Reply to Ewell and Abbotts.

to the editor: We have reviewed the letter submitted by Ewell and Abbotts (1) regarding our article entitled “Recent COVID-19 vaccination has minimal effects on the physiological responses to graded exercise in physically active healthy people” (2) that was recently published in the Journal of Applied Physiology. The main point raised in this letter is that data collected from nonathletic healthy participants should be interpreted with caution when applied to the context of elite sport. Although we agree with this assertion and had stated in our manuscript that these experiments should be replicated in elite/highly trained athletes, we have some misgivings about the “considerations” proposed by Ewell and Abbotts (1). The core point of their argument is that the participants used in our study had “poor” levels of aerobic fitness in accordance with American College of Sports Medicine (ACSM) criteria and that “elite athletes have more robust immune responses compared to poorly conditioned individuals.” It is important to highlight that the statement by Ewell and Abbotts (1) that “the mean predicted maximal oxygen uptake (V̇o2max) of the study population was 40.7 mL/kg/min and would be considered ‘Poor’ according to the American College of Sports Medicine (ACSM)” only applies to healthy men aged 20–29 yr (3). It appears, therefore, that Ewell and Abbotts (1) have overlooked two facts. First, that V̇o2max scores are categorized according to age and sex; and second, that we recruited equal numbers of male and female participants aged 24– 43 yr. In this context, a V̇o2max value of 40.7 mL/kg/min would actually be considered “good” and “excellent” for women aged 20–29 and 30–39 yr, respectively (2). Given that approximately three-fourth of our participants had age- and sex-adjusted V̇o2max scores considered “good,” “excellent,” or “superior,” we contend that our participants were not “poorly conditioned” as implied by Ewell and Abbotts (1). We do agree that elite athletes, or at least very highly trained individuals, have more robust immune responses than poorly conditioned individuals. However, there is very little evidence to indicate that elite athletes have more robust immune responses than nonelite, physically active individuals with above-average scores of cardiorespiratory fitness, akin to our study participants. This is particularly apparent when a reliable measure of global immune competency (e.g., response to vaccination) is used as the defining criteria (4). On the contrary, there is some evidence that global immune competency is impaired in athletes who are “overreaching” or preparing for major sporting events (4), and it is plausible that vaccines will have more discernible effects on physiological responses to exercise if administered during these times.

Ewell and Abbotts (1) rightly pointed out that elevations in the heart rate response to exercise after COVID-19 vaccination should have been accompanied by either increases in cardiac output or reductions in stroke volume. We believe that this is due to the methods used to measure stroke volume and cardiac output during exercise, which were estimated indirectly using heart rate and oxygen uptake by respiratory gas exchange. We did, however, find a trend for stroke volume to be lower during exercise after vaccination (P = 0.08), which may have been significant had we used a more sensitive technique (e.g., echocardiography). We should also point out that the primary goal of this research was to determine immune responses to exercise before and after COVID-19 vaccination, and the description of the physiological responses was a secondary outcome. This is why we did not specifically recruit an athletic population or include a performance measure (e.g., time trial) in our study design. Although we agree that a more detailed temporal analysis of physiological responses to exercise after vaccination is needed in both elite and nonelite participants, we purport that these findings, despite the acknowledged study limitations, are still informative to athletes performing at the elite level. One also has to consider the timing of when these data were published and our duty as researchers to quickly release information to the public during an ongoing global pandemic. This paper was published online in December 2021, a few months after the summer Olympics in Tokyo and ∼4 wk before the start of the Winter Olympics in Beijing, when there was no scientific data available on the effects of COVID-19 vaccination on athletic performance or physiological responses to exercise. Given that many high-profile athletes worldwide were expressing concerns about the COVID-19 vaccine and the possibility that its side effects may hinder athletic performance, it would have been remiss of us not to publish this work due to the mere issue that our participants were not “elite” athletes. We also considered the influence that elite sports men and women have over the general public, particularly health and fitness enthusiasts, and wanted to demonstrate how COVID-19 vaccination can affect physiological responses to exercise in normal physically active people. Notwithstanding, we agree with Ewell and Abbotts (1) that there is a critical need to fully understand the impact of vaccination on athletic performance and the physiological responses to exercise and future studies in elite athletes will be required to acquire this knowledge.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

H.B. drafted manuscript; R.J.S. edited and revised manuscript; H.B. and R.J.S. approved final version of manuscript.