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Does It Matter What Time of Day You Schedule Your SARS-CoV-2 Vaccine?

Scientists are exploring whether and how SARS-CoV-2 vaccination timing affects the body’s ability to mount an immune response. If vaccinating during a certain time of day yields a more robust effect, providers and patients might opt for a targeted appointment schedule.


But research into vaccine timing appears, at present, to be mixed. Based on research published in 2016 into vaccine timing for the influenza virus in older adults, where it was found that, “morning vaccination may be beneficial for the influenza antibody response,” a team of Chinese researchers investigated the potential impact of SARS-CoV-2 vaccine delivery timing on immune responses through a prospective cohort study. Sixty-three health care workers “received vaccination in the morning (9 am–11 am) or afternoon (15 pm–17 pm).” The participants all received an inactivated SARS-CoV-2 vaccine at day 0 and day 28.


The researchers remarked that, “The immune system is influenced by circadian rhythm, and immune response varies at different times of day” and they “observed that immune cells in the blood changed from morning to afternoon.” They concluded, “Our study provides evidence that morning vaccination might enhance serological response and could therefore enhance the efficacy of vaccination (in health care workers).”


However, because the vaccine studied by the Chinese team was inactivated virus, and none of the vaccines currently administered in the United States are similarly produced, these findings are of limited utility for anyone receiving an mRNA (Moderna/Pfizer) or adenovirus (Johnson & Johnson) vaccine in the United States. However, a study published in December 2021 might provide some clue into whether and how vaccine timing impacts mRNA and Adenovirus vaccine efficacy.


In "Time of Day of Vaccination Affects SARS-CoV-2 Antibody Responses in an Observational Study of Health Care Workers,” published in Journal of Biological Rhythms, a team of scientists, “quantified the influence of SARS-CoV-2 vaccination time, vaccine type, participant age, sex, and days post-vaccination on anti-Spike antibody responses in health care workers.” They found that, “The magnitude of the anti-Spike antibody response is associated with the time of day of vaccination, vaccine type, participant age, sex, and days post-vaccination.”


Specifically, the researchers concluded that, “anti-Spike responses were higher in those who were vaccinated later in the day.” Additionally, their “(A)nalysis of 2784 health care workers reveals a significant effect of the time of vaccination on anti-Spike antibody levels following the administration of two alternative SARS-CoV-2 vaccines (mRNA or Adenovirus based).” They acknowledge these findings contrast with both of the previous studies mentioned above and also highlight limitations of their study. For example, studying health care workers means their data set excluded children, elderly or the immunocompromised.


So what does all this mean for individuals looking to maximize the impact of their SARS-CoV-2 vaccine? Should they schedule their vaccine appointment for the afternoon? The researchers stop short of making that recommendation. Instead, they call for more study into the way circadian rhythms might affect both natural and vaccine-induced SARS-CoV-2 immunity. As GlobalCures founder, Vidula Sukhatme reflects, “Conflicting data is how science proceeds, so at this point, we cannot make a recommendation based on timing for SARS-CoV-2 vaccination. But we will continue to monitor the data and share any new findings with our community.”


Global Cures recommends all eligible individuals receive their vaccinations and boosters as per CDC guidelines, which can be found here.

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