Pfizer Inc. and its German vaccine partner said their booster tailored to the latest omicron variants raised more antibodies against the dominant strains of COVID-19 when compared with the original shot designed to fight the form of the virus.

Blood from 80 volunteers collected seven days after the booster shot showed an increase in neutralizing antibodies against the BA.4 and BA.5 subvariants in a study, Pfizer and BioNTech SE said in a statement Thursday.

The vaccines were authorized without data showing their performance in humans. Pfizer and BioNTech plan to release additional data in coming weeks measuring immune responses one month following administration of the new bivalent booster. They have not shared data on the shot’s efficacy, which would offer a better measure of protection against widely circulating variants.

“While we expect more mature immune-response data from the clinical trial of our omicron BA.4/BA.5-adapted bivalent vaccine in the coming weeks, we are pleased to see encouraging responses just one week after vaccination in younger and older adults,” Pfizer Chief Executive Albert Bourla said in the statement. “These early data suggest that our bivalent vaccine is anticipated to provide better protection against currently circulating variants than the original vaccine, and potentially help to curb future surges in cases this winter.”

The vaccine partners studied two cohorts that had been given the new vaccine: one included those ages 18 to 55, and the other including those 55 and up. The older cohort that demonstrated a weaker antibody response against BA.4/BA.5 compared to the younger cohort, the companies said.

The U.S. fall booster campaign has thus far faltered. Only 11.5 million Americans have been administered a newly modified vaccine from Pfizer-BioNTech or Moderna Inc., a fraction compared to previous booster campaigns. On Wednesday, U.S. regulators expanded access to the new bivalent booster shots to include children ages 5 and up.

Pfizer shares fell 0.4% at the New York market open, while BioNTech’s American depositary receipts fell 2.5%.

The World Health Organization is tracking more than 300 sub-lineages of omicron at the moment. The BA.5 subvariant that swept across the globe this summer still dominates, accounting for some four-fifths of the samples that are genetically sequenced and identified.