The intradermal route requires a very fine needle and some specialized training. Some healthcare providers are familiar with the method as it’s used for the tuberculosis skin test. In that case, tuberculin, a purified protein, is injected under the skin, which can trigger a swelling that is considered a positive TB test.
Research indicates that intradermal administration is a safe and effective vaccination method for rabies, influenza, and the hepatitis B virus.
There’s not a lot of data proving the effectiveness of the monkeypox vaccine when given intradermally. Some preliminary studies have shown it’s about as protective as the traditional subcutaneous injections, Adalja said.
The main issue is in the actual delivery of the vaccine, Schaffner said. “It’s an art form,” he said. “It’s so much more difficult to do correctly each and every time.” Providers need to be specially trained; if nurses who are familiar with doing TB tests are brought out of retirement to help, they need to have the correct licensure to avoid liability.
The FDA has been getting questions from pharmacists about how many times they can use a vial and how long someone should wait before the second shot, which Schaffner believes the FDA can answer.
Will stretching monkeypox vaccines in general work?
The CDC says that the intradermal method should be as effective as a regular shot, pointing to a 2015 study that found that intradermal influenza vaccines in men with HIV were as effective as intramuscular flu vaccines. Another study that looked specifically at the Jynneos monkeypox vaccine found similar immune responses between the intradermal and subcutaneous administrations.
Some cities, including New York City and Washington, DC, have already been administering only one subcutaneous dose of the Jynneos vaccine and waiting for more supplies before administering the second shot.
Although the highest level of antibody immunity comes after the second dose, this approach allows more people to potentially build at least partial immunity, said Dr. Cesar A. Arias, chief of the infectious disease division at Houston Methodist Hospital.
Exactly how much immunity, no one can say, but it seems unlikely that delaying the second dose will affect overall protection once you get the second shot, Schaffner said.
Administering a single subcutaneous shot is “a very reasonable scientific plan at this point in time,” said Dr. Aaron Glatt, a spokesperson for the Infectious Diseases Society of America and chair of medicine at Mount Sinai South Nassau.
Why is there a vaccine shortage?
There are actually two monkeypox vaccines available in the US, said Arias, who is also codirector of the Center for Infectious Diseases Research at the Houston Methodist Research Institute.
Although the US at one point had millions of doses of the Jynneos vaccine, the only one fully approved by the FDA, the “doses have a limited shelf life and can no longer be used,” said Dr. Mark Loafman, a board-certified family physician with Cook County Health in Illinois.
A second product, the ACAM2000 smallpox vaccine, can be used for monkeypox in certain people under the FDA’s expanded access protocol for so-called investigational new drugs.
“It’s known to be effective against monkeypox but was not licensed for that use,” Loafman said. Although the ACAM2000 vaccine has the advantage of being a single dose, it’s not safe for pregnant people or people with weakened immune systems, eczema, and other skin conditions.
People with weakened immune systems need to get the larger dose, as there isn’t enough data on how they react to intradermal delivery, Schaffner said.
It is difficult to administer the vaccine for older people who have thin skin as a result of certain skin conditions. “It’s easier to go through the skin and, if the skin is very thin, it may not contain the fluid and it leaks out,” Schaffner said.
The side effects are similar with both Jynneos vaccine administration methods, although you can have more redness and itching with intradermal vaccines, and once it heals up there may be some discoloration, Schaffner said.
The 2016 clinical trial comparing the two methods found that intradermal administration resulted in “some mild to moderate side effects like redness, firmness, itchiness, and swelling at the injection site,” FDA Commissioner Robert Califf said at a press briefing this week. “These were all manageable.”