LONDON — 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 say 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.

GSK and Sanofi, based in London and Paris, respectively, say 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 say.