Interim results of the phase II trial showed 95% to 100% seroconversion following a second injection in all age groups (18 to 95 years old) and across all doses (5, 10, and 15 µg). The vaccine candidate was acceptably tolerated and didn't trigger any safety concerns. The neutralizing antibody levels were comparable with those generated by natural infection. However, higher neutralizing antibody levels were seen in younger adults (18 to 59 years old). Also, after a single injection, high neutralizing antibody levels were seen in study participants with evidence of prior SARS-CoV-2 infection.

The phase III study will be randomized and double-blind using a 10-µg dose in combination with GSK's pandemic adjuvant involving more than 35,000 adults from a variety of countries. The study will assess the efficacy of two vaccine formulations including the D614 (Wuhan) and B.1.351 (South African) variants.

Sanofi and GSK also intend to conduct booster studies with various variant formulations to assess the ability of a lower vaccine dose to generate a strong booster response regardless of the initial vaccine platform the study participant received.

If the phase III study goes well, the firms expect vaccine approval in the fourth quarter of this year.