Under any other circumstances, the speed at which Sanofi and GSK have moved — bringing a vaccine to human testing less than five months after joining forces in an unusual alliance — would be unprecedented.

But so far, the two companies, each one of the world’s top vaccine makers, have been overshadowed by companies using newer technologies that have been advanced more quickly.

In the U.S., Moderna as well as a partnership of BioNTech and Pfizer, each utilizing similar, unproven platforms, have paced those efforts. Both could be in position to produce initial data from Phase 3 trials within months, and seek initial clearances before the end of the year. Vaccines in China and Russia, meanwhile, have been approved for use before testing proved they work.

The first vaccines to arrive, however, may be in a difficult position — particularly in the U.S. Experts have expressed concern about vaccine hesitancy among the general public, a sentiment that has heightened as the process has become more politicized. The White House, for instance, has reportedly made overtures about fast-tracking a vaccine before the presidential election, and Food and Drug Administration commissioner Stephen Hahn has said he’d clear a vaccine before Phase 3 trials are complete.

Reports emerged Monday that government officials are preparing states to be ready to distribute vaccines by Nov. 1. It’s unclear whether data will be sufficient to warrant even an emergency authorization — a lower bar than a standard drug approval — by then. Long-term safety and efficacy results will take more time to accrue.

The challenges will be amplified by the fact the lead vaccine developers in the U.S. are testing first-of-their-kind products. Moderna’s vaccine, for instance, uses messenger RNA molecules to deliver genetic instructions, a method that has never produced an approved medicine. It’s unclear how effectively an mRNA vaccine will prevent disease compared to traditional vaccine approaches.

All of which leaves the second wave of vaccine developers, among them Sanofi and GSK, with a real shot to make a significant difference in curbing the pandemic. Their vaccine pairs an approach used in Sanofi’s flu vaccine FluBok with an immune-boosting adjuvant from GSK. Sanofi has spoken confidently of the approach, believing it will lead to superior results.

“We’re the only vaccine in the race that’s off a proven platform that works at scale,” CEO Paul Hudson has said. “We’re a little bit slower, but we [have] a much higher likelihood of success.”

The two have yet to publish detailed results from animal tests backing up those claims; they plan to do so later this year. In a statement, Sanofi and GSK said those studies showed two injections of the vaccine produced levels of neutralizing antibodies “comparable” to those seen in humans who have recovered from infection, a bar developers are using to predict their shots will be effective. As with their rivals, Sanofi and GSK are scaling up manufacturing before knowing whether their vaccine works, planning to produce as many as 1 billion doses in 2021.

The U.S. government has secured 100 million doses of the shot, with an option to buy up to 500 million more. The two companies have also agreed to supply up to 60 million doses to the U.K., and a “significant portion” of their worldwide supply in 2021 and 2022 to CoVax, a collaboration between various governments and health organizations to equitably distribute vaccines and COVID-19 treatments across the globe.

Sanofi, meanwhile, has a second coronavirus vaccine in development through a partnership with biotech Translate Bio. The first human test of that vaccine is expected in the fourth quarter.