Julia Ma did something last week that the majority of people in the United States cannot: she received an authorized second COVID-19 booster shot after only one dose of Johnson & Johnson’s vaccine.

Ma’s J&J shot was followed by a Pfizer-BioNTech booster dose five months later. Her third Pfizer shot put her in the same category as the 86 million Americans who have received three doses of either the Pfizer or Moderna COVID-19 vaccine, as a two-dose series plus a booster.

The federal government allows only J&J recipients who are immunocompromised to receive a single initial shot and two boosters. However, the San Francisco Department of Public Health approved a second booster dose for everyone who received J&J as their first shot earlier this month, making it the only known location in America where a third shot is officially permitted for those with normal immune protection.

Ma, 37, said she’s not concerned about COVID-19 because she’s fully vaccinated, but she’s hoping to travel next month and wanted to make sure she wouldn’t be a vector for others while omicron surged.

The third dose was described as “an accommodation” by the San Francisco Department of Health, based on recent studies that found a third dose of an mRNA COVID-19 vaccine, such as those made by Pfizer and Moderna, is required to induce adequate protection as immunity waned. The third dose reduces the omicron variant’s symptomatic infection, hospitalization, and severe outcomes.

The second booster is only available to San Francisco residents or those who received their initial J&J vaccination there.

“From a scientific standpoint, San Francisco’s Department of Public Health has always been a vanguard of the way things should be,” said Dr. Peter Chin-Hong, a professor of medicine and infectious disease at the University of California, San Francisco.

The department stated in a release that while the research has so far focused on people who received vaccines from Pfizer or Moderna, “we believe that similar studies in persons whose primary series was a single J&J vaccine dose would yield similar results showing that three doses are required for optimal protection.”

Such studies are not available, in part because the J&J vaccine has only been administered to a small number of people. According to Chin-Hong, 119 million people in the United States have been fully vaccinated with Pfizer, 74 million with Moderna, and 16 million with J&J.

“While there is understandable frustration with the lack of data, I’m not a fan of the decision to wing it ahead of data,” said Dr. Kelly Moore, CEO of Immunize.org, which educates health care professionals about US vaccine recommendations.

Federal regulators approved a single booster shot two months after the initial J&J shot in October. It is one of the mRNA vaccines, according to the Centers for Disease Control and Prevention. A mix and match vaccine study published last week in the New England Journal of Medicine discovered that when J&J recipients were followed by an mRNA vaccine, the highest levels of antibodies were produced.

“If you get either Moderna or Pfizer as your second dose, you’re going to get much higher antibody levels, and we have a good sense that higher antibody levels are advantageous,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University School of Medicine in Nashville, Tennessee.

A booster is recommended for people who received Pfizer or Moderna five months after the last dose of their original two-dose series. Multiple studies show that three doses of an mRNA vaccine are far more protective against omicron than two doses.

A Yale study of 37,877 people in Connecticut who had PCR COVID-19 tests between December 12 and 26 discovered that those who had received two mRNA doses five months prior had omicron positivity rates of 4.2 percent, while those who had received a third (booster) shot had positivity rates of 2.2 percent.

The J&J vaccine continues to be highly protective against severe COVID-19 disease and death, which was always the goal of all three vaccines.

However, pharmacists in her state may soon be unable to ask if she has been previously vaccinated. A proposed law there would make it a crime for any public, private, or nonprofit entity to ask anyone about their vaccination status, with fines of up to $14,000 or a year in prison.