After eight hours of non-stop debate, a high powered US vaccine advisory panel endorsed mass use of a single dose Johnson & Johnson Covid-19 vaccine, putting America just one step away from its third shot against a virus that has killed more than 509,000 Americans in 12 months.

In a 21-0 vote, the usually obscure Vaccines and Related Biological Products Advisory Committee (VRBPAC), decided that the J&J shot is safe in people aged 18 and older.

Shortly after 5 p.m. EST, the panel voted resoundingly on a single question: Based on the totality of scientific evidence available, do the benefits of the Janssen Covid-19 vaccine outweigh its risks for use in individuals 18 years of age and older?

It's now over to the US Food and Drug Administration for the final greenlight. If the FDA clears the J&J shot for US use, only a few million doses are expected to be ready for shipping in the first week. J&J told US Congress that it expects to deliver 20 million doses by the end of March and 100 million by June. The company aims to produce around a billion doses by the end of the year.

America, which began its historic vaccination drive on December 14 last year, has delivered a total of more than 70 million shots in arms across first and second doses combined. More than 47 million people in the US have received at least one shot. The two vaccines currently in use - Pfizer and Moderna - are both two shot vaccines spaced about 3-4 weeks apart.

US FDA scientists have already confirmed that the Johnson & Johnson vaccine is safe and is about 66 per cent effective at preventing moderate to severe Covid-19, and about 85 percent effective against serious illness. Similar to the other Covid-19 vaccines in the US, the main side effects of the J&J shot are pain at the injection site and flu-like fever, fatigue and headache.

J&J tested its single-dose option in about 44,000 adults in the US, Latin America and South Africa with a 2-month median follow-up. "The analysis supported a favourable safety profile with no specific safety concerns identified that would preclude issuance of an EUA," the US FDA said of the J&J vaccine.

The FDA summary noted that there were no Covid-19-related deaths and no Covid-19 cases requiring medical intervention 28 days or more post-vaccination among participants age 60 years or older with medical comorbidities in the vaccine group.