Pfizer and BioNTech on Tuesday launched a clinical trial to evaluate the safety and effectiveness of a vaccine that targets the Covid omicron variant, as concerns grow that the current shots aren’t holding up against infections and mild illness caused by the strain discovered just over two months ago.

Pfizer CEO Albert Bourla told CNBC earlier this month that the company will have a vaccine that targets omicron ready by March. The vaccine will also target the other Covid variants that are circulating, Bourla said.

BioNTech CEO Ugur Sahin said there’s growing data indicating that the efficacy of the current vaccine against infection and mild-to-moderate disease from omicron is waning more rapidly compared with prior strains of the virus. The goal is to develop a vaccine that provides durable protection against omicron, Sahin said in a statement Tuesday.

The Centers for Disease Control and Prevention found in a study published last week that a booster dose of Pfizer’s vaccine was 90% effective at preventing hospitalization from omicron 14 days after the third shot was administered.

Booster doses are also up to 75% effective at preventing symptomatic infection from omicron two to four weeks after the third shot, according to data from the U.K. Health Security Agency published earlier this month. However, the study found that boosters weaken substantially after about 10 weeks, providing 45% to 50% protection against symptomatic infection.

“While current research and real-world data show that boosters continue to provide a high level of protection against severe disease and hospitalization with omicron, we recognize the need to be prepared in the event this protection wanes over time and to potentially help address omicron and new variants in the future,” Kathrin Jansen, head of vaccine development at Pfizer, said in a statement.

Pfizer and BioNTech’s clinical study will evaluate up to 1,420 participants ages 18 to 55. In addition to evaluating the omicron vaccine, some people will receive a fourth dose of the current vaccine.

Bourla told CNBC earlier this month he doesn’t know if an omicron-specific vaccine is needed right now or how it would be used. However, Pfizer will have the vaccine ready as many countries are asking for it as soon as possible, he said.

“The hope is that we will achieve something that will have way, way better protection — particularly against infections,” Bourla said.

The omicron variant has dozens of mutations, many of them on the spike protein that the virus uses to invade human cells. The current vaccines, developed in 2020 against the original virus strain, target the spike. It becomes more difficult for vaccine-induced antibodies to block the virus as the spike mutates further and further from the original strain detected in Wuhan, China.

Omicron, first detected in Botswana and South Africa in November, has spread faster than earlier variants, causing an unprecedented wave of infection around the world.

World Health Organization Director-General Tedros Adhanom Ghebreyesus said Monday there have been more than 80 million Covid cases reported to the WHO since the omicron variant was identified just nine weeks ago — more than were reported in all of 2020.

However, people generally don’t get as sick from omicron compared with the delta variant. But because omicron has mutated so far away from the original strain vaccines were developed to fight, it’s causing more mild breakthrough infections, raising concern that it will result in disruptions to essential services as many people call out sick.