US regulators are strictly limiting who can receive Johnson & Johnson's COVID-19 vaccine due to the ongoing risk of rare but serious blood clots.

The Food and Drug Administration on Thursday said the vaccine should only be given to adults who cannot receive a different serum or specifically request J&J's vaccine.

Authorities for months have recommended Americans get Pfizer or Moderna shots instead of J&J's vaccine.

In Australia, the single dose J&J vaccine -- COVID-19 vaccine Janssen -- has been provisionally approved but is not currently in use in the vaccine rollout, according to government agency Health Direct.

FDA's vaccine chief Dr Peter Marks said the agency decided to restrict the vaccine after more research on the risks of life-threatening blood clots and concluding they are limited to J&J's vaccine.

"If there's an alternative that appears to be equally effective in preventing severe outcomes from COVID-19, we'd rather see people opting for that," Dr Marks said.

"But compared to no vaccine. This is still a better option."

The problem occurs in the first two weeks after vaccination, he added.

The FDA authorised J&J's shot in February last year for adults 18 and over.

The vaccine was initially considered an important tool in fighting the pandemic because it required only one shot. But the single-dose option proved less effective than two doses of Pfizer and Moderna vaccines.

In December, the Centers for Disease Control and Prevention recommended Moderna and Pfizer jabs over J&J's due to its safety issues.

As of mid-March, the FDA identified 60 cases of the side effect, including nine that were fatal. That amounts to 3.23 blood clot cases per one million J&J shots. The problem is more common in women under 50, where the death rate was about one per million shots.

The FDA found the problem was not connected to other issues such as women taking birth control medications that raise the risk of clotting.

The J&J vaccine will carry a stronger warning about potential "long-term and debilitating health consequences" of the side effect.

Under the new FDA instructions, J&J's vaccine could still be given to people who had a severe allergic reaction to one of the other vaccines. J&J's shot could also be an option for those who refuse the mRNA vaccines from Pfizer and Moderna, and therefore would remain unvaccinated.

A J&J spokesman said: "Data continue to support a favourable benefit-risk profile for the Johnson & Johnson COVID-19 vaccine in adults, when compared with no vaccine."

COVID-19 causes deadly blood clots but the vaccine-linked kind is believed to form because of a rogue immune reaction to the J&J and AstraZeneca vaccines because of how they are made. Clots form in unusual places, such as veins that drain blood from the brain.