U.S. drugmaker Pfizer Inc and its German partner BioNTech SE have started an early-stage study to evaluate a combination vaccine targeting COVID-19 and influenza, the companies said on Thursday.

The single-dose vaccine candidate is a combination of Pfizer's mRNA-based flu shot and the companies' omicron-tailored COVID-19 booster shot.

The early-stage study, which is being sponsored by BioNTech, aims to evaluate the combination shot's safety, tolerability and immunogenicity, or the ability to generate immune response.

The trial is being conducted in the United States and the companies aim to enroll 180 volunteers between the ages of 18 and 64. The first participant in the study was dosed earlier this week.

Rivals Moderna Inc and Novavax Inc are also developing combination vaccines targeting both COVID-19 and influenza.