Moderna Inc said on Tuesday it was moving forward the supply target for the second 100 million of its COVID-19 vaccine doses to the United States by a month to May end.

The biotech firm said short-term delays in the final stages of production and release of filled vials at Moderna's fill and finish contractor Catalent Inc have recently delayed the release of some doses.

Moderna said these delays are expected to be resolved in the near term and not expected to impact monthly delivery targets.

The United States, which is fighting supply constraints for its large-scale efforts to vaccinate most Americans, earlier this month bought an additional 100 million vaccine doses from Moderna, taking the total ordered doses to 300 million.

The company reiterated it was on track to deliver the first 100 million doses by the end of March and also moved forward the delivery of its third 100 million vaccine doses by two months to July end.

The US government has so far administered about 25.5 million doses of the two-dose vaccine, with 45.4 million doses supplied, Moderna said.

An additional 33.2 million doses of the vaccine have been filled in vials and are at various stages of final production and testing before release to the US government, according to the company.

Moderna expects to deliver an average of 30 million to 35 million doses per month for February and March, and 40 million to 50 million doses a month from April through July-end.

The company said it would ship doses to the US as they are produced instead of stockpiling first, potentially affecting its weekly supply numbers.