British regulators on Monday approved the country’s first Covid-19 booster vaccine to target two coronavirus variants, the original virus and the Omicron variant.

Half of each dose of the vaccine, or 25 micrograms, will target the original variant, and the other half will target Omicron. In clinical trials, the vaccine, an updated version of Moderna’s original Covid vaccine, generated a good immune response to these two variants, as well as the BA.4 and BA.5 subvariants in adults, researchers found.

Dr. June Raine, the chief executive of Britain’s Medicines and Healthcare Products Regulatory Agency, said she was pleased that the new booster vaccine met the regulator’s standards of safety, quality and effectiveness. The decision was endorsed by Britain’s independent expert scientific advisory body, the Commission on Human Medicines.

“The first generation of Covid-19 vaccines being used in the U.K. continue to provide important protection against the disease and save lives,” Dr. Raine said. “What this bivalent vaccine gives us is a sharpened tool in our armory to help protect us against this disease as the virus continues to evolve.”

Side effects were the same as those seen for the original Moderna booster dose and were typically mild, with no serious safety concerns, British regulators said.

The emergence of highly contagious Omicron subvariants this spring have appeared to reduce the protection offered by the Pfizer-BioNTech and Moderna vaccines against Covid hospitalizations, with more vaccinated people admitted to the hospital with Covid than they had been during the winter Omicron wave. But booster shots have raised people’s levels of protection, scientists from the U.S. Centers for Disease Control and Prevention said last month. The C.D.C. recommends that people receive booster shots as soon as they are eligible.