Two of the world's largest pharmaceutical companies are teaming up in the hunt for a coronavirus vaccine.

The European drugmakers Sanofi and GlaxoSmithKline said on Tuesday that they would work together on a vaccine and aimed to start human testing later this year.

The companies estimated that if their trials are successful, the vaccine could be available in the second half of 2021.

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Two of the world's largest drugmakers are working together to develop a coronavirus vaccine.

Sanofi and GlaxoSmithKline said on Tuesday that they were aiming to start testing a potential vaccine in humans later this year to be ready for use in the second half of 2021. Financial terms of the deal weren't disclosed.

The two European pharmas each bring decades of experience in vaccine development. Their combined market value exceeds $200 billion, and they employ roughly 200,000 workers.

"As the world's leading vaccine manufacturer, our No. 1 focus is to help to develop a vaccine," GlaxoSmithKline CEO Emma Walmsley said on a Tuesday call with reporters. "This is, of course, core to the exit plan that the world needs."

Sanofi will use an antigen targeting the coronavirus' signature spike protein, and GSK will provide an adjuvant, which can reduce the amount of vaccine needed per dose, the companies said. The US Biomedical Advanced Research and Development Authority has partially funded Sanofi's vaccine research.

BARDA Director Rick Bright said in a statement that this collaboration "holds the potential to lower the vaccine dose to provide vaccine to a greater number of people to end this pandemic, and help the world become better prepared or even prevent future coronavirus outbreaks."

Read more: There are more than 40 potential coronavirus vaccines in the works. Here are the top efforts to watch, including the 8 vaccines set to be tested in people this year.

There are now 70 vaccine research efforts underway, according to the World Health Organization. Even with the resources of the two pharma giants, the world is likely to need multiple vaccines to meet the unprecedented global demand for a coronavirus vaccine, GSK executives said.

GlaxoSmithKline CEO Emma Walmsley. Thomson Reuters

"Both companies bring significant manufacturing capacity," Walmsley said. "While we have a lot of work to do, given that this is in an early stage of development, we believe that if we're successful, we'll be able to make hundreds of millions of doses annually by the end of next year."

Sanofi and GSK said that they were "committed to making any vaccine that is developed through the collaboration affordable to the public" and that they planned to "offer fair access for people in all countries." Walmsley added that this would likely include donations to some of the poorest countries.

A sharpening distinction between new- and old-school vaccine approaches

The deal also shows the contrast in approaches between some pharma giants using well-known vaccine technologies versus smaller biotechnology firms applying unproven, novel approaches against the coronavirus.

These biotechs have sped past Big Pharma into clinical testing. Moderna and Inovio Pharmaceuticals have both already started dosing healthy volunteers with their experimental vaccines that leverage new genetic technologies.

Read more: A top Wall Street analyst says there's a less than 20% chance of having a widely available coronavirus vaccine by next year. He told us why he's 'deeply skeptical' of Anthony Fauci's 18-month timeline.

Moderna is testing a messenger-RNA vaccine that instructs human cells to produce virus-fighting proteins called antibodies. Inovio developed a DNA vaccine to deliver optimized DNA into cells to spur an immune response.

Both approaches are based on decades of research but have yet to lead to any vaccines approved by the US Food and Drug Administration. Both platforms have the advantage of using only genetic code, instead of requiring samples of the live virus. That helps speed up much of the early-stage development work.

Sanofi CEO Paul Hudson. Gonzalo Fuentes/Reuters

Sanofi and GSK are using a well-established platform of a protein-based vaccine plus an adjuvant. While it will take longer to begin human testing, the companies argued that they could progress quickly, given the familiarity of this technology.

Sanofi is also working with a tiny Massachusetts biotech called Translate Bio to advance an mRNA coronavirus vaccine. And GSK is supplying its adjuvant technology to other vaccine programs from the University of Queensland, the Chinese biotech Clover Biopharmaceuticals, and Xiamen Innovax Biotech.

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This article has been updated with additional information about the collaboration.