GlaxoSmithKline and Sanofi on Monday said they had started a new clinical trial of their protein-based Covid-19 vaccine candidate, reviving their efforts against the pandemic after a setback in December delayed the shot's launch.

The British and French drugmakers aim to reach final testing in the second quarter, and if the results are conclusive, hope to see the vaccine approved by the fourth quarter after having initially targeted the first half of this year.

In December, the two groups stunned investors when they said their vaccine would be delayed towards the end of 2021 after clinical trials showed an insufficient immune response in older people.

Disappointing results were probably caused by an inadequate concentration of the antigen used in the vaccine, Sanofi and GSK said, adding that Sanofi has also started work against new coronavirus variants to help plan their next steps.

Global coronavirus infections have exceeded 110 million as highly transmissible variants of the virus are prompting vaccine developers and governments to tweak their testing and immunisation strategies.

GSK and Sanofi's vaccine candidate uses the same recombinant protein-based technology as one of Sanofi's seasonal influenza vaccines. It will be coupled with an adjuvant, a substance that acts as a booster to the shot, made by GSK.

"Over the past few weeks, our teams have worked to refine the antigen formulation of our recombinant-protein vaccine," Thomas Triomphe, executive vice president and head of Sanofi Pasteur, said in a statement.

The new mid-stage trial will evaluate the safety, tolerability and immune response of the vaccine in 720 healthy adults across the United States, Honduras and Panama and test two injections given 21 days apart.

Sanofi and GSK have secured deals to supply their vaccine to the European Union, Britain, Canada and the United States. It also plans to provide shots to the World Health Organization's COVAX programme.

To appease critics after the delay, Sanofi said earlier this year it had agreed to fill and pack millions of doses of the Pfizer/BioNTech vaccine from July.

Sanofi is also working with Translate Bio on another COVID-19 vaccine candidate based on mRNA technology.