Pfizer Inc and its German partner BioNTech said on Thursday that their omicron-adapted COVID-19 vaccine booster generated a strong immune response against the currently circulating BA.5 and the BA.4 subvariants of omicron.

The companies said data from a trial in adult patients showed that the booster dose led to a substantial increase in neutralizing antibody levels against the BA.4/BA.5 variants.

Omicron-tailored shots made by Pfizer and Moderna Inc have already been greenlit by several countries, including in the United States for adults and, more recently, for children as young as 5 years.

But, that U.S. authorization was based on the safety and effectiveness of the original vaccine, and not the BA.4/BA.5-tailored shots.

The preliminary data on Thursday also suggested that the bivalent vaccine was likely to provide better protection against the omicron subvariants than the original vaccine.