U.S. health regulators delayed the review of Pfizer Inc's COVID-19 vaccine for children under 5 years of age because its two-dose regimen did not work well against the omicron variant, the Wall Street Journal reported on Friday.

Last week, the U.S. Food and Drug Administration (FDA) said it needed more data on the vaccine, delaying its decision for using the vaccine in children 6 months through 4 years of age for at least two months.

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 report said, citing people familiar with the FDA's decision.

However, since the overall COVID-19 cases were low, the small number of omicron cases made the vaccine appear less effective in an early statistical analysis, the report added.

The FDA did not respond to a Reuters request for comment.

Pfizer and BioNTech had submitted data on the first two doses of a planned three-dose regimen earlier this month at the request of the FDA. Pfizer did not disclose efficacy data.

The submission was surprising because in December, the companies said early trial results of the two low doses of the vaccine fell short of expectations, and the clinical trial was amended to test a three-dose version.