The White House monkeypox response team on Tuesday announced a new vaccine strategy less than a week into the US national public health emergency for the outbreak.
The US Food and Drug Administration issued an emergency use authorisation for healthcare providers to immunise people against monkeypox with a fraction of the full Jynneos vaccine amount through a new injection technique.
"With today's announcement, those 400,000 vaccine vials have the potential to provide up to 2 million doses to Americans," the team's co-ordinator Bob Fenton said.
The initial Jynneos regimen requires two doses inserted into deep tissue in the arm, sometimes four weeks apart.
The new strategy would use a fifth of the normal dose injected just under the skin. It still requires two doses four weeks apart.
US President Joe Biden's administration has faced criticism for being slow to respond to the country's rapidly growing outbreak, with limited vaccine supply and high demand.
Demand was so stretched in some US cities that leading health departments were forced to give priority to first doses.
"We encourage jurisdictions to utilise the alternative dosing method as quickly as possible," Mr Fenton said.
The New York Times reported on Monday that the new strategy is being based on a single 2015 study by the National Institutes of Health suggesting it would be effective.
But there is some criticism there is not enough evidence of its efficacy. The NIH will have results from further studies sometime this autumn or winter.
The US has committed 1.1 million vaccines and has sent out 670,000 thus far. The Centres for Disease Control and Prevention say between 1.6 million and 1.7 million people are at high risk of contracting monkeypox.
Monkeypox: everything you need to know
“In recent weeks the monkeypox virus has continued to spread at a rate that has made it clear our current vaccine supply will not meet the current demand,” FDA Commissioner Robert Califf said.
There are more than 8,900 presumed monkeypox cases and no deaths from the virus in the US, CDC data shows.
The outbreak is mainly affecting gay men and other members of the LGBTQ community.
Monkeypox can spread through sex, close contact, and interacting with cloth used by infected people.
Symptoms include painful rashes and lesions, requiring isolation for up to a month.
