The US Food and Drug Administration on Thursday authorised a booster dose of Covid-19 vaccines from Pfizer Inc and Moderna Inc for people with compromised immune systems.

A few other countries, such as Israel and Germany, plan to or have already administered the third shot to avoid another crisis due to the contagious Delta variant of the coronavirus.

Scientists are still divided over the broad use of Covid-19 vaccine boosters among those without underlying problems as benefits of the boosters remain undetermined.

Pfizer has said the efficacy of the vaccine it developed with BioNTech drops over time, citing a study that showed 84% effectiveness from a peak of 96% four months after a second dose.

Moderna has also said it sees the eventual need for booster doses, especially since the Delta variant has caused "breakthrough" infections in fully vaccinated people.

The US health regulator on Thursday amended the emergency use authorisations for the vaccines to allow an additional dose in certain individuals, specifically for recipients of solid organ transplant or those diagnosed with conditions that are considered to have an equivalent level of immunocompromise.

Reports of infections among vaccinated people and concerns about diminishing protection have galvanized wealthy nations to distribute booster shots, even as many countries struggle to access first vaccine doses.

The World Health Organization last week called for a moratorium on Covid-19 vaccine booster shots until at least the end of September.

Spurred by the Delta variant, coronavirus cases in the United States have spiked to their highest levels in more than six months, according to a Reuters tally.

Those with weak immune systems may not be sufficiently protected by their existing Covid-19 vaccinations, US health officials have said.

US regulators must fully authorize the Covid-19 vaccines or amend their emergency use approvals before officials can recommend additional shots. A panel of advisers to the US Centers for Disease Control and Prevention will meet on Friday to discuss eligibility of immunocompromised individuals for booster doses.

Wall Street analysts expect the authorization of a booster dose for a broad population to bolster profits of Covid-19 vaccine makers, as hundreds of millions of fully inoculated people will come back for an extra dose.