Results from a US trial of AstraZeneca's Covid-19 vaccine may have used "outdated information", according to federal health officials in the country.

The Data and Safety Monitoring Board (DSMB) said it was concerned that AstraZeneca may have provided an incomplete view of the efficacy data.

AstraZeneca reported on Monday that its Covid-19 vaccine provided strong protection among adults of all ages in a long-anticipated US study, a finding that could help rebuild public confidence in the jab around the world and move it a step closer to clearance in the US.

In the study of 30,000 people, the vaccine was 79% effective at preventing symptomatic cases of Covid-19 - including in older adults.

There were no severe illnesses or hospital admissions among vaccinated volunteers, compared with five such cases in participants who received dummy shots - a small number, but consistent with findings from Britain and other countries that the vaccine protects against the worst of the disease.

AstraZeneca also said the study's independent safety monitors found no serious side-effects, including no increased risk of rare blood clots like those identified in Europe, a scare that led numerous countries to briefly suspend vaccinations last week.

But just hours after those encouraging results were reported, the US National Institute of Allergy and Infectious Diseases said the DSMB "expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data".

"We urge the company to work with the DSMB to review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible," the statement added.

The company aims to file an application with the Food and Drug Administration in the coming weeks, and the government's outside advisers will publicly debate the evidence before the agency makes a decision.

Authorisation and guidelines for use of the vaccine in the United States will be determined by the Food and Drug Administration and Centres for Disease Control and Prevention after a thorough review of the data by independent advisory committees.