Pfizer Inc.'s push for health regulators to authorize a third dose of its COVID-19 shot is not yet backed by evidence, despite the fast-spreading delta variant, vaccine experts said.

Pfizer and German partner BioNTech SE said last week that they will ask U.S. and European regulators within weeks to authorize a booster dose due to an increased risk of infection after six months.

The companies did not share the data showing that risk, but said it would soon be made public. A meeting with federal health officials to discuss the matter was scheduled for Monday, Pfizer said.