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.

So few study subjects, whether vaccinated or unvaccinated, developed Covid-19 during testing thus far that the small number of Omicron cases made the vaccine appear less effective in an early statistical analysis, the people said.

As more cases emerge, Pfizer’s shot might wind up providing stronger protection against Omicron, the people said, if the bulk of infections are in unvaccinated subjects.... Read More: WSJ