LONDON (AP) — Drugmakers GlaxoSmithKline and Sanofi say their potential COVID-19 vaccine won’t be ready until late next year as they seek to improve the shot’s effectiveness in older people.

The companies said Friday that early-stage trials showed the vaccine produced an “insufficient” immune response in older adults, demonstrating the need to refine the product so it protects people of all ages. They said they now expect the vaccine to be available in the fourth quarter of 2021.

GSK and Sanofi, based in London and Paris, respectively, said they were confident of the vaccine’s ultimate success due to positive results from other tests.

In adults 18 to 49, the vaccine produced an immune response comparable to patients who had recovered from COVID-19, the companies said. In addition, they reported positive results from a “challenge study” in which non-human primates were intentionally exposed to the virus.

“We have identified the path forward and remain confident and committed to bringing a safe and efficacious COVID-19 vaccine,” Thomas Triomphe, head of Sanofi’s vaccine unit, said in a statement. “Following these results and the latest encouraging new preclinical data, we will now work to further optimize our candidate to achieve this goal.''