Key TakeawaysA new study suggests that the time of day may influence the effectiveness of the COVID-19 vaccine.Getting the Pfizer COVID-19 vaccine midday might be associated with fewer breakthrough infections, especially for younger children and older adults, but more studies are needed to confirm these findings.COVID-19 vaccines are safe and effective no matter when you get it.

Key Takeaways

A new study suggests that the time of day may influence the effectiveness of the COVID-19 vaccine.Getting the Pfizer COVID-19 vaccine midday might be associated with fewer breakthrough infections, especially for younger children and older adults, but more studies are needed to confirm these findings.COVID-19 vaccines are safe and effective no matter when you get it.

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 theJournal 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.

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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 tocircadian 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.

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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.”

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What This Means For YouWhile the time of day may play a role in the body’s response to the COVID-19 vaccine, more research is needed to confirm the association. Ultimately, it’s most important to get vaccinated no matter what time works best for you.

What This Means For You

While the time of day may play a role in the body’s response to the COVID-19 vaccine, more research is needed to confirm the association. Ultimately, it’s most important to get vaccinated no matter what time works best for you.

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 ourcoronavirus news page.

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

5 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Hazan G, Duek OA, Alapi H, et al.Biological rhythms in COVID-19 vaccine effectiveness in an observational cohort study of 1.5 million patients.J Clin Invest. Published online April 25, 2023. doi:10.1172/JCI167339Zhang Z, Zeng P, Gao W, Zhou Q, Feng T, Tian X.Circadian clock: a regulator of the immunity in cancer.Cell Commun Signal. 2021;19(1):37. doi:10.1186/s12964-021-00721-2National Institute of Allergy and Infectious Disease.Vaccine adjuvants.Long JE, Drayson MT, Taylor AE, Toellner KM, Lord JM, Phillips AC.Morning vaccination enhances antibody response over afternoon vaccination: A cluster-randomised trial.Vaccine. 2016;34(24):2679-2685. doi:10.1016/j.vaccine.2016.04.032Abbaspour S, Robbins GK, Blumenthal KG, et al.Identifying modifiable predictors of COVID-19 vaccine side effects: a machine learning approach.Vaccines. 2022;10(10):1747. doi:10.3390/vaccines10101747

5 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Hazan G, Duek OA, Alapi H, et al.Biological rhythms in COVID-19 vaccine effectiveness in an observational cohort study of 1.5 million patients.J Clin Invest. Published online April 25, 2023. doi:10.1172/JCI167339Zhang Z, Zeng P, Gao W, Zhou Q, Feng T, Tian X.Circadian clock: a regulator of the immunity in cancer.Cell Commun Signal. 2021;19(1):37. doi:10.1186/s12964-021-00721-2National Institute of Allergy and Infectious Disease.Vaccine adjuvants.Long JE, Drayson MT, Taylor AE, Toellner KM, Lord JM, Phillips AC.Morning vaccination enhances antibody response over afternoon vaccination: A cluster-randomised trial.Vaccine. 2016;34(24):2679-2685. doi:10.1016/j.vaccine.2016.04.032Abbaspour S, Robbins GK, Blumenthal KG, et al.Identifying modifiable predictors of COVID-19 vaccine side effects: a machine learning approach.Vaccines. 2022;10(10):1747. doi:10.3390/vaccines10101747

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Hazan G, Duek OA, Alapi H, et al.Biological rhythms in COVID-19 vaccine effectiveness in an observational cohort study of 1.5 million patients.J Clin Invest. Published online April 25, 2023. doi:10.1172/JCI167339Zhang Z, Zeng P, Gao W, Zhou Q, Feng T, Tian X.Circadian clock: a regulator of the immunity in cancer.Cell Commun Signal. 2021;19(1):37. doi:10.1186/s12964-021-00721-2National Institute of Allergy and Infectious Disease.Vaccine adjuvants.Long JE, Drayson MT, Taylor AE, Toellner KM, Lord JM, Phillips AC.Morning vaccination enhances antibody response over afternoon vaccination: A cluster-randomised trial.Vaccine. 2016;34(24):2679-2685. doi:10.1016/j.vaccine.2016.04.032Abbaspour S, Robbins GK, Blumenthal KG, et al.Identifying modifiable predictors of COVID-19 vaccine side effects: a machine learning approach.Vaccines. 2022;10(10):1747. doi:10.3390/vaccines10101747

Hazan G, Duek OA, Alapi H, et al.Biological rhythms in COVID-19 vaccine effectiveness in an observational cohort study of 1.5 million patients.J Clin Invest. Published online April 25, 2023. doi:10.1172/JCI167339

Zhang Z, Zeng P, Gao W, Zhou Q, Feng T, Tian X.Circadian clock: a regulator of the immunity in cancer.Cell Commun Signal. 2021;19(1):37. doi:10.1186/s12964-021-00721-2

National Institute of Allergy and Infectious Disease.Vaccine adjuvants.

Long JE, Drayson MT, Taylor AE, Toellner KM, Lord JM, Phillips AC.Morning vaccination enhances antibody response over afternoon vaccination: A cluster-randomised trial.Vaccine. 2016;34(24):2679-2685. doi:10.1016/j.vaccine.2016.04.032

Abbaspour S, Robbins GK, Blumenthal KG, et al.Identifying modifiable predictors of COVID-19 vaccine side effects: a machine learning approach.Vaccines. 2022;10(10):1747. doi:10.3390/vaccines10101747

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