US-based biotechnology company Moderna's Covid-19 vaccine demonstrated 94.1 per cent efficacy, showed results from the primary analysis of a phase-3 trial.

The study, published in The New England Journal of Medicine, provides evidence that the vaccine, mRNA-1273, can prevent symptomatic infection.

The results are similar to the first interim analysis of the study announced by Moderna in November which showed vaccine efficacy of 94.5 per cent.

The new paper published on Wednesday said that among the more than 30,000 participants randomised to receive the vaccine or a placebo, 11 of those in the vaccine group developed symptomatic Covid-19 compared to 185 participants who received the placebo, demonstrating 94.1 per cent efficacy in preventing symptomatic COVIDsevere Covid-19 occurred only in participants who received the placebo.

"Our work continues. Over the next months, we'll have increasing amounts of data to better define how this vaccine works, but the results so far show a 94.1 percent efficacy. These numbers are compelling," said Lindsey Baden, an infectious diseases specialist at Brigham and Women's Hospital in the US.

"And, importantly, the data suggest protection from severe illness, indicating that the vaccine could have an impact on preventing hospitalisations and deaths, at least in the first several months post-vaccination."

The study enrolled 30,420 adult participants at 99 US sites, including over 600 participants enrolled at the Brigham.

Eligible participants were 18 years old or older with no known history of SARS-CoV-2 infection, and whose locations or circumstances put them at appreciable risk of SARS-CoV-2 infection and/or high risk of severe Covid-19.

Participants received their first injection between July 27 and October 23, 2020, followed by a second injection 28 days later.

In the placebo group, 185 participants developed symptomatic Covid-19 illness; in the vaccine group, 11 participants did.

Thirty participants had severe Covid-19 -- all in the placebo group, said the study.

Overall, reactions to the vaccine were mild -- about half of the recipients experienced fatigue, muscle aches, joint pain and headaches, more so after the second dose,it added.

"While these results are encouraging, they are limited by the short duration of follow-up so far. Longer term data from the ongoing study may allow us to more carefully evaluate the vaccine's efficacy among different groups, determine the impact on asymptomatic infection, understand when immunity wanes, and determine whether vaccines affect infectiousness," said Baden.