ZURICH, Jan 29 (Reuters) - Swiss drugmaker Novartis has signed an initial agreement to fill vials for Pfizer and BioNTech's COVID-19 vaccine, a move aimed at helping boost production as supplies fall behind demand.

Subject to reaching a final deal, Novartis plans to start production in the second quarter of 2021, the Basel-based company said on Friday, adding it would use aseptic facilities at its site in Stein, Switzerland, near the Rhine River on the German border.

It will fill vials with vaccine then ship them back to BioNTech for distribution.

Three vaccines from Western companies Pfizer and BioNTech, AstraZeneca and Moderna have been approved by regulators in various countries, but limited production and delayed deliveries have hampered efforts to quickly vaccinate people most at risk for COVID-19.

Novartis's announcement that it was joining the push to remedy the supply shortfall follows a move by Sanofi this week to work with BioNTech to supply 125 million doses of its mRNA vaccine, based on genetic technology, to the European Union.

"Novartis has been mobilizing on multiple fronts to support the global pandemic response," said Steffen Lang, head of Novartis Technical Operations.

A spokesman didn't provide volumes to be filled by the Swiss plant.

Novartis added it was in "advanced discussions" with other companies to take on production of mRNA, therapeutic protein and other raw materials for vaccines and therapeutics. (Reporting by John Miller; Editing by Michael Shields and Pravin Char)