NEW YORK (REUTERS) - A large plant being used to manufacture Johnson & Johnson's Covid-19 vaccine was cleared by US regulators on Tuesday (March 23), setting the stage for the weekly US supply to surge more than 20 per cent.

About 27 million Covid-19 vaccine doses will be allocated to US states and other localities this week, including 4 million from J&J, White House spokeswoman Jen Psaki told reporters.

That is the largest allocation yet, up from 22 million last week.

Earlier, the Indiana plant at which Catalent Inc is helping to manufacture the J&J vaccine received US regulatory authorisation, the companies said.

J&J's shipments had slowed considerably since the first week of the month, but the new authorisation will enable it to ship out millions of doses.

J&J tapped contract manufacturers Catalent and Emergent BioSolutions Inc to scale up production and meet its global supply targets.

Catalent provides the final stage - called fill and finish - while Emergent makes the drug substance.

The US Food and Drug Administration authorised the one-shot J&J vaccine in February, but only for its production facility in the Netherlands and a small fill-and-finish plant in the United States.

Based on that authorisation, the company shipped nearly 4 million doses in the beginning of March. Shipments dropped sharply since then as J&J awaited the additional authorisations.

The company had previously promised to deliver 20 million vaccine doses by the end of March.

As of Tuesday morning, 82.7 million people in the United States had received at least one vaccine dose, around a quarter of the population, according to data from the Centres for Disease Control and Prevention.