In the final push to end polio, global health planners are embarking on an unthinkably ambitious and potentially risky move. They’re switching 155 countries—a good portion of the world—from one polio vaccine to another.

This will require moving millions of doses of a new vaccine into place over the course of two weeks in late April, while sequestering the remaining stocks of the old one.

And that’s only one of the many maneuvers necessary to truly end polio, which in the 1980s caused more than 350,000 cases of paralysis a year. So far in 2016, there have been only nine cases in two countries: Afghanistan and Pakistan.

It looks like the goal is in sight. But polio has slipped from control before.

“This is the largest, the