Requiring three doses for a single course of vaccination isn’t unusual. “Most viral vaccines are three doses,” Jeffrey Klausner, a professor of medicine and public health at UCLA who previously worked at the CDC, told The Daily Beast. But if you’re going to stick with just two doses, you’d want to wait more than three weeks between shots.

A longer wait between jabs would give your immune system ample time to produce a “primary immune response” from the first shot before you get a second shot for an even stronger “secondary immune response.” With a strong secondary immune response, you might not need a third shot—a booster—until many months later, if ever.

The FDA’s front-loaded vaccine strategy, absent a mandatory third dose, isn’t ideal for long-term immunity. But there’s a reason the agency pushed it. Getting both doses of a two-shot vaccine just three weeks apart produces a rapid, short-term build-up in antibodies.