London — Two of the world’s biggest vaccine makers are joining forces to tackle the coronavirus as the number of confirmed infections approaches 2-million worldwide.

Sanofi will test its experimental coronavirus vaccine with GlaxoSmithKline (GSK) supplying adjuvants, which are additional ingredients that improve efficacy and make it easier to produce shots in larger quantities, the companies said on Tuesday. The drugmakers said they plan to start human trials in the second half of this year, with the goal of having a vaccine available by the second half of 2021 if the studies are successful.

Vaccines are “core to the exit plan that the world needs”, said GSK CEO Emma Walmsley said on a call. “If we’re successful, we’ll be able to make hundreds of millions of doses annually by the end of next year.”

The collaboration brings together a pair of pharmaceutical giants with manufacturing might in the race to deliver a Covid-19 vaccine. Dozens of companies from Moderna to Johnson & Johnson, along with universities, are pursuing a vaccine to halt the rapidly spreading pathogen. Even if developers can meet a target of having a vaccine available in 12 to 18 months, it’s an open question as to whether they will be able to keep up with global demand.

“This is a global challenge of epic proportions,” Walmsley said in an interview with Bloomberg Television. “The world will need more than one vaccine.”

If GSL and Sanofi manage to come up with a vaccine, Glaxo may manufacture the adjuvant in sites in the UK, Europe and the US. GSK has also committed to re-invest any short-term profit from a vaccine into Covid-19 research and long-term pandemic preparedness, she said.

J&J said last month that it would begin a more-than $1bn effort with the US government to develop and produce a vaccine. The company said it plans to begin building manufacturing capacity for a billion doses and could have some ready for emergency use as soon as January.

Sanofi jumped into the fray in February, betting that earlier work in pursuit of a SARS vaccine could accelerate its effort. The Paris-based company joined with the US Biomedical Advanced Research and Development Authority (Barda), a government agency that funds research and development efforts for health threats.

In a collaboration with the Coalition for Epidemic Preparedness Innovations (Cepi), London-based GSK agreed the same month to share its know-how with other vaccine developers, starting with the University of Queensland in Australia. The British pharmaceutical giant now has seven vaccine partnerships for the coronavirus.

Adjuvants are added to some vaccines to enhance the immune response and create stronger protection. They may reduce the amount of vaccine protein required per dose, raising the likelihood of having a vaccine that can be produced in large quantities, Sanofi and GSK said.

They also pledged in the statement to make any vaccine that is developed affordable and accessible worldwide.