US biotech firm Moderna on Monday announced its experimental vaccine against Covid-19 was 94.5 percent effective, marking a second major breakthrough in the vaccine hunt.

Moderna released early results from a clinical trial with more than 30,000 participants, after US pharmaceutical company Pfizer and its German partner BioNTech last week said their vaccine was 90 percent effective.

Both vaccine frontrunners are based on a new platform called messenger RNA, which is faster to produce than traditional vaccines and effectively turn human cells into vaccine factories.

"This positive interim analysis from our Phase 3 study has given us the first clinical validation that our vaccine can prevent COVID-19 disease, including severe disease," said Stephane Bancel, Moderna's CEO.

The company plans to submit applications for emergency approval in the US and around the world within weeks, and says it expects to have approximately 20 million doses ready to ship in the US by the end of the year.

Global infections from Covid-19 have soared past 54 million with more than 1.3 million deaths since the virus emerged in China late last year.

The Moderna vaccine, which was co-developed by the US National Institutes of Health, is given in two doses 28 days apart, and the preliminary results are based on 95 volunteers of the 30,000 who fell ill with Covid-19.

Of the 95, 90 had been in the trial's placebo group, and five in the group that received the drug, called mRNA-1273.

'Tremendously exciting'

There were 11 people who fell severely ill, all of whom were in the placebo group. The vaccine was well tolerated, with the majority of side-effects classed as mild or moderate.

After the first dose, about three percent of people had injection site pain classed as severe. Among side-effects classed as severe after the second dose, about 10 percent had fatigue, nine percent had muscle pain, five percent had joint pain or headaches, four percent had other pain and two percent had redness at the injection site.

These adverse events were "short lived," according to a statement. "This news from Moderna is tremendously exciting and considerably boosts optimism that we will have a choice of good vaccines in the next few months," said Peter Openshaw, a professor of experimental medicine at Imperial College London.

Crucially, Moderna also announced that its vaccine can remain stable at standard refrigerator temperatures of 2 degrees Celsius to 8 degrees Celsius (36 degrees Fahrenheit to 46 degrees Fahrenheit) for 30 days.

The company added it could be kept in long-term storage at standard freezer temperatures of -20 degrees Celsius (-4 degrees Fahrenheit) for up to six months.

Pfizer's vaccine, on the other hand, needs to be stored in deep-freezer conditions which could complicate supply chain logistics, particularly in less developed countries.

It is not yet clear how long lasting the protection will be from either the Moderna or Pfizer vaccines, nor how well they work for the elderly, the age-group at highest risk from Covid-19.