By Michael BurgessMay 8 2021

Countries like the UK, Israel, and the USA are now in the thick of successful vaccine campaigns against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the COVID-19 pandemic.

As the vaccine rollout continues, there have been thousands of anecdotal reports of receivers suffering symptoms of mild fatigue, achiness, and fever in the few days after dosage, to the point where members of the public that do not suffer such symptoms worry the vaccine hasn't worked for them.

Although widely publicized, evidence for physiological responses to the vaccines is currently scarce in published literature. Giogiro Quer and Matteo Gadaleta of the Scripps Research Translational Institute in California, USA, led an investigation to obtain tracking data for physiological responses to vaccine programs.

Using a smartphone-based app that reported physiological activities from smartwatches, Quer and Gadaleta found that resting heart rate increases after receiving a vaccination, especially those who received the Moderna vaccine, previously had contracted COVID, or were under the age of 40.

A pre-print version of the research paper is available to read in full on the medRxiv*server.

In the United States, two two-dose mRNA vaccines (Pfizer-BioNTech & Moderna) and a single-dose vaccine (Janssen / Johnson & Johnson) are concurrently being administered to the general public. The Centers for Disease Control and Prevention (CDC) found that 69% percent of the nearly 2 million individuals who have received a second mRNA vaccine dose reported some fever-like side effect in the following days, between perhaps as many as 53.9% of those started just a day after receiving the jab.

Mean and 95% confidence interval of the absolute individual changes in resting heart rate (in BPM) with respect to the individual baseline around the date of vaccination (day 0), for the first dose of the vaccine (a) and for the second dose (b). The cumulative distribution of the maximal variation in resting heart rate in the 2 days after the vaccines after the first (c) and second (d) vaccine dose.

Quer and Gadaleta's team collected data from over 4,000 American citizens using the DETECT software, a smartphone app that consensually syncs with a user's Fitbit or Apple watch device to record a participant's heart rate, sleeping patterns, and various other physiological functions.

Participants were those who had received at least one dosage of either the Pfizer or Moderna mRNA vaccines. All participants' resting heart rate was recorded from prior to receiving dosage, the day of, and the days afterward, and were split into three age groups: <40, 40-60, and >60.

After adjusting for confounding variables, the research team analyzed the resting heart rates of participants to infer physiological symptoms to the vaccines.

They found that resting heart rate increased in participants perhaps up to as much as an additional 1.5 beats per minute after receiving a dosage. Average heart rate additionally did not return to normal levels until four days after a primary jab and six days following a secondary jab.

Although no difference in gender was observed, previous infection by SARS-CoV-2 and the ages of participants did affect heart rate. Indeed, individuals in the under-40s group had the most significant increase in heart rate, and all those under-60 experience significantly higher heart rates than the over-60s group after the second dosage. Previous infection of COVID additionally was associated with higher heart rates, though only after the first jab. Finally, the Moderna vaccine was associated with higher rates of heart rate increase than the Pfizer vaccine, and similarly, this correlated negatively with age.

Normal activity and sleep patterns were slightly disturbed on the first night following the first dosage, with the researchers noting a significant decrease in activity and an increase in sleeping. These returned to nominal levels by the second day after vaccination, however.

This study provides comprehensive and easily accessible evidence of subtle physiological symptoms of the COVID mRNA vaccines - a slight increase in heart rate in the days following a dose, with a minor increase in resting the day afterward. The authors are confident in their findings, though caution that the Fitbit and Apple watch technologies are demographically biased, with a strong bias toward younger users. Additionally, the current vaccine rollout in America has targeted older and at-risk individuals rather than the general population.

However, this wearable sensor technology has provided at least a glimpse at the effects of the vaccines otherwise overlooked. It may allow for future more in-depth assessments to be conducted on citizens in the future.

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.