CAMBRIDGE, MASSACHUSETTS (BLOOMBERG) - Moderna Inc said it would produce as many as three billion doses of its Covid-19 vaccine next year as it makes new investments to bolster output at several factories in the United States and Europe.

The biotech company said it would increase supply by 50 per cent at its plant in Norwood, Massachusetts, which makes much of the vaccine substance used in shots for the US market.

The investments would also enable partner Lonza Group AG, which is making supply for foreign markets, to double its output at a factory in Switzerland that makes vaccine substance.

Vaccine output at a third factory in Spain operated by another partner, Laboratorios Farmaceuticos Rovi SA, would also more than double under the plan.

The increased production from the company-owned and partner factories is expected to ramp up in late 2021 and early 2022, Moderna said.

The exact amount of doses Moderna produces next year will depend on the mix of production between the current vaccine, which uses a higher dose, and paediatric vaccines and booster shots now in human trials that are likely to use smaller doses, it said.

"We are almost doubling" the capacity for the next year, said Moderna chief executive officer Stephane Bancel, in an interview.

Mr Bancel said the Cambridge, Massachusetts-based company will spend billions of dollars on the expansion, without providing exact numbers.

As viral variants continue to spread worldwide, he said he has been besieged by calls from other countries in recent weeks wanting to order more of the vaccine.

"My phone has gone crazy in the last month," he said.

Moderna had previously projected it would produce 1.4 billion doses of its current vaccine in 2022, or as many as 2.8 billion doses next year if it switched its production entirely to a booster shot.

Moderna also raised the low end of its supply projections for this year to 800 million doses from 700 million doses. The high end estimate for 2020 is still one billion doses.

The new investments are in addition to a recent agreement with French drugmaker Sanofi to help perform "fill-and-finish" work at a plant in New Jersey to package up to 200 million doses of Moderna's messenger RNA vaccine, starting in September.