The most proven Covid-19 vaccines require two doses. Research shows patients are really bad at following up. | Dan Charity/AFP via Getty Images

The Covid-19 vaccine requires two doses. Studies show as many as 50 percent of people could miss the second one.

When we talk about the Covid-19 vaccines, we aren’t necessarily talking about people getting one shot, then being done and good to go. Both the Pfizer and Moderna vaccines — which are the closest to approval — have two-dose regimens. You get one shot, and then a few weeks later, you’re supposed to get another one.

Among all the logistical challenges in distributing and administering a Covid-19 vaccine, this looks like one of the biggest.

That’s because, based on research that evaluated compliance with other multi-dose vaccines, patients are really, really bad at getting their second dose. Bad