US health officials on Wednesday authorized updated Covid-19 vaccinations by Moderna and Pfizer-BioNTech that specifically target the latest strains of the Omicron variant, with the goal of jump-starting a new booster campaign.

The two updated booster shots aimed at providing "better protection against Covid-19 caused by the Omicron variant" are approved for people age 12 and above for the Pfizer shot and 18 and older for Moderna, the Food and Drug Administration (FDA) said in a statement.

This new generation of anti-Covid vaccines targets both the original strain of coronavirus and the BA.4 and BA.5 lineages, the subvariants of Omicron that are causing the most cases in the United States and which the FDA predicts will circulate in the coming months.

Earlier this summer the US health department announced it had purchased 105 million doses from Pfizer and 66 million from Moderna for use over the fall and winter.

The vaccines have yet to be recommended by the Centers for Disease Control and Prevention, the nation's health protection agency. But an independent panel of experts is scheduled to be convened by the CDC on Thursday to discuss the updates.

CDC director Rochelle Walensky will be in charge of giving the final green light.

The new versions of the vaccines could potentially be available in the United States as early as next week.

"Receiving a booster that specifically targets the Omicron BA.4/.5 variant, currently the most prevalent strain of SARS-CoV-2, is an important public health measure that people can take to help protect themselves, especially as we head into a season filled with indoor gatherings," Moderna chief executive Stephane Bancel said in a statement.

The vaccines currently in circulation target the initial strain of the virus that first appeared in Wuhan, China. But they have gradually proven to be less effective against the variants that have appeared over time, due to rapid evolution of the virus.

In contrast to the Alpha and Delta variants, which eventually waned, Omicron and its subvariants have come to dominate infections worldwide in 2022.

Pfizer and Moderna have also filed for approval of their updated vaccines with the European Medicines Agency.