A healthcare worker prepares a Pfizer Covid-19 booster shot at a vaccination clinic at Union Station in Los Angeles, California, USA, 30 March 2022. EPA-EFE/CAROLINE BREHMAN 
The U.S. Food and Drug Administration had in late March authorized a second booster dose of the Moderna MRNA.O and Pfizer PFE.N/BioNTech SE 22UAy.DE vaccines for people aged 50 and older, citing data showing waning immunity and the risks posed by Omicron variants of the virus.
"With regard to a fourth dose for those under the age of 50, that is going to require action from the FDA, and we're in conversations there," U.S. Centers for Disease Control and Prevention Director Rochelle Walenksy said on Wednesday.
Cases have steadily risen over the past five weeks, Walensky said at a White House briefing, with the the current seven-day average of daily cases rising 26% from the previous week to 94,000 cases per day and up threefold over the last month. The seven-day average for hospitalizations was up 19% to about 3,000 per day and the average for deaths was 275 per day, she said.
"We of course must remember that each person lost to Covid-19 is a tragedy and that nearly 300 deaths a day is still far too many," said Walensky.
By Ahmed Aboulenein and Jeff Mason
(Reporting by Ahmed Aboulenein and Jeff Mason; Additional reporting by Susan Heavey; Editing by Alison Williams)