Pfizer Inc. and BioNTech SE have started a trial evaluating an adapted version of their Covid-19 vaccine that targets the Omicron variant of the coronavirus.

The drugmakers said Tuesday they began enrolling adults ages 18 to 55 in the U.S. and South Africa to examine the safety, tolerability and immune response generated by the vaccine if it is given either as a primary series or as a booster dose.

One...

Pfizer Inc. and BioNTech SE have started a trial evaluating an adapted version of their Covid-19 vaccine that targets the Omicron variant of the coronavirus.

The drugmakers said Tuesday they began enrolling adults ages 18 to 55 in the U.S. and South Africa to examine the safety, tolerability and immune response generated by the vaccine if it is given either as a primary series or as a booster dose.

One subject has already received the shot, Pfizer said.

Initial study results are expected in the first half of the year, Pfizer said. The drugmaker could ask U.S. regulators for authorization and begin distribution in March, should the Omicron-targeted shot prove to work safely, Chief Executive Albert Bourla has said.

Should the vaccine be needed, Pfizer and BioNTech would still be able to manufacture 4 billion doses of the shot this year, the companies said.

Amid a surge in cases, some countries are handing out second booster shots. In Israel, early data suggest a fourth vaccine dose can increase antibodies against Covid-19, but not enough to prevent infections from Omicron. WSJ explains. Photo composite: Eve Hartley/WSJ The Wall Street Journal Interactive Edition

Moderna Inc. plans to start a clinical trial of its Omicron-specific vaccine candidate within days, a company spokeswoman said.

Vaccine makers have been racing to tweak their Covid-19 shots to target Omicron since the variant began sweeping the globe and causing a surge in cases.

Laboratory studies have found that Covid-19 vaccines provide less protection against Omicron than previous strains. Yet a third dose of certain vaccines, administered as a booster, neutralized Omicron in lab tests.

U.S. health authorities have said that Omicron-targeted shots might not be needed, citing the activity of boosters found so far against Omicron in lab tests. Another variable: Future variants might not be descendants of Omicron.

Pfizer and BioNTech say they want to be prepared with an Omicron vaccine, should regulators and researchers determine one is needed.

“Staying vigilant against the virus requires us to identify new approaches for people to maintain a high level of protection, and we believe developing and investigating variant-based vaccines, like this one, are essential in our efforts to work towards this goal,” said Dr. Kathrin Jansen, who leads vaccine R&D at Pfizer.

Pfizer and BioNTech have studied versions of their vaccines targeting the Beta and Delta variants, although they haven’t been cleared for use by regulators.

In the new study, the companies said they would test how the vaccine performs in three groups of volunteers. Some of the subjects will come from the study that led to the current vaccine’s clearance in 2020.

Researchers plan to enroll up to 615 people who received the two-dose primary series of the current vaccine three to six months before enrollment. The subjects will receive either one dose of the Omicron-based vaccine, two doses four weeks apart from each other, or a third dose of the current vaccine.

Additionally, researchers will enroll up to 600 people who received three doses of the current vaccine three to six months ago. These subjects will receive either one dose of the Omicron vaccine or a fourth dose of the current shot.

About 200 additional subjects who haven’t received any vaccine will receive two doses of the Omicron-based vaccine three weeks apart, and a third dose six months later.

—Peter Loftus contributed to this article.

Write to Jared S. Hopkins at jared.hopkins@wsj.com