WASHINGTON — Results from a U.S. trial of AstraZeneca’s COVID-19 vaccine may have included “outdated information” and that could mean the company provided an incomplete view of efficacy data, American federal health officials said early Tuesday.

AstraZeneca said in a statement that the data it released Monday included cases up to Feb. 17, as the study rules specified, and that it was continuing to analyze cases that have occurred since then. The company said that a preliminary analysis of data that has continued to roll in was consistent with what it had already reported. It promised an update within 48 hours.

AstraZeneca reported Monday that its COVID-19 vaccine provided strong protection among adults of all ages in a long-anticipated U.S. study, a finding that some experts hoped would help rebuild public confidence in the shot around the world and move it a step closer to clearance in the U.S.

In the study of more than 30,000 people, the company reported that the vaccine was found to be 79% effective at preventing symptomatic cases of COVID-19 — including in older adults. There were no severe illnesses or hospitalizations 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 U.S. National Institute of Allergy and Infectious Diseases issued an unusual statement.

The agency said the Data and Safety Monitoring Board “expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data.”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.

In previous vaccine trials, as is the situation here, cases have continued to accrue, even as companies began to release their data. But the AstraZeneca circumstance is unusual in drawing the concern of the data monitoring board.

The Food and Drug Administration reviews the most up-to-date trial findings when it assesses the vaccines.

The company has said it aims to file an application with the FDA in the coming weeks, and the government’s outside advisers will then publicly debate the evidence.

Authorization and guidelines for use of the vaccine in the United States will be determined by the FDA and Centers for Disease Control and Prevention after thorough review of the data by independent advisory committees.