Double-Dose Covid Shots Face History of People Skipping Last Jab

The new Covid-19 vaccines’ success hinges on getting people to come back for a second shot after the first dose. That’s been a challenge in the past, and scientists fear skipped inoculations will drag out the pandemic.

Historically, a significant portion of people fail to follow through on vaccine schedules that require multiple doses; sometimes fewer than half of participants returned for the second shot. The leading Covid-19 vaccine candidates from Pfizer and Moderna both require two shots. If enough people skip the second injection, that risks prolonging the pandemic.

Scientists aren’t sure how effective only one dose of ...