The study is one of the first late-stage trials that combine tests for boosters and variants as the companies adapt their strategies to deal with an evolving coronavirus.
Earlier this month, the companies reported positive interim results from the Phase 2 study showing "strong immune responses" in early tests of their COVID-19 vaccine.
"In a two-stage approach, the study will initially investigate the efficacy of a vaccine formulation targeting the original virus, while a second stage will evaluate a second formulation targeting the B.1.351 virus," Sanofi said.
The virus lineage known as B.1.351 was first detected in South Africa.
"Recent scientific evidence shows that antibodies created against the B.1.351 variant may provide broad cross-protection against other more transmissible variants," Sanofi said.
Sanofi also confirmed it would begin clinical studies in the coming weeks to assess the vaccine's efficacy as a booster, regardless of what vaccine a subject may have received first.
Pending a positive Phase 3 outcome, the vaccine could be approved in the fourth quarter of 2021.
Sanofi and GSK were forced to restart their trial in December when the vaccine showed a low immune response in older adults due to a weak antigen formulation.
Price Action: SNY shares are down 0.37% at $53.71, and GSK stock is down 0.57% at $38.49 during the premarket session on the last check Thursday.
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