This much is clear: Some people in the United States with medical conditions that have compromised their immune systems are now eligible for a third dose of Covid-19 vaccine.

But the new policy from U.S. health officials, announced late last week, came together in a hurry. It addresses some questions but leaves a number unanswered, in some cases because there are no data on which to base a decision.

Some experts believe the new policy may also have created a loophole big enough to allow easier access to a third dose for non-immunocompromised people who are trying to find ways to get one.

“I worry very much that this is going to lead to vaccination chaos,” Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, told STAT.