The companies said early 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. London-based GSK and Paris-based Sanofi, now expect the vaccine to be available in the fourth quarter of 2021.

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

While researchers around the world are racing to develop COVID-19 vaccines, the setback announced by Sanofi and GSK shows the challenges scientists face as they try to condense 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, which has killed more than 1.5 million people worldwide, because of the challenges in rapidly producing and distributing enough doses to vaccinate billions of people.

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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.

GSK and Sanofi said they were confident of their 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.

“Following these results and the latest encouraging new preclinical data, we will now work to further optimize our candidate to achieve this goal,” said Thomas Triomphe, head of Sanofi’s vaccine unit. “No single pharma company can make it alone. The world needs more than one vaccine to fight the pandemic.”

Researchers at the University of Queensland and Australian biopharmaceutical company CSL said Friday that they would not move forward with their own vaccine candidate because a protein in the shot triggered false positive results for HIV. Follow up tests confirmed that no HIV was present, the researchers said.

The vaccine had proven safe and produced a “robust response” to COVID-19 during Phase One trials, researchers said in a statement. While it would have been possible to re-engineer the vaccine to avoid the problem with false positives, this would have taken too much time in the midst of the pandemic, they said.

“Doing so would set back development by another 12 or so months, and while this is a tough decision to take, the urgent need for a vaccine has to be everyone’s priority,” said Professor Paul Young, co-leader of the project.

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