Pfizer and BioNTech have started clinical trials of a new Covid vaccine which targets the Omicron variant.
The companies plan to test the protection gained from the new vaccine as a booster jab and as three separate jabs in unvaccinated people.
More than 1,400 adults are expected to be enrolled in the trial, likely to be in the United States.
US company Moderna is planning to begin trials of its own Omicron-specific shot soon.
Oxford University and AstraZeneca have also started working on a new version of their vaccine.
Vaccine developers had always planned to tweak their original Covid vaccines as new variants emerged, but the arrival of the fast-spreading Omicron strain in the last two months has hastened that process.
Many countries have now offered a booster or third dose of their original vaccine, which has been shown to provide a good level of protection against serious illness and death – even against Omicron.
But protection against infection and mild symptoms is much lower and could decline more quickly.