Very few people who have gotten monkeypox vaccine shots in the United States have received the second dose needed for full protection, U.S. Centers for Disease Control and Prevention (CDC) Director Rochelle Walensky said on Friday.

She told a White House briefing that nearly 97 percent of the shots administered were first doses, and although many people are eligible for a second dose, "very few" have been given them so far.

Walensky said the CDC expected to gain more insight into the trend. Vaccine scarcity may have caused delays in second shots previously but that was no longer the case, she said.

More than 207,000 doses of Bavarian Nordic's Jynneos vaccine were given as of Aug. 23 in the 19 jurisdictions reporting data to the CDC, Walensky said.

The vaccine needs to be given in two doses, four weeks apart, to be fully effective.

To expand access to the vaccine, the U.S. Food and Drug Administration (FDA) on August 9 authorized the "dose-sparing" approach, which allowed physicians and others to use a shallower injection under the skin and therefore splitting a single vial into five doses.

Health officials pointed to a study published in 2015 that found that Jynneos vaccine administered that way was as effective at stimulating the immune system as when the needle plunged deeper into other tissue.

The company, however, raised doubts about the safety of the method when it was announced by the FDA, citing a lack of evidence and the possible increased risk of reactions compared to injecting it into muscle. 

During the briefing on Friday, officials said providers are reporting getting 4.5 to 5 doses per vial, but several experts told Reuters the figure is lower.

"The vast majority of health departments that I've talked to in the last week are feeling fortunate if they can get four doses out of a vial, and in some cases three," Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, said in an interview.

Dr. Umair Shah, secretary of health for Washington state, said on Thursday that many providers were getting three or four doses per vial.

Walensky confirmed that some U.S. jurisdictions were reporting a downward trend in monkeypox cases and that health officials were watching this with cautious optimism.

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(With input from Reuters)