Americans should be able to receive Johnson & Johnson's COVID-19 vaccine within the next 24 to 48 hours, its chief executive said on Monday after U.S. regulators approved the vaccine, making it the country's third available one for the novel coronavirus.

The drugmaker was still on track to deliver 4 million vaccine doses this week, and 100 million doses by June, J&J CEO Alex Gorsky told NBC News' Today program in an interview.

"Within the next 24 to 28 hours, Americans should start receiving shots in arms. They're literally rolling out with the trucks as we speak," he said.

Shares of the pharmaceutical company were up 2.9% in premarket trading after the U.S. Food and Drug Administration approved its one-shot COVID-19 vaccine on Saturday. McKesson Corp earlier on Monday said it had begun distributing it.

Pfizer-BioNTech and Moderna Inc have FDA-approved two-dose vaccines. Unlike those vaccines, J&J's version requires no refrigeration and only a single dose is needed, potentially facilitating wider use.