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

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

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

But they felt sufficiently encouraged from human study results on a similar shot targeting the BA.1 subvariant and on the established vaccine based on the original virus detected in China in late 2019. “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,” said Pfizer Chief Executive Albert Bourla.

While the European Medicines Agency has also cleared shots that address the BA.1 subvariant, the U.S. Food and Drug Administration has focused its response for immunity against the fast spreading Omicron variant only on BA.4/5-adapted shots.

Both are known as bivalent in that they target both the original virus as well as Omicron subvariants.

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 subvariant than the established shot based on the initial form of the virus..