(CNN)In an effort to stretch the limited supply of the Jynneos monkeypox vaccine, federal health officials this week authorized giving smaller doses using a different method of injection.
With more than 10,000 monkeypox cases in the United States and vaccine demand remaining high, Jynneos has been in short supply for weeks. The new injection strategy allows health-care providers to give shallow injections intradermally, in between layers of the skin, with one-fifth the standard dose size instead of subcutaneously, into the fatty layer below the skin, with the larger dose.
Yet the specific efficacy data for the vaccine is not clear, and some experts argue that not enough is known about how the low-dose intradermal strategy might affect the protection it provides in the real world.
Even at the original dosage size, "there is no traditional assessment of this vaccine," Dr. Robert Califf, commissioner of the US Food and Drug Administration, said Tuesday. It's approved for emergency purposes based on studies of immune responses, not clinical outcomes, because there haven't been smallpox or monkeypox outbreaks large enough to test it.
In a series of tweets Thursday, Califf said the alternative dosing regimen will allow more people to get vaccinated and protected.
"In authorizing an unapproved use of an already approved product, the FDA is ensuring the vaccine meets high standards for safety, immune response and manufacturing quality," he wrote.
The White House cited a clinical study published in 2015 that showed that two doses given intradermally at one-fifth the full amount produced a similar immune response to two full doses given subcutaneously, meaning people in both groups responded to vaccination in a similar way.
The intradermal route "may increase the number of available doses in an emergency situation," the study's findings say. Last week, the United States declared monkeypox to be a national public health emergency.
US officials said Tuesday that there are plans to collect real-world data on the vaccine and that the National Institutes of Health will launch a clinical trial.
The new federal strategy still calls for two doses of the vaccine given 28 days apart, and it says people who get their first dose subcutaneously can receive their second dose either intradermally or subcutaneously.
'Some reservations'
There are some concerns about the shift to a low-dose intradermal strategy.
"While we appreciate that the federal government is finally willing to look for solutions to help rectify the shortage of vaccines, no effort to protect people's health should come without proper due diligence and research," David Harvey, executive director of the National Coalition of STD Directors, said in a statement Tuesday.
"We have grave concerns about the limited amount of research that has been done on this dose and administration method, and we fear it will give people a false sense of confidence that they are protected. This approach raises red flag after red flag, and appears to be rushed ahead without data on efficacy, safety, or alternative dosing strategies."
The chief executive officer of Bavarian Nordic, the company that manufactures the vaccine, has told federal officials that the company also has reservations about the White House's plan.
"Bavarian Nordic (BN) is dedicated to assisting Governments around the globe to control the current monkeypox outbreak and is fully supportive of dose-sparing approaches, such as delaying the second vaccination. However, we do have some reservations on the [intradermal] approach," CEO Paul Chaplin wrote in a letter Tuesday to Califf and US Department of Health and Human Services Secretary Xavier Becerra.
Those reservations include "the very limited safety data available" on this vaccination strategy for monkeypox, the higher reactogenicity compared with the standard dose and subcutaneous route, and the fact that a relatively high percentage of people, 20%, did not get their second shot during a controlled clinical study.
"While we have certain reservations, we are trying to find the best way to support the EUA by collecting additional data and aligning on the responses to assist state officials in the rollout," Chaplin said. "We are also investing in expanding the manufacturing capacity at both BN and external facilities, with likely more announcements soon to come."
The Chicago Department of Public Health said Wednesday that it will be going along with the federal plan to stretch out the supply of monkeypox vaccine by changing to smaller doses given in a different way -- but it won't happen overnight.
"I do think it'll be a couple of weeks until we're rolled out more broadly," Chicago Department of Public Health Commissioner Dr. Allison Arwady said.
Other city health departments, like Columbus Public Health in Ohio, are still reviewing the new federal strategy and what it means for their local response.
"Because this guidance is new, we are currently reviewing it to determine the impact on our vaccine effort, staffing and supplies," Kelli Newman, a spokesperson for Columbus Public Health, wrote in an email to CNN on Wednesday.
A need for 'formal clinical efficacy data'
The efficacy of Jynneos for the prevention of monkeypox originally was tested in animals that were exposed to the virus, and it can be inferred from antibody responses seen in people who participated in smallpox clinical studies.
But the Jynneos vaccine was originally developed to contribute to the nation's defense against smallpox as a bioterrorism response effort -- so there still isn't much data on exactly how well it protects against monkeypox infection, even with a subcutaneous dose.
"There is no formal clinical efficacy data with the monkeypox vaccine -- but that's not a criticism of the vaccine. In fact, I think it's a testament to the vaccine development efforts that we actually have a vaccine for monkeypox already, largely before this major outbreak occurred," said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston.
