US drugmaker Johnson & Johnson aims to begin clinical testing of its COVID-19 vaccine in the coming weeks and produce billions of doses in 2021, chief executive officer Alex Gorsky said, but cautioned that it will take more than one vaccine to rein in the virus.

Gorsky said it was a good thing other companies are experimenting with different approaches to vaccines, in part because different vaccines might work better in different groups of people.

“None of us should be thinking that this is the single solution that is going to take us back to the old normal,” Gorsky told Fortune’s Brainstorm Health conference. “I think it’s going to take multiple vaccines.”

He also stressed the importance of developing drugs that treat patients who have already contracted the virus.

Even if several vaccines are approved next year, the industry will still need to develop distribution systems, train medical personnel, and research ideal dosage levels for different patients, he said, adding the global scale of the pandemic poses additional challenges.

“The world has never attempted something quite that large, quite that complex,” Gorsky said. “That’s why I think a vaccine, while a very critical element to bringing an end to this pandemic, is part of the puzzle.”

He added that Johnson & Johnson has not yet made a decision about whether it will conduct challenge trials, in which volunteers are purposefully exposed to the virus, but is “l(fā)ooking at all options.”

More than 100 vaccines are being developed and tested around the world to stop the COVID-19 pandemic, which has infected more than 11 million people worldwide and led to more than half a million deaths.