The U.S. Food and Drug Administration on Friday recommended that Covid vaccine manufacturers make single-strain shots for the fall that target omicron subvariant XBB.1.5, the dominant strain of the virus nationwide.

“Based on the totality of the evidence, FDA has advised manufacturers who will be updating their COVID-19 vaccines, that they should develop vaccines with a monovalent XBB 1.5 composition,” the agency said in a release, referring to pharmaceutical companies Pfizer, Moderna and Novavax.

Monovalent means a shot is designed to protect against one variant of Covid.

XBB.1.5 is a descendant of the omicron variant, which caused cases in the U.S. to spike to record levels early last year. It is also one of the most immune-evasive strains to date.

XBB.1.5 accounted for nearly 40% of all Covid cases in the U.S. in early June, according to data from the Centers for Disease Control and Prevention. That proportion is slowly declining, while cases of the related variants XBB.1.16 and XBB.2.3 are rising.

The FDA’s decision is consistent with what an advisory panel to the agency recommended on Thursday.

That panel unanimously voted that new jabs should be monovalent and target a member of the XBB family. Advisors also generally agreed that targeting XBB.1.5 would be the most ideal option.

The FDA’s selection is also good news for Pfizer, Moderna and Novavax.

The three companies have already been developing updated versions of their shots that target XBB.1.5. Preliminary data each company presented on Thursday suggests that those jabs produce strong immune responses against all XBB variants.

“Novavax is encouraged by today’s FDA announcement, and the company’s XBB 1.5 COVID vaccine candidate is being manufactured at commercial scale with the intent to be in market for the fall vaccination campaign,” a Novavax spokesperson said in a statement to CNBC.

Pfizer said it will be able to deliver a monovalent shot targeting XBB.1.5 by July. Moderna and Novavax have not provided specific timelines for delivery.

Dr. Peter Marks, head of the FDA’s vaccine division, suggested on Thursday that the updated vaccines could be available to the public around September.