US drug manufacturer Moderna has released data that shows its experimental COVID-19 vaccine was 94.5 percent effective, based on interim data from a late-stage trial.

Moderna's interim analysis was based on 95 infections among trial participants who received either a placebo or the vaccine. Its findings were not published in a peer-reviewed journal paper.

Of those, only five infections occurred in those who received the vaccine, which is administered in two shots 28 days apart.

This makes the company the second US manufacturer to report results that far exceed expectations, following Pfizer's announcement of promising vaccine test results last week.

Both potential vaccines still need to pass more safety data and regulatory reviews.

But Moderna expects to have enough safety data required for US authorisation in the next week or so and the company expects to file for emergency use authorisation (EUA) in the coming weeks.

"We are going to have a vaccine that can stop COVID-19," Moderna President Stephen Hoge said in a telephone interview.

In a tweet, US President Donald Trump also attempted to claim credit for the results on Tuesday.

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How does it work?

A vaccine's 'efficacy' doesn't measure how well it stops the SARS-CoV-2 virus entering a vaccinated person's body.

Instead, it's a measure of stopping — or at least reducing the severity of — COVID-19.

Unlike more traditional vaccines, which contain a weakened version of the actual virus or portions of it, both Moderna and Pfizer's vaccines are based on a new technology that involves taking genetic material from the virus and delivering it to healthy cells.

Using messenger RNA (or mRNA), the vaccines carry instructions to the body to create the SARS-CoV-2 protein the vaccine is aiming to protect against.

The idea is that the body detects the genetic material and new protein as foreign, and mounts an immune response — producing antibodies that learn to remember and fight the virus if the body encounters it again.

One advantage of Moderna's vaccine is that it does not need ultra-cold storage like Pfizer's, making it easier to distribute.

Moderna expects it to be stable at normal fridge temperatures of 2 to 8 degrees Celsius for 30 days and it can be stored for up to 6 months at -20C.

Pfizer's vaccine must be shipped and stored at -70C, the sort of temperature typical of an Antarctic winter.

"Having more than one source of an effective vaccine will increase the global supply and, with luck, help us all to get back to something like normal sometime in 2021," said Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh.

What did the data show?

The data from Moderna's trial involving 30,000 volunteers also showed the vaccine prevented cases of severe COVID-19, a question that still remains with the Pfizer vaccine.

Of the 95 cases in Moderna's trial, 11 were severe and all 11 occurred among volunteers who got the placebo.

The 95 cases of COVID-19 included several key groups who are at increased risk for severe disease, including 15 cases in adults aged 65 and older and 20 in participants from racially diverse groups.

Research has suggested that black and minority ethnic patients are at higher risk of death from COVID-19.

"We will need much more data and a full report or publication to see if the benefit is consistent across all groups, notably the elderly, but this is definitely encouraging progress, said Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

One unknown with this vaccine, and all the others currently being tested, is whether they will stop COVID-19 spreading.

"It is likely that vaccines that prevent symptomatic disease will reduce the duration and level of infectiousness, and thus reduce transmission, but we don't yet know if this effect will be large enough to make any meaningful difference to the spread of the virus within communities," Dr Riley said.

What about the side effects?

Most side effects of the study were reportedly mild to moderate.

A significant proportion of volunteers, however, experienced more severe aches and pains after taking the second dose, including about 10 per cent who had fatigue severe enough to interfere with daily activities while another 9 per cent had severe body aches.

Most of these complaints were generally short-lived, Moderna said.

"These effects are what we would expect with a vaccine that is working and inducing a good immune response," said Peter Openshaw, professor of experimental medicine at Imperial College London.

If it passes further tests, how soon could it be rolled out?

The vaccine could enter Australia via the COVAX Facility, which is a coalition of 188 countries allowing members to purchase COVID-19 vaccines if and when they become available. Moderna's vaccine is one of ten candidates the facility is currently eyeing.

As part of membership, wealthy and poorer countries have agreed to pay different amounts to participate, ensuring lower-income nations also get fair and equitable access to any future vaccines.

The Pfizer trial vaccine is also one of four other potential vaccines the Australian Government has signed an agreement to purchase,

Moderna's candidate is part of the US Government's Operation Warp Speed program, and they expect to produce about 20 million doses for the US this year, millions of which the company has already made and is ready to ship if it gets US Federal Drug Administration (FDA) authorisation.

Moderna has received nearly $US1 billion ($1.4 billion) in research and development funding from Washington and has a $US1.5 billion deal for 100 million doses.

The company hopes to produce between 500 million and 1 billion doses in 2021, split between its US and international manufacturing sites, dependent in part on demand.

Moderna also said it would use its data to seek authorisation in Europe and other regions.

Europe's health regulator said on Monday it had launched a real-time "rolling review" of Moderna's vaccine, following similar reviews of vaccines from Pfizer and AstraZeneca.

ABC/Reuters