Drugmaker Moderna asked the Food and Drug Administration on March 17 to authorize a fourth shot of its Covid-19 vaccine as a booster dose for all adults. The request is broader than rival pharmaceutical company Pfizer's request earlier this week for the regulator to approve a booster shot for all seniors.

In a press release, the company said its request for approval for all adults was made 'to provide flexibility' to the Centers for Disease Control and Prevention and medical providers to determine the 'appropriate use' of a second booster dose of the mRNA vaccine, 'including for those at higher risk of Covid-19 due to age or comorbidities." U.S. officials have been laying the groundwork to deliver additional booster doses to shore up the vaccines' protection against serious disease and death from Covid-19.