Drugmaker Moderna asked the Food and Drug Administration on March 17 to authorise 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.

The White House has been sounding the alarm that it needs Congress to urgently approve more funding for the federal government to secure more doses of the COVID-19 vaccines, either for additional booster shots or variant-specific immunisations.