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

The companies said early trials showed the vaccine produced an “insufficient” immune response in people over 60 because it didn’t contain enough of the material that triggers the production of disease-fighting antibodies. They said they plan to reformulate the vaccine and do more testing, which is likely to delay approval to the fourth quarter of 2021 from the middle of the year.

“The results of the study are not as we hoped,’’ Roger Connor, president of London-based GSK Vaccines, said in a statement.

The delay is a setback for plans to vaccinate large numbers of people around the world amid a pandemic that has already taken more than 1.5 million lives. GSK and Sanofi plan to produce up to 1 billion doses of their COVID-19 vaccine annually, and they have signed agreements to supply millions of doses to the U.S., European Union, Canada and developing countries.

The difficulties underscore the challenges scientists face as they race to develop COVID-19 vaccines, condensing a process that usually takes years into a matter of months. Researchers in Australia said Friday they were abandoning their own vaccine candidate because it produced false positive results to HIV tests.

Public health experts say several vaccines will be needed to end the pandemic, because of the challenges in rapidly producing and distributing enough doses to vaccinate billions of people.

Dr. Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, said the setback is even more significant because Sanofi is a leading producer of flu vaccines and has large-scale manufacturing capacity.

The delay “will impact their ability to (make) the millions of doses that have been pre-ordered because they have to sort out this dose-response effect question,” she said. “Very unfortunate, I would say.’’

A vaccine produced by U.S. drugmaker Pfizer and German partner BioNTech has been authorized for widespread use in the U.K. and a handful of other countries. A U.S. government advisory panel on Thursday endorsed use of the Pfizer vaccine, putting the country one step away from launching its own mass vaccination program.

Sanofi and GSK confirmed that volunteers in their study received less of the vaccine than intended because of incorrect readings produced by two of the commercial compounds used to test the amount of active ingredient in each dose. The lower dose was enough to produce a good immune response in younger people, but the response was much weaker in those over 60, they said.