NEW YORK (REUTERS) - Johnson & Johnson's one-shot Covid-19 vaccine received US authorisation on Saturday (Feb 27) and will start being administered to Americans within days.

The following is how it fits into the vaccination effort with the two other authorised immunisations from Pfizer-BioNTech and Moderna.

Is J&J'S vaccine as effective as the other two?

The Pfizer-BioNTech and Moderna vaccines, which both use new messenger RNA technology, showed efficacy of around 95 per cent in preventing illness in large trials, while J&J's showed efficacy of around 66 per cent.

However, the trials had important differences, making comparison difficult. Pfizer-BioNTech and Moderna focused on efficacy against mild-to-moderate sickness, while J&J's trial looked for efficacy against moderate-to-severe Covid-19.

J&J's trial was conducted later and included significant numbers of participants in regions where highly transmissible new variants of the virus were circulating, potentially subjecting its shot to a more rigorous test.

Importantly, J&J's vaccine was about as effective against severe illness and hospitalisation as the other vaccines and, like the others, was 100 per cent effective in preventing Covid-19-related death in the trial.

All three vaccines have a similar range of short-lived side effects, including fatigue, aches, injection site soreness, nausea and fever, mostly mild-to-moderate.

If J&J's shot is less effective, why should i get it?

J&J's vaccine is a single shot. Pfizer-BioNTech and Moderna vaccines each require two shots three or four weeks apart.

J&J's shot is extremely effective at preventing people exposed to Covid-19 from getting severely ill or dying, as well as offering protection against milder cases.

"These vaccines are the path to getting your life back," said senior scholar Amesh Adalja from the Johns Hopkins Centre for Health Security. "These are vaccines you should not hesitate to get when offered."

J&J's vaccine is one of the few that has been tested in clinical trails against the worrisome virus variant first discovered in South Africa. It demonstrated an ability to provide protection against severe disease and death even for those infected with that variant.

Pfizer-BioNTech and Moderna have only tested their vaccines against the variant in the lab and believe they will remain protective.

How many doses of J&J's new shot will be available in the US?

J&J said it expects to ship nearly 4 million doses in the first week, and shipments could begin late Sunday or early Monday.

J&J expects to deliver a total of 20 million shots by the end of March and as many as 100 million doses by mid-year.

The federal government, through distributor McKesson , will ship J&J's shots to states and territories around the US. It will also deliver them to select pharmacies and community health centres to make sure they reach underserved populations.

With J&J, Pfizer-BioNTech and Moderna, the US is on track to receive 240 million doses by the end of March, enough to fully inoculate 130 million Americans. The drugmakers have agreed to send 700 million doses of their vaccines by the end of July, more than enough to vaccinate all Americans.