The FDA approved the Jynneos vaccine in 2019 to prevent smallpox and monkeypox disease, but the approval was mostly to offer a smallpox vaccine option that does not contain live virus. Before then, Sanofi Pasteur Biologics' ACAM2000 was the only FDA-approved vaccine for the prevention of smallpox. Jynneos has been the preferred vaccine in the current outbreak because ACAM2000 has the potential for more side effects and is not recommended for people with severely weakened immune systems.
"Jynneos was a second-generation smallpox vaccine whose use case was to be a backup vaccine for individuals who could not get ACAM2000 in the event of a smallpox biological weapons attack," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security at Johns Hopkins Bloomberg School of Public Health.
"There was no intention of using ACAM2000 or Jynneos for monkeypox," he said. "Even though they were effective against it, it wasn't considered something that they were developing it for."
When Jynneos was approved, FDA officials said its effectiveness for the prevention of smallpox was determined by a clinical study of about 400 healthy adults. Half of them received ACAM2000, and half got Jynneos.
The US Centers for Disease Control and Prevention estimates that the Jynneos and ACAM2000 smallpox vaccines are at least 85% effective in preventing monkeypox.
"Although we don't have clinical efficacy data, it is likely that Jynneos will be highly effective, because of what we know about the smallpox vaccine and because of the efficacy with Jynneos against monkeypox in animals," Barouch said.
He added that "in general, vaccines are usually more effective at preventing the severe consequences of the disease than preventing acquisition of infection with the pathogen."
"We can be confident that this will likely be a highly effective vaccine, but the specifics of it in terms of the exact numerical efficacy -- and whether that will be prevention of infection or prevention of severe disease -- those details are really not known at this point," Barouch said. "I do think that high-risk people should be vaccinated, but right now, I don't think it's possible to give very precise guidance as to exactly what that protection will look like."
A chance to 'greatly improve' availability
The uncertainties surrounding the effectiveness of the Jynneos vaccine will be magnified by the change to a lower dose of the vaccine in the real world, outside of a clinical trial, Dr. Philip Krause, former deputy director of the FDA's Office of Vaccines Research and Review, and infectious disease physician Dr. Luciana Borio wrote in Stat News on Tuesday.
"If the vaccine can be administered at lower doses at no risk to effectiveness, lower-dose intradermal injections make sense," Krause and Borio wrote. "But this strategy could also backfire. If there is a risk to effectiveness, it may be better to focus the available full doses of vaccine on individuals at highest risk — men who have sex with multiple male partners — to provide the best chance of getting the outbreak under control."
Throughout the outbreak, the vaccine has been given to health-care workers treating people with monkeypox, to close contacts of patients and to people who have been able to find doses at local clinics. The CDC has estimated that at least 1.5 million people in the US are eligible for monkeypox vaccination.
"We have a line out around the corner," Dr. Stacy Lane, a clinician at the Central Outreach Wellness Center in Pittsburgh, said Tuesday of the demand for the monkeypox vaccine at her clinic.
HHS has delivered more than 670,000 doses of the Jynneos vaccine from the Strategic National Stockpile to support local monkeypox response efforts on the ground.
"In addition, the SNS is preparing to distribute approximately 400,000 additional vials to states and jurisdictions as part of the next phase of the national vaccine strategy," according to the White House. "Jurisdictions that administer 90% of their current vaccine supply may request additional doses sooner."
And because of the authorization of intradermal injections, "the 400,000 vials of vaccine in the SNS's inventory that have been allocated but not yet distributed hold the potential to provide up to 2 million doses using intradermal administration. Additionally, vaccines that have been received by jurisdictions, but not yet administered, are eligible for intradermal administration."
Lane said the shift to an intradermal strategy could "greatly improve" the availability of vaccine. Her clinic has ordered the shorter syringes needed to administer the vaccine intradermally, the same ones used for tuberculosis testing, she said.
"I ordered those this afternoon," she said Tuesday, before the FDA issued its authorization.
Lane said many of her patients are worried about getting monkeypox, and she refers them to the ACAM2000 vaccine data as a reference point for how well the Jynneos vaccine may protect against monkeypox.
"What we're educating people on is that we know from historical data that the ACAM2000 vaccine that was historically given in Africa was about 85% protective against monkeypox. We know that these orthopox families of viruses tend to be responding to the same treatment, as well as the vaccine being somewhat effective," Lane said.
She added that she does not anticipate any concern around the new vaccination strategy.
"We're not really seeing any vaccine hesitancy, at least at this point," she said.
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Dr. Paulette Grey Riveria agreed that the intradermal strategy could be an effective way to stretch the supply.
"Given that less vaccine volume is needed for an intradermal versus a subcutaneous dose, intradermal Jynneos injections could be a reasonable option to maximize supply while not compromising vaccine protection," Grey Riveria, family medicine physician with the virtual health platform PlushCare and a regional medical director for the state of Louisiana, wrote in an email to CNN.
"Scientists are actively studying to determine if immune cells under the skin can activate an antibody response with equal or greater efficiency compared to the subcutaneous route."
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