With capacity stretched and billions of doses required, COVID-19 vaccine makers cheer each other on

Pfizer's site in Kalamazoo, Michigan, will play a role in the company's COVID-19 vaccine rollout if the program succeeds. (Pfizer)

Only a handful of COVID-19 vaccines are in early stages of testing in humans, and most projects have yet to reach the clinic. But that’s not stopping some of biopharma's top minds from thinking ahead to eventual mass vaccination campaigns—and the enormous production capacity they'll require.

The CEO of Moderna Therapeutics, which has made routine headlines with its mRNA program, is actually rooting for some of the company's rivals to succeed. Speaking at a CNBC virtual conference, Stephane Bancel said he hopes that “three, four, five vaccines” against COVID-19 succeed, “because no manufacturer can make enough doses for the planet.” 

Moderna has inked a 10-year supply partnership with Lonza, and the partners aim to make up to 1 billion vaccine doses per year. Still, with a global population of more than 7.6 billion, there likely won’t be enough vaccine doses to reach herd immunity for “several years,” the Washington Post reported. That’s why experts are hoping multiple vaccines show they can work.

Execs at Johnson & Johnson, Pfizer and Sanofi each have their own ideas about allocating an eventual vaccine supply.

Johnson & Johnson, which has inked deals with Emergent BioSolutions and Catalent for supply, plans to distribute its potential vaccine where it would provide the most benefit, R&D chief Paul Stoffels told the newspaper. 

“We think honestly … that the priority should go to the people who need it most," he said, including healthcare workers and those at a high risk.

Meanwhile, as Sanofi advances its program in partnership with GlaxoSmithKline—and with early funding from BARDA—its CEO is pledging to allocate its shot to the U.S. first in recognition of that financial support, Bloomberg reports.

AstraZeneca chief Pascal Soriot says something similar about the University of Oxford vaccine his company has agreed to manufacture. Those doses will go first to the U.K., the news service says.

By contrast, Pfizer vaccine R&D head Kathrin Jansen told the Post that groups such as the World Health Organization should determine where initial vaccine doses are shipped. She added that “by the time we will face the issue, I’m very confident there will be plans in place, to make sure that there’s an equitable rollout.” 

Regeneron CEO Len Schleifer has voiced some of his own concerns about COVID-19 vaccine manufacturing capacity. Distribution channels to deliver medicines and vaccines are adequate, he recently said on CNN, but manufacturing capacity is “limited.”

RELATED: Sanofi, GSK tie up for COVID-19 vaccine work with eyes on possible 2021 rollout 

The supply concerns come as numerous vaccine programs race ahead, with at least eight in human testing, NIAID director Anthony Fauci told a Senate hearing Tuesday. He's "cautiously optimistic" one or more will succeed.

In recent days and weeks, companies in the COVID-19 vaccine race have continuously updated the public about their manufacturing plans and inked collaborations to help with mass production—even ahead of seeing clinical data. Pfizer has tagged three U.S. sites—plus one in Belgium—for its rollout. The company aims to deliver millions of doses by the end of 2020 and hundreds of millions next year. 

Through its two manufacturing partnerships—with Emergent BioSolutions and with Catalent—J&J is setting up for a big push; Stoffels recently told ABC the company plans to supply 1 billion doses next year. 

And part of the reason vaccine giants Sanofi and GSK decided to partner in COVID-19 was to combine their global scale, execs said. The companies believe they can deliver hundreds of millions of doses annually starting in 2021.  

Novavax, which has been involved in other emerging disease vaccine research but hasn’t brought any products to market, says it’s aiming to produce 100 million doses by the end of the year and more than 1 billion in 2021. The company recently scored up to $384 million from the Coalition for Epidemic Preparedness Innovations, the group’s largest grant to date. 

RELATED: With 'multiple shots on goal,' NIAID Director Fauci is 'cautiously optimistic' about COVID-19 vaccine success 

Last month, billionaire philanthropist Bill Gates said his foundation aims to invest billions in COVID-19 vaccine factories ahead of seeing data. That way, he said on the Daily Show with Trevor Noah, experts will be able to launch much faster than on traditional timelines.