The CEO of the company that makes the only vaccine approved by the Food and Drug Administration (FDA) to prevent monkeypox told Biden administration health officials he was concerned about a new strategy to split doses and change the way the vaccine is injected.

In a letter shared with The Hill, Bavarian Nordic CEO Paul Chaplin said the company has “some reservations” about the new approach, “due to the very limited safety data available,” as well as the fact that more people experienced adverse reactions after vaccination.

“This may have a negative impact on vaccine uptake and coverage,” Chaplin wrote.

The letter to FDA Commissioner Robert Califf and Health and Human Services Secretary Xavier Becerra was first reported by the Washington Post. 

Chaplin said the company fully supports approaches to conserve limited supply, but “it would have been prudent” to roll out the new strategy with much more comprehensive guidance that would allow additional safety data to be collected.

The letter was dated Aug. 9, the day the administration announced a major change to its monkeypox vaccination strategy. The new method splits up the doses and uses one-fifth as much vaccine per shot. The partial dose of the vaccine is injected into the upper layer of skin, rather than the full dose into the underlying fat, which is how shots are typically administered.

The new approach, called an intradermal injection, uses a smaller needle and likely will require additional training for people giving the shot. The request to change the vaccination method was made by the National Institutes of Health, not the manufacturer.

Chaplin said the company has been “inundated with calls from U.S. state government officials with questions and concerns” since last Thursday regarding the new vaccination plan.

“We will of course align our responses with our colleagues at the CDC, but we believe this alignment would have been better served before any announcement to ensure the best rollout,” Chaplin wrote.

The new strategy is an attempt to stretch the U.S.’s limited supply of Bavarian Nordic’s Jynneos vaccine. There are more than 10,000 confirmed cases of monkeypox in the country and demand for the vaccine far exceeds supply. 

Peter Marks, the FDA’s top vaccine official, wrote in a memo Tuesday that approximately 1.6 to 1.7 million people are currently estimated to be at elevated risk of monkeypox in the U.S. and may need vaccination.

Since Jynneos requires two doses, that means the country needs 3.2 to 3.4 million doses. However, Marks said only about half that number are currently estimated to be available before the end of 2022. 

Marks said other methods of conserving or stretching doses were considered, including delaying second doses of Jynneos or using an older smallpox vaccine, but the intradermal method was the only one deemed acceptable. 

Under the new plan, the country’s remaining 441,000 doses could be stretched to give 2.2 million shots. 

“In the current setting of a monkeypox outbreak that is continuing to spread in the context of a limited number of vaccine doses, the additional doses of vaccine that will be made available may also help to benefit public health measurably by assisting in containment efforts,” Marks wrote.

Chaplin said Bavarian Nordic was open to delaying second doses in order to vaccinate more people with a first dose, a strategy employed by several cities as well as the United Kingdom. 

But Marks said there was no evidence a single dose provided enough protection, while there was a clinical trial that showed intradermal administration was effective.