TORONTO -- A new study suggests that the time of day a person receives their COVID-19 vaccine could have an impact on the body's immune response.

Researchers from Harvard University and the University of Oxford published their findings in the Journal of Biological Rhythms on Saturday. They looked at the antibody levels in blood samples from 2,190 health-care workers in the U.K. after receiving their first doses of either a Pfizer or an AstraZeneca COVID-19 vaccine between December 2020 and February 2021.

The researchers found that antibody responses were highest among people who were vaccinated between 3 p.m. and 9 p.m. when compared to those vaccinated earlier in the day.

“Our observational study provides proof of concept that time of day affects immune response to SARS-CoV-2 vaccination, findings that may be relevant for optimizing the vaccine’s efficacy,” said co-senior author Elizabeth Klerman in a news release.

Previous studies looking into other diseases and medications have also shown evidence that the body's circadian rhythm, or biological clock, has an impact on symptom severity.

For example, Klerman points to trials that have shown that some chemotherapy drugs will effectively target cancer cells but limit toxicity to other cells if administered at a specific time of day. A 2008 study looking into the influenza vaccine also found that elderly men who were vaccinated in the morning had higher antibody levels compared to those who were vaccinated in the afternoon.

However, the results of the 2008 flu vaccine study seem to contradict the findings of Klerman's research, which found that vaccinating in the afternoon resulted in higher antibody levels rather than the other way around.

“The SARS-CoV-2 vaccine and the influenza vaccine have different mechanisms of action from each other, and antibody response may vary greatly depending on whether the immune system recognizes the pathogen from earlier infections, such as influenza, or whether it is confronted by a novel virus,” Klerman explained.

The study isn't without its limitations. The authors noted that they lacked data on the participants' medical and medication history. In addition, the sleep and shift-work patterns of the health-care workers were not taken into account.

Klerman calls her study an important "first step" in demonstrating the importance of time-of-day responses to the COVID-19 vaccines, but says more research needs to be done before she can recommend that people schedule their vaccine for the afternoon to receive an extra boost.

"We need to replicate our findings and develop a better understanding of the underlying physiology of SARS-CoV-2 and the body’s response to vaccination," she said.

Klerman and her team are also looking into how the time of day could affect the vaccine's side-effects.

“If antibody levels are higher when people receive the vaccine in the afternoon, we may see that side-effects are also greater,” said Klerman.