The trial, which aims at evaluating the safety, tolerability and immune response, will be conducted on 440 healthy adults across 11 investigational sites in the US.
Results are expected in December, with the pair looking to start the final phase of trials in the same month.
If data is enough to apply for approval, it will be requested in the first half of 2021.
The candidate uses the same recombinant protein-based technology as one of Sanofi's seasonal influenza vaccines with GSK's established pandemic adjuvant technology.
Meanwhile, both drugs giants are scaling up manufacturing to reach 1bn doses next year.
Pre-clinical data collected by Sanofi showed acceptable tolerability, with results to be published later this year.
Shares advanced 1% to 1,498.4p on Thursday at the opening bell.