Intradermal injection: 3 notes on the US monkeypox vaccine approach

The U.S. is now allowing fractional doses of the Jynneos monkeypox vaccine to be administered via an injection that delivers the dose between, rather than under, layers of skin — an effort that will stretch low vaccine supplies, according to federal health officials. 

The alternative dosing regimen was greenlit by the FDA through an emergency use authorization on Aug. 9 and allows a single vial of the Jynneos vaccine to be split into up to five different doses and be administered through an intradermal injection rather than the subcutaneous (under the skin) injection, for which it was originally approved. 

"Because of today's announcement, the 400,000 vials of vaccine in the [Strategic National Stockpile's] inventory that have been allocated but not yet distributed hold the potential to provide up to 2 million doses using intradermal administration," the White House said in a statement following the announcement from HHS and the FDA. 

Three things to know about the intradermal injection approach: 

1. Data from a 2015 study on the monkeypox vaccine showed those who received one-fifth of the usual dose intradermally had a similar immune response to those who received the vaccine through the usual subcutaneous injection, "meaning individuals in both groups responded to vaccination in a similar way," the FDA said. The study found the intradermal route led to more redness and irritation at the injection site, though less pain. Federal health officials' Aug. 9 decision to move forward with the approach was largely based on this study. 

2. The approach has been used before, but requires education and training. Intradermal shots have been previously used for tuberculosis tests and in polio vaccination campaigns. It uses a thinner needle and must be guided into a thin space, presenting complications for vaccinators, experts told The New York Times

The CDC has released interim clinical considerations for how to administer the vaccine intradermally, and plans to host trainings and webinars on intradermal administration for public health officials and providers, the White House said. 

3. Health experts' reactions to the dose-sparing strategy are split. Michael Osterholm, MD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis, said he has concerns regarding protection the strategy offers to people with immunocompromising conditions, such as HIV. 

"We don't yet have the data to know how effectively an intradermal-administered vaccination will protect immunocompromised persons," he told The Washington Post. . He also acknowledged that, as a state epidemiologist, he saw many public health workers struggle with intradermal injections used for tuberculosis testing. "This is a well-recognized issue where intradermal injections can be tough to do, without adequate training," he said. 

"The upside is you can stretch out doses," John Moore, PhD, a virologist at Weill Cornell Medicine in New York City, told the Times. "The downside is, if you cut it too far or take too many liberties, you reduce the efficacy. And how are you going to know that? It's educated guesswork."

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