The new method will use just a fifth of the previously administered dosage, but “you have to get it just right,” and that means ensuring health providers are trained in the precision nuances, said Peter Chin-Hong, an infectious disease expert with UCSF.

“There is no other choice,” he added. “Given the anemic trickling in of vaccine doses, there are not going to be enough for many, many months.”

California health department officials projected it will take considerable time to implement the new emergency-use authorization, granted on Tuesday by the Food and Drug Administration, to administer smaller doses of the Jynneos vaccine by using an injection between layers of skin rather than into deeper fat tissue.

Studies have shown this method can trigger a stronger immune response, but it is not commonly used in the U.S., necessitating clinician training before it becomes widely available.

“It would allow a smaller dose be given intradermally,” Dr. Erica Pan, the state epidemiologist told health care professionals during a Tuesday briefing. “We’ll have to work closely with all of you and health departments to implement this.”

That makes it difficult to know when enough providers will be trained up to realize a five-fold increase in the state’s monkeypox vaccine supply.

“I hope they can do it as soon as possible,” said Chin-Hong, noting the injection method has been extensively studied for polio and yellow fever. “We’re on the steep curve of people getting ill, infected and spreading it. On the hospital side where we see the most serious cases, the number of patients has doubled in the last week.”

California reported a cumulative 1,733 monkeypox cases as of Tuesday, with nearly a third of those concentrated in San Francisco and Alameda counties. Although anyone can get monkeypox, the outbreak is still largely contained within networks of men who have sex with men and is believed to be spreading primarily through prolonged skin-to-skin contact, including sexual activity.

The San Francisco Department of Public Health said it welcomes “every possible avenue that would allow us to get more people vaccinated against monkeypox as soon as possible.”

But as of Wednesday the department was awaiting federal and state guidance on the injection changeover, including instructions on dose extraction and administration, and stated, “SFDPH needs to understand adjustments that would need to be made to operations to ensure San Francisco’s vaccine providers continue to reach as many people who need the vaccine as quickly and equitably as possible. ”

It also anticipates instructions from state health officials on how to account for and register each dose when multiple doses are taken from single vials.

“We are also aware that the intradermal method of administering the vaccine, in which a dose is injected between layers of skin, will require updated protocols and additional training for many vaccinator staff and may be slower to administer,” the California Department of Public Health said in a statement to The Chronicle.

Across California, 98% of cases where there is available demographic data have been among men, and 96% are gay or bisexual, according to state data tallied in The Chronicle’s monkeypox tracker. Monkeypox, which can produce painful lesions, is not a sexually transmitted infection, but sustained contact through intimate encounters can result in transmission. Only 3% of those infected have required hospitalization.

Alameda County’s health department also said it is awaiting state and federal guidance before making any change in how it administers monkeypox vaccinations.

“Efforts to increase access to the monkeypox vaccine are appreciated and would be implemented, pending guidance, if it is determined to be in the best interest of Alameda County communities impacted by this epidemic,” Neetu Balram, the county’s public information manager, said.

White House officials said Tuesday that the new method is a “game changer” and would allow them to stretch the 400,000 available vials of the vaccine into about 2 million smaller doses.

“It’s safe, it’s effective, and it will significantly scale the volume of vaccine doses available for communities across the country,” Robert Fenton, the White House’s monkeypox response coordinator, told reporters.

While it would not completely alleviate the current demand, they said, it will at least help ease some of the strain.

About 1.6 million to 1.7 million Americans are considered by federal officials to be at the highest risk of the disease, primarily men with HIV or men who have a higher risk of contracting it. Vaccinating that group would require more than 3.2 million shots, with full vaccination consisting of two shots per person.

“Gay, bisexual and other men who have sex with men in recent history have demonstrated significant confidence in vaccines. … I imagine that we will likely still run out of vaccines before we run out of arms,” Dr. Demetre Daskalakis, the White House monkeypox response deputy coordinator, told reporters.

Scarcity of vaccine supply has made the rollout to the eligible population frustratingly slow for many. On Aug. 4, the Biden administration declared a national monkeypox health emergency. San Francisco and California issued similar proclamations days earlier.

California has only received 109,000 vaccine doses to date, Pan said, although the state expects another allocation next week. Jurisdictions can order additional vaccine supply starting on Aug. 15 or when they have used 90% of their current vaccine allotment, according to the White House.

“One worry I have with these delays is equity,” said Chin-Hong. “You have to be electronically savvy and able to take time off to line up for vaccines. That doesn’t work for gig workers or closeted with their sexuality. Even if they do with the five-to-one vaccine, there are going to be holes they have to figure out.”

Aidin Vaziri (he/him) is a San Francisco Chronicle staff writer. Email: avaziri@sfchronicle.com