The US FDA advised Pfizer, Moderna, and Novavax in June 2023 to develop vaccines targeting the omicron subvariant XBB.1.5, which allegedly has been the most immune-evasive Covid variant so far. That particular strain was reportedly responsible for nearly 40% of all Covid cases in the US in early June, according to the CDC. While the CDC suggests that its prevalence is slowly declining, the FDA still deemed it necessary to create a specific vaccine for this variant.
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