A San Francisco hospital is offering a dose of Pfizer's or Moderna's vaccine to people who got J&J

• A San Francisco hospital is letting people who got Johnson & Johnson's vaccine get a dose of Pfizer's or Moderna's.
• J&J's shot may be the least effective against the Delta variant.
• But US regulators haven't recommended extra shots or boosters for any Americans yet.

It's not a booster shot, the Zuckerberg San Francisco General Hospital says - it's a "supplemental dose." Starting later this week, the hospital will allow people who received Johnson & Johnson's single-dose vaccine to get a second shot of either Pfizer or Moderna.

The hospital made the decision jointly with San Francisco's Department of Public Health, $4.

In a statement on Tuesday, the health department said it was "accommodating special requests" from J&J vaccine recipients, many of whom have already consulted their doctors about the additional dose. San Francisco residents will get first priority, but residents of other counties could also receive an extra shot if there's enough supply.

"We continue to align with the Centers for Disease Control and Prevention guidance and $4," the department added.

Dr. Chris Colwell, chief of emergency medicine at San Francisco General, $4 that the supplemental dose doesn't qualify as a booster "because it's not specific for some of the variants, which the booster ultimately will be."

Both Pfizer and Moderna have indeed $4 of their mRNA shots, customized to fight some of the most concerning variants. The companies are still testing those, as well as an approach that simply administers a third dose of the same formulation to see if that amplifies protection. Both are considered "boosters," according to the companies.

So far, vaccines continue to offer strong protection against severe disease or death. But J&J's shot may be the least effective against breakthrough infections caused by Delta, the most transmissible coronavirus variant to date.

J&J $4 in early July that its shot generated a strong immune response against Delta based on blood samples from vaccinated people. But $4 from New York University (though not yet peer reviewed) found that antibodies produced by J&J's shot were less effective at neutralizing Delta over time. (Antibodies aren't the only form of immunity, though - B-cells and T-cells are $4 against the virus.)

"The message that we wanted to give was not that people shouldn't get the J&J vaccine, but we hope that in the future, it will be boosted with either another dose of J&J or a boost with Pfizer or Moderna," Nathaniel Landau, a virologist who led the NYU study, told $4.

At least 2 public-health experts have already 'topped off' their J&J shot with an mRNA vaccine

Researchers haven't determined yet whether booster shots are necessary for the general public - though some countries have already $4, for whom vaccines are often less effective.

Both Pfizer and Moderna have shown that a third dose increases antibody levels relative to a second dose. But $4 from The Rockefeller University that hasn't been peer reviewed found that a third dose of these vaccines doesn't offer any greater protection against variants than the standard vaccine regimen.

Even before the Delta variant became widespread, however, J&J's shot was less effective than mRNA shots: The vaccine was found to cut the risk of moderate and severe COVID-19 by $4 globally in clinical trials, whereas Pfizer's and Moderna's shots lowered the risk of symptomatic COVID-19 by roughly 95%.

John Moore, a vaccine expert at Cornell University, $4 that despite J&J's one-shot approach, "it's really, to my mind, a two-dose vaccine."

Indeed, two public-health experts who received J&J have already taken it upon themselves to get an additional mRNA dose.

Angela Rasmussen, a virologist at the University of Saskatchewan, $4 that she got a Pfizer shot to "top off" the J&J vaccine she received in April. Jason Gallagher, an infectious-disease expert at Temple University, $4 that he got an extra Pfizer dose after participating in J&J's trial in November.

Preliminary evidence suggests that $4 and may improve vaccine protection, but more data is needed.

Have you opted to get an extra shot on top of your initial vaccine? Email covidtips@businessinsider.com with the subject line "Multi-vax"