Health regulators in the United States have delayed the review of Pfizer’s coronavirus vaccine for children under the age of five due to the fact that the two-dose regimen did not perform well against the omicron variant of the illness, according to a report from the Wall Street Journal on Friday.

Just last week, the Food and Drug Administration stated that it needs more information on the shots, delaying its decision for using the vaccine in kids that are aged 6 months through 4-years-old for 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,” Newsmax said.

Here’s more from their report:

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.

Back in December the companies stated that the early results of the trial for the two doses of the vaccine fell short of their expectations and the clinical trial was then amended for a three-dose version, which makes this submission quite surprising.