stanford, an expert in obesity medicine and nutrition at Harvard Medical School. “It doesn’t tell me if that’s fat mass that’s causing inflammation. It doesn’t tell me if that’s water weight, it doesn’t give me those types of specific details.” (Dr. Stanford also noted, to NPR, that B.M.I. can be helpful in determining vaccine priority.)

For those who do meet the B.M.I. requirement for the vaccine, that measurement has presented a rare opportunity. William Antonelli said that once his sister realized that she qualified for the vaccine because of her B.M.I., she set up an appointment for him, too. A few days later, Mr. Antonelli, 24, an editor at Insider, received his first vaccine jab.

“When it comes to a disease like this, there really is not a wrong person to vaccinate,” he said. “The problem is not me applying for something I am eligible for, it’s the rollout. The issue lies with the government system that has led us to this point.”