According to data from the CDC, 8% of people who have received a first dose of the Pfizer or Moderna vaccines have missed their second scheduled dose. That amounts to five million Americans, and infectious disease experts worry these individuals’ immunities may wane much more quickly.

According to a report in the New York Times, the reasons people are skipping second doses include issues with second dose appointments, people already feeling protected with just one shot, or if they felt sick after the first dose.

Doctor Emily Landon, an infectious disease specialist at the University of Chicago Medicine, said people are considered unvaccinated until they’ve had both shots. “The way that the vaccine is supposed to work is the first shot is sort of like your body being told ‘Here is a picture of a spike protein. This is what you’re looking for, and you need to take care of it if you see it.’ And often times, our bodies react the same way that we react, and they say, ‘Ok, got it, sounds good.’ But then the second shot is like a practice run. It sort of tricks your body and says, here’s that spike protein again, and your body jumps into action, and it does everything that it needs to do to fight off that spike protein. And that creates that big boost in antibodies that last longer,” she said.