Experts weigh in on timing for second covid-19 vaccine doses Health experts are now weighing in on how long one should wait in between vaccine shots. WASHINGTON - Just how important is timing when it comes to the second dose of the Pfizer and Moderna vaccines? Walgreens is now adjusting its vaccine scheduling process after being asked to do so by the CDC. They were spacing Pfizer vaccines 28 days apart instead of recommended 21 days, because it was just easier to schedule them the same as the Moderna shot. So what’s the big deal, why ask Walgreens to change their process?