A panel of outside advisers to the United States Food and Drug Administration (FDA) on Thursday overwhelmingly endorsed Moderna Inc’s COVID-19 vaccine for emergency use, a step that paves the way for its authorisation across the country.

The decision comes one week after the same panel backed another COVID-19 vaccine from Pfizer and the company’s German partner, BioNTech, leading to an FDA emergency use authorisation a day later.

The FDA is expected to grant the authorisation as early as late Thursday or Friday. The Moderna vaccine is set to begin distribution as soon as the FDA gives the green light.

US officials have said they expect to have 40 million doses of the Pfizer-BioNTech and Moderna vaccines by the end of the year 

Both vaccines were about 95 percent effective at preventing illness in pivotal clinical trials with no serious safety issues.

Moderna’s vaccine does not require specialised refrigeration for transportation or storage, unlike the Pfizer-BioNTech vaccine, which must be kept at -70 Celsius (-94) and could make storage in rural, lower-income areas more difficult.

The Moderna vaccine requires temperatures of around -20C for shipping - similar to a normal freezer.

The Pfizer vaccine requires temperatures closer to -75C, making transport logistics much more difficult.

Like Pfizer, the Moderna vaccine also requires a second booster shot. Moderna's second injection comes 28 days after the first.