(Bloomberg) -- Two doses of the Pfizer Inc. and AstraZeneca Plc. vaccines induced lower levels of antibodies against the omicron variant, increasing the risk of Covid infection, according to researchers from the University of Oxford.

Blood samples collected from people vaccinated with a combination of the two different shots and tested against the new strain showed a substantial drop in neutralizing antibodies, a proxy for protection, the scientists said Monday in a paper.

The results echo other findings released by the U.K. government last week emphasizing the need for booster shots, especially amid evidence of omicron’s ability to drive a tidal wave of infections. The country’s Health Security Agency said Friday the basic course of shots from Astra and Pfizer provided much lower defenses against symptomatic infection with omicron compared with the delta strain, while a booster lifted protection to 70% to 75%.

“These data will help those developing vaccines, and vaccination strategies,” Gavin Screaton, head of Oxford’s medical-sciences division and lead author of the paper, said in a statement. Despite a lack of evidence about omicron’s ability to cause severe disease, Screaton called for remaining “cautious, as greater case numbers will still place a considerable burden on health-care systems.”

The blood samples were taken from the Oxford-led Com-Cov2 study looking at how mixing and matching vaccines with different intervals impacts the immune response to Covid-19. The findings were published on the pre-print server medRxiv. 

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