Because other shots provide less protection, a consortium of advocates and academics on Wednesday said the world needs 22 billion shots of mRNA vaccines in 2022 to stem the pandemic — a threshold that is not achievable under current projections.

“We’re in for a rough road,” said C. Sola Olopade, a physician, dean of academic affairs and director of international programs at the University of Chicago Pritzker School of Medicine. “As long as there is this disparity and inequity in being able to vaccinate the whole world, we all are not going to be safe, because this virus is going to keep mutating and we won’t be able to predict how dangerous the next mutant is going to be.”

Vaccine makers, which developed shots in record time, have registered mixed results in efforts to meet the challenge.