In Phase 3 clinical trials, the companies said, a two-dose regime showed nearly 58% efficacy against symptomatic disease, 100% efficacy against severe disease and hospitalization and 75% efficacy against moderate to severe disease.

The two drugmakers said they are in discussion with both the US Food and Drug Administration and the European Medicines Agency (EMA) and plan to also send the health regulators data on using their shot as a booster. The delay has put so much distance between the French-British duo and mRNA vaccine makers that there may not be much demand for another primary vaccine series.

The Phase 3 study that evaluated a 10 µg antigen formulation also looked at the effect of giving a Sanofi-GSK booster vaccine to people who received other products for the first series. In adults aged 18 to 55, the companies said the booster increased neutralizing antibodies by nine-fold in recipients of Moderna’s vaccine and as much as 84-fold in those who received the Sanofi-GSK vaccine. The range of antibody increases in older adults spanned a range of 11- to 53-fold.

Across both the primary series and booster vaccinations, Sanofi and GSK said their vaccine was well-tolerated in younger and older adults with no safety concerns.

While efficacy figures for the initial series were below the more than 90% protection shown in earlier trials with the mRNA vaccines made by Pfizer/BioNTech and Moderna, experts pointed out that the results cannot be formally compared, as the mRNA trials were completed before the appearance of variants.

During the Phase 3 Sanofi-GSK study, the delta and omicron variants were dominant. While sequencing is still in progress, the partners said early data indicate 77% efficacy against any Delta variant-associated symptomatic Covid-19 disease, in line with expected vaccine effectiveness. Parallel to its own trials, Sanofi  toll-manufactured the BioNTech/Pfizer, Moderna and Johnson & Johnson vaccines.

Commenting on the trial results, Thomas Triomphe, executive vice president of Sanofi Vaccines, said efficacy of the adjuvanted vaccine as a primary series was” robust in today’s challenging epidemiological environment. No other global phase 3 efficacy study,” he said, “has been undertaken during this period with so many variants of concern, including omicron, and these efficacy data are similar to the recent clinical data from authorized vaccines.”

Full results of the study are planned to be published later this year.

Sanofi received funding for its Covid vaccine development from a number of agencies and programs steered by the US federal government, including the Biomedical Advanced Research and Development Authority (BARDA), which works together with the US Department of Health and Human Services in collaboration and the US Department of Defense.

Author: Dede Williams, Freelance Journalist