U.S. health regulators delayed their review of Pfizer Inc.’s Covid-19 vaccine in children under 5 years old because the initial two-dose series so far wasn’t working well against the Omicron variant during testing, people familiar with the decision said.
An early look at data showed the vaccine to be effective against the Delta variant during testing while that was the dominant strain, but some vaccinated children developed Covid-19 after Omicron emerged, the people said.