Pfizer and its German partner BioNTech said their COVID-19 vaccine booster, adapted for the BA.4 and the BA.5 sub-variants of Omicron, generated a strong immune response and was well-tolerated in testing on humans.

With the results, which the companies described as consistent with pre-clinical data, the partners are following up with human trial data that had previously been missing from their successful filings for regulatory approval.

In a joint statement on Thursday, the companies said data from roughly 80 adult patients showed the booster dose led to a substantial increase in neutralizing antibody levels against the BA.4 and BA.5 variants after one week.

They did not disclose the levels of antibodies generated in the preliminary analysis of the study.

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

Healthcare regulators for the European Union and the United States had approved the upgraded shot last month, even though trial data from testing on humans had at the time not been available.

The preliminary data on Thursday also showed that, in the age group of adults older than 55, the new bivalent shot triggered a better neutralizing antibody response against the Omicron BA.4/5 sub-variant than the established shot based on the initial form of the virus.

(With input from Reuters)