WASHINGTON: Frozen vials of Pfizer's Covid-19 vaccine may be stored at temperatures commonly found in pharmaceutical freezers for a period of up to two weeks, the US Food and Drug Administration said Thursday.

The move loosens a previous requirement that the vaccine should be stored at ultra-low temperatures, between -112 and -76 degrees Fahrenheit (-80 to -60 degrees Celsius).

"The alternative temperature for transportation and storage will help ease the burden of procuring ultra-low cold storage equipment for vaccination sites and should help to get vaccine to more sites," said Peter Marks, director of the FDA's Center for Biologics Evaluation and Research.

The FDA said it would update its fact sheet for health care providers accordingly. Pharmaceutical freezers commonly operate at around -4 degrees Fahrenheit (-20 degrees Celsius).

The move came after Pfizer submitted a request based on its research on the vaccine's stability under the warmer temperature.

The Pfizer vaccine is based on new technology that uses synthetic mRNA (messenger ribonucleic acid) molecules to deliver the genetic instructions for human cells to create a part of the coronavirus.

The mRNA molecules are encased in particles of fat to protect them, but they still degrade more quickly than traditional vaccines and so require stricter storage measures.

Once thawed, the vials can be stored at fridge temperatures of 35 to 46 degrees Fahrenheit (2 to 8 degrees Celsius) for up to five days.

They can be kept at room temperature for no more than two hours.