Johnson & Johnson has said that its COVID-19 booster shot is 94 percent effective against symptomatic infection when administered two months after the first dose in the United States.

It provided 75 percent protection against moderate-to-severe Covid globally, J&J said.

It also increases antibody levels by four to six times compared with one shot alone, J&J said.

A J&J booster dose given six months from the first shot appears to be potentially even more protective against COVID-19, the company said, adding that it generated antibodies twelvefold higher four weeks after the booster.

This shows a big improvement in protection with the longer interval between the doses.

While the company stresses on the durability of its single dose vaccine, the data will help J&J make its case to the US regulators for a booster shot.

"Our single-shot vaccine generates strong immune responses and long-lasting immune memory. And, when a booster of the Johnson & Johnson COVID-19 vaccine is given, the strength of protection against COVID-19 further increases," Dr. Mathai Mammen, global head of Janssen Research & Development, was quoted as saying by CNN.

Side effects with two doses were comparable to those seen in studies with the single-dose vaccine.

The data is yet to be peer reviewed but will be submitted for publication in the coming months.

Last month, the US announced plans to roll out booster shots for people who received the Moderna and Pfizer vaccines. An FDA advisory committee recommended Pfizer booster shots to people age 65 and older and other vulnerable Americans.

(With inputs from CNN.)