I cried at my desk this week. I could probably start every story I’ve written since February with that sentence but this was different. They were tears of joy and hope as I watched the first people in the world receive the newly approved COVID-19 vaccines.

Now, those patients have an important task ahead of them: returning in a few weeks for a second dose. Two the leading vaccine candidates — Pfizer’s and Moderna’s — require two doses to be effective, a daunting problem when you’re trying to vaccinate entire nations. How do you make sure everyone is coming back for their second shot? Our ability to stop the spread of COVID-19 depends on the answer. If people only get one dose, they will not be fully protected, but may behave as if they are. And even worse, there’s a fear that if enough people get only the first injection, the virus could develop resistance to the vaccine.

But this isn’t our first time trying to get people to take multiple doses of a new vaccine: take the vaccine for human papillomavirus (HPV), for example. The way officials ensured most people made it to that all-important follow-up appointment for that vaccine may hold important lessons for our current efforts.

Read the full article about COVID-19 vaccine by Kaleigh Rogers at FiveThirtyEight.