Is There a Best Time of Day to Get Your COVID Shot?

If you’re planning to get a COVID-19 vaccine or booster, the time of day might make a difference in its effectiveness, according to a study published in the Journal of Clinical Investigation.

The observational study included more than 1.5 million people in Israel who mostly received the Pfizer mRNA COVID-19 vaccine. Those who got the shot between 10 a.m. and 2 p.m. seemed to experience fewer breakthrough infections.

Jeff Haspel, MD, PhD, a co-author of the study and an associate professor of medicine at the Washington University School of Medicine in St. Louis, said that mass vaccination campaigns could use this information to prioritize particular groups to get the jab between late morning and early afternoon.

“It might not be a terrible idea to think about getting it during this time, because there’s actually very little downside,” Haspel told Verywell.

The correlation was the most significant in children younger than 20 and adults over 50. But the study results would have to be confirmed by clinical trials.

Why Does the Time of Day Matter When We Get a Vaccine?

Although the researchers don’t know exactly why the time of day might affect vaccine efficacy, it might be related to circadian rhythms—the body’s internal clock that regulates the sleep-wake cycle and the immune system.

“When we get a vaccine, what we’re doing is harnessing our body’s immune system,” said Stuart Campbell Ray, MD, a professor of medicine in the division of infectious diseases at Johns Hopkins University in Baltimore, MD.

The mRNA in Pfizer’s COVID-19 vaccine can teach the body how to build the spike protein that’s on the surface of the coronavirus and trigger an immune response. But the vaccine also contains something called an adjuvant, which helps stimulate the immune cells to react to the spike protein. It’s plausible that these two arms of the vaccine could be affected by circadian rhythms, Ray said.

Scientists have been studying how circadian rhythms can affect vaccine response. A 2016 study of around 300 older adults found that getting the flu shot in the morning instead of the evening might offer greater protection from the influenza virus.

A 2022 study also suggested the time of day when someone got the COVID-19 shot was associated with certain side effects from the vaccine. For instance, people who got the vaccine in the morning were more likely to report experiencing non-allergic side effects, such as headaches and fatigue.

Elizabeth B. Klerman, PhD, MD, a professor of neurology at Harvard Medical School who co-authored the study, told Verywell that circadian rhythms can change many aspects of physiology. Although the side effects from the vaccine were minimal, researchers should look into how healthcare providers could potentially optimize the timing of vaccination, she said.

“If you can reduce the side effects, then do what you can so that people don’t miss work and they can still do family obligations,” Klerman said. “Or they can be more willing to actually get vaccinated.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

Correction - May 15, 2023: This article was updated to more accurately explain the function of an adjuvant.