The US Food and Drug Administration has approved Johnson and Johnson's Covid-19 vaccine that works with just one dose instead of two for emergency use, giving America its third jab to fight the pandemic that has claimed over half a million lives in the country.

President Joe Biden described the FDA's approval as an encouraging development in efforts to end the Covid-19 crisis and said this is exciting news for all US citizens.

The Johnson and Johnson vaccine, which works with one dose instead of two, got approval on Saturday, the third vaccine cleared in the country after emergency use authorisation (EUA) was given to two-dose shots from Pfizer and Moderna in December last year.

Unlike Pfizer's and Moderna's vaccines that require two shots within two weeks, J&J's one-dose regimen eliminates the need for patients to return for a second dose and it can be stored at refrigerator temperatures for months.

"Today, after a rigorous, open, and objective scientific review process, the Food and Drug Administration (FDA) issued an emergency use authorisation for a third safe and effective vaccine to help us defeat the Covid-19 pandemic - the Janssen Covid-19 (Johnson and Johnson) vaccine," Biden said.

The EUA allows the Janssen Covid-19 vaccine to be distributed in the US for use in individuals of 18 years of age or older. Unlike the other two approved vaccines, the Johnson and Johnson vaccine is administered as a single dose.

Preliminary results from a large trial in the US, Latin America and South Africa suggested J&J's vaccine was safe and offered strong protection against moderate to severe Covid-19.

The FDA said J&J's vaccine offers strong protection against what matters most: serious illness, hospitalisations and death. One dose was 85 per cent protective against the most severe Covid-19 illness, in the massive study that spanned three continents - protection that remained strong even in countries such as South Africa, where the variants of most concern are spreading.

J&J is also seeking authorisation for emergency use of its vaccine in Europe and from the World Health Organisation. The company aims to produce about 1 billion doses globally by the end of the year. On Thursday, the island nation of Bahrain became the first to clear its use.

The J&J vaccine is easier to handle, lasting three months in the refrigerator compared to the Pfizer and Moderna options, which must be frozen.

"The authorisation of this (Johnson and Johnson) vaccine expands the availability of vaccines, the best medical prevention method for Covid-19, to help us in the fight against this pandemic, which has claimed over half a million lives in the US," Acting FDA Commissioner Janet Woodcock said.

The FDA said the totality of available data provides clear evidence that the Janssen Covid-19 vaccine may be effective in preventing the infection.

Data also shows that the vaccine's known and potential benefits outweigh its known and potential risks, supporting the company's request for the vaccine's use in people of 18 years of age and older, it said.

"We know that more people that get vaccinated, the faster we will overcome the virus, get back to our friends and loved ones, and get our economy back on track," President Biden said, adding that the country is moving in the right direction.

He said at the same time, this fight is far from over.

"Though we celebrate today's news, I urge all Americans - keep washing your hands, stay socially distanced, and keep wearing masks. As I have said many times, things are still likely to get worse again as new variants spread, and the current improvement could reverse. My administration will not make the mistake of taking this threat lightly, or just assuming the best: that's why we need the American Rescue Plan to keep this fight going in the months ahead," Biden said.

There is light at the end of the tunnel, but people cannot let their guard down now or assume that victory is inevitable, the president said.

"We must continue to remain vigilant, act fast and aggressively, and look out for one another - that is how we are going to reach that light together," Biden said.