It comes down to global supply. Already, by one CDC account reported by ABC News, a million people in the United States have successfully obtained a third dose of a two-dose vaccine. Now there’s some risk that vaccinated Americans with healthy immune systems will make a new run on the country’s pharmacies and clinics.

The FDA’s booster authorization “should not become a slippery slope where boosters start getting administered to people who are not at inordinate risk,” Irwin Redlener, the founding director of Columbia University’s National Center for Disaster Preparedness, told The Daily Beast…

The U.S. is flush with doses, but most countries aren’t. And that’s a good reason for Americans with healthy immune systems not to get an unnecessary third shot and potentially divert a limited supply of doses. If demand for jabs spikes in the United States because millions of people are getting boosters they don’t need, it could slow shipments to countries that are still trying to give people their first dose. After all, the contracts the U.S. government signed with the major vax-producers put the United States at the head of the line for supply.