Sanofi and GlaxoSmithKline (GSK) have reemerged as COVID-19 vaccine contenders after their reformulated candidate delivered a strong immune response across all age groups in a phase 2 study.

The adjuvanted vaccine suffered a serious setback in December when a phase 1/2 trial showed the candidate delivered only a weak response in older adults. But the companies said their refined antigen formulation induced a strong immune response in individuals age 18 to 95, with neutralizing antibodies comparable to those found in patients after recovery from SARS‑CoV‑2 infection.

The mid-stage study enrolled 722 volunteers age 18 years and older in the U.S. and Honduras to evaluate the two-dose regimen given 21 days apart.

Sanofi and GSK now plan to enroll more than 35,000 participants in a phase 3 study to evaluate the vaccine’s efficacy against the original SARS‑CoV‑2 virus and the variant first identified in South Africa. And if the results are positive, the companies expect to seek approval from regulators in the fourth quarter.