A two-dose COVID-19 vaccine series doesn’t generate sufficient neutralizing antibodies against the Omicron strain, said Oxford University researchers.

The scientists assessed blood samples from people vaccinated with either the AstraZeneca or Pfizer/BioNTech vaccines and then exposed the samples to the Omicron variant. They saw a “substantial decrease” in the level of neutralizing antibodies against the variant.

The findings are in line with newly published data from the UK Health Security Agency, which showed reduced vaccine effectiveness against Omicron vs. the Delta variant. However, the health agency found that a third, booster dose helped increase protection.