Booster doses of Johnson & Johnson's one-shot coronavirus vaccine generated a big spike in antibodies, the frontline immune system defences against infection, the company reported Wednesday.

People who received a booster six to eight months after their initial J&J shots saw antibodies increase nine-fold higher than 28 days after the first shot, Johnson & Johnson said.

The data comes from two Phase 2 studies conducted in the United States and Europe, the company said in a statement. Some of the 2,000 or so people in the studies got booster doses six months after their first doses of J&J's Janssen vaccine.

"New interim data from these studies demonstrate that a booster dose of the Johnson & Johnson COVID-19 vaccine generated a rapid and robust increase in spike-binding antibodies, nine-fold higher than 28 days after the primary single-dose vaccination," the company said in its statement.

"We have established that a single shot of our COVID-19 vaccine generates strong and robust immune responses that are durable and persistent through eight months. With these new data, we also see that a booster dose of the Johnson & Johnson COVID-19 vaccine further increases antibody responses among study participants who had previously received our vaccine," Dr Mathai Mammen, global head of research and development for Janssen, said in a statement.

J&J said it was in discussions with the US Food and Drug Administration, US Centers for Disease Control and Prevention, European Medicines Agency, World Health Organization and other health authorities about the need for offering a booster dose of the Janssen vaccine.

"We look forward to discussing with public health officials a potential strategy for our Johnson & Johnson COVID-19 vaccine, boosting eight months or longer after the primary single-dose vaccination," Mammen added.

Many people who received the J&J vaccine have been clamouring for information about whether they will need a booster shot. US federal government officials have said they are preparing to start offering a booster dose to people who got Moderna's or Pfizer's Covid-19 vaccine after data showed boosters can amp up the antibody response -- and after studies started showing an uptick in infections in both vaccinated and unvaccinated people. The more transmissible Delta variant is partly to blame, experts say, as is a waning immune response.