The U.S. Food and Drug Administration (FDA) said Thursday that vaccine manufacturers should update their COVID-19 booster shots to target emerging subvariants of the Omicron strain ahead of the fall vaccination campaigns.

Dr. Peter Marks, head of the agency's vaccine division, said that manufacturers should update their vaccine formulas to target Omicron strains BA.4 and BA.5--which are rapidly spreading across the world. According to the U.S. Centers for Disease Control and Prevention (CDC), Omicron BA.4 and BA.5 now make up more than 50% of COVID infections in the United States.

"Currently available vaccines have helped reduce the most serious outcomes (hospitalization and death) caused by COVID-19, but results from post-authorization observational studies have shown that effectiveness of primary vaccination wanes over time against certain variants, including Omicron," said Marks in a statement. "While initial booster doses have helped restore protections against severe disease and hospitalization ... studies have also indicated waning effectiveness of first booster doses over time."

The FDA predicts that a next-gen booster targeting these two sub variants will provide more durable protection against infection heading into the colder months later this year--when COVID spreads more easily as people spend more time indoors.

Despite the FDA's recommendation, vaccine makers are falling behind the rapidly evolving virus. Pfizer (PFE  ) and its partner BioNTech (BNTX  ) and Moderna (MRNA  ) are currently testing updated booster shot candidates that target the Omicron strain, but early data shows they did not provide as strong of an immune response against the subvariants.

In order to keep up with the virus, the FDA will not require vaccine manufacturers to conduct new clinical trials for shots targeting BA.4 and BA.5, a FDA official told Reuterson Thursday. Instead, the agency will use clinical trial data of variant-specific boosters, manufacturing data, and animal studies to evaluate the shots, Reuters reports.

Marks said the FDA will review new vaccine candidates for safety, efficacy and manufacturing quality to ensure they meet the agency's highest standards before they are authorized for public use.

All currently authorized vaccines in the U.S. solely target the original SARS-CoV-2 virus first discovered in Wuhan, China, in 2019. While these vaccines no longer provide strong protection against infection, they still prevent most severe disease. For this reason, the FDA is not recommending an update to the current two-dose primary series for most vaccines.