AstraZeneca, whose vaccine has not been approved in the United States, said it will work with President Joe Biden's administration to find an alternative site to produce its vaccine.
Under the current plan international travel will not resume until May 17 at the earliest. The Financial Times said Johnson was not expected to set out a specific timeframe.
The Department of Health and Human Services facilitated the move, the health official said in an email, asking not to be named due to the sensitivity of the matter.
Young people have been less likely to suffer severe disease and more likely to have asymptomatic infection, allowing them to unwittingly transmit COVID-19 to others.
The overall figure was revised down dramatically after the Anglo-Swedish pharma giant AstraZeneca slashed the number it was providing from a contracted 120 million doses.
Germany on Tuesday became the latest in a series of countries to advise against using the AstraZeneca jab for younger people after rare reports of clotting.
"Everything is based on one principle and that is trust," Merkel said at a news conference. "Confidence arises from the knowledge that every suspicion is counted in every individual case."