US regulators’ decision on whether to approve Moderna Inc.’s vaccine for RSV has been delayed due to “administrative constraints” at the Food and Drug Administration, as the company works to get its second product to market.

The FDA hasn’t informed Moderna of any issues related to vaccine safety, efficacy or quality that would prevent approval, Moderna said Friday in a statement. The agency is now working to conclude its review of the vaccine by the end of May, the company said. A decision had been expected by May 12.

Moderna’s sales have fallen drastically as the market fades for Covid shots. The company has looked to the RSV vaccine as an entry into another promising area. RSV shots from GSK Plc and Pfizer Inc. that gained US clearance last year generated about $2.4 billion in combined sales, with GSK leading the market.

The FDA didn’t ask for any major changes to Moderna’s license application or issue a continuing response letter, which would have indicated that it wouldn’t be approved in its current form, said Michael Yee, a Jefferies analyst.

“We don’t expect major delays from here onwards,” he said in a note. Wall Street’s fears of a “worst case scenario” should now be lifted, he said.

If Moderna’s shot were approved by the end of May, the company would still be able to participate in a meeting of government vaccine advisers scheduled for late June that would be required for a recommendation for use, Myles Minter, an analyst with William Blair, said in note.

Moderna’s shares fell as much as 3.8% at the US market open.