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The next COVID vaccines should exclude the original strain of SARS-CoV-2 and exclusively target the Omicron XBB subvariants, according to FDA staff members.

They unveiled their recommendation in a ahead of Thursday's meeting of the FDA's , stating that "the totality of available evidence suggests that a monovalent XBB-lineage vaccine is warranted for the 2023-2024 vaccination campaign."

XBB strains are more immune evasive than prior Omicron subvariants and currently account for about 95% of COVID-19 cases worldwide, the agency staffers noted.

In the U.S., eight XBB strains account for over 98% of the current cases, according to tracker, including three sublineages under consideration for the fall vaccine: XBB.1.5 in 40%, and the newer XBB.1.16 in 18% and XBB.2.3 in 6% of cases.

Spike proteins of these three sublineages "are similar with few amino acid differences ... and available studies suggest little to no further immune evasion from these new substitutions in the XBB.1.16 spike compared to XBB.1.5," noted FDA staff.

According to the briefing document, "the current trajectory of virus evolution suggests that XBB.1.16 could be dominant by fall 2023 but that XBB.2.3 and other XBB sublineages could also continue to increase in proportion as the virus evolves."

The new plan follows the agency's strategy laid out last year, to annually update the COVID-19 vaccine strain ahead of the fall, based on the circulating and emerging variants at the time.

While mRNA vaccine makers Moderna and Pfizer last year initially began developing monovalent and bivalent Omicron BA.1-targeted vaccines, VRBPAC last June ultimately recommended the companies pivot to create bivalent vaccines that targeted the original SARS-CoV-2 strain along with BA.4/BA.5.

"Although real-world effectiveness studies suggest that the current bivalent vaccines continue to provide protection against other circulating sublineages of Omicron, including the XBB-XBB.1.5, there appears to be an inverse relationship between the time since vaccination and vaccine effectiveness," wrote agency staff.

Recent vaccine effectiveness (VE) estimates in adults suggested that with the currently available bivalent shots dropped from 62% in the 2 months post-vaccination to 24% at 4 months and beyond. In light of the declines, the agency recently recommended extra doses for high-risk groups.

A significant change for the next vaccine composition is the recommendation to drop the original SARS-CoV-2 from the upcoming fall booster. This move stemmed from a meeting of the International Coalition of Medicines Regulatory Authorities, according to the briefing document, where the FDA met with other regulators across the world to discuss the global alignment and preferred strains for subsequent vaccines.

"Based on the global dominance of the XBB descendent lineages, in conjunction with the high level of baseline immunity to the ancestral virus strain, regulators concluded that a monovalent XBB vaccine would be an adequate candidate for a COVID-19 vaccine composition update and would likely increase the chances of matching more closely virus variants in the immediate future," FDA staff wrote.

At VRBPAC, the agency's advisors will vote on whether to recommend an XBB-lineage monovalent vaccine for the 2023-2024 campaign, and they will also be asked to discuss the available data on COVID variants along with the following:

• VE and clinical immunogenicity data of the current bivalent products against recent SARS-CoV-2 strains
• Antigenic characterization of circulating variants
• Preclinical immunogenicity data on new vaccine candidates incorporating updated spike components

While the FDA is not bound to follow the recommendations of its advisory committees, it typically does.

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