Impact of Mild to Moderate COVID-19 Infection on Cardiorespiratory Fitness in Active Firefighters


Cardiorespiratory fitness (CRF) is a critical factor in maintaining optimal health, especially for individuals engaged in physically demanding occupations like firefighting. High CRF levels have been consistently linked to a reduced risk of cardiovascular disease, mortality, and certain cancers.

Given the strenuous nature of firefighting tasks and the elevated risk of sudden cardiac events in this profession, understanding factors influencing firefighters’ CRF is crucial. Recent research has shed light on the potential impact of COVID-19 on CRF, with evidence suggesting decreased fitness levels, particularly in those who experienced severe illness.

This study aims to investigate the changes in CRF among active firefighters who had mild to moderate COVID-19 infection and assess whether elapsed time since diagnosis is a significant predictor of CRF changes.

Literature Review

Previous studies have demonstrated a link between COVID-19 infection and decreased CRF, with more pronounced effects in individuals who experienced severe illness. While some recovery occurs over time, persistent declines in CRF have been reported months after infection.

However, these studies often rely on observational cohort designs, utilizing control groups or population-based norms due to the limited use of cardiopulmonary exercise testing (CPET) in the general population. Furthermore, existing research primarily focuses on hospitalized subjects or those with long-term symptoms, lacking representation of individuals with mild to moderate illness – the majority of COVID-19 cases.

Research Gap

Despite the evident importance of CRF in firefighting, there is a notable gap in research regarding the effects of COVID-19 on firefighters’ fitness levels. Given that the vast majority of COVID-19 cases fall into the mild to moderate category, understanding the impact on this specific group is essential for comprehensively assessing the risks and implications for firefighter health and safety.

Objectives of the Study

  • Examine changes in CRF among active, career firefighters who experienced mild to moderate COVID-19 infection.
  • Identify whether elapsed time since COVID-19 diagnosis is a significant predictor of change in CRF.


This study employed a longitudinal, single-group design, assessing pre- and post-COVID-19 infection CRF in active, career firefighters. Participants underwent an annual occupational medical exam, including CPET, both before and after COVID-19 infection. Elapsed time since COVID-19 diagnosis was also considered in the analysis.


Preliminary findings indicate significant declines in CRF from pre- to post-COVID-19 infection among the firefighter sample. The analysis revealed that elapsed time since COVID-19 diagnosis was a significant predictor of CRF changes, emphasizing the importance of considering the recovery period in understanding the long-term effects on fitness.


Novel Contribution to Knowledge

This study marks a pioneering effort in examining the longitudinal changes in cardiorespiratory fitness (CRF) among actively employed firefighters following mild to moderate COVID-19 infection. To our knowledge, no previous research has focused on this specific occupational cohort, shedding light on the potential impacts of the pandemic on the health and performance of these essential frontline workers.

Key Findings and Implications

The central finding of this study reveals a noteworthy decrease in CRF, specifically peak VO2, averaging 2.6 ml·kg−1·min−1 (7.3%) among firefighters approximately 110 days after reporting mild to moderate COVID-19 infection. Furthermore, a significant but modest relationship between days post-COVID-19 and peak VO2 change suggests a time-dependent recovery pattern. This reduction in CRF holds significance for firefighters, as it may compromise their ability to perform strenuous duties effectively and potentially elevate their health risks.

Comparative Insights

Comparisons with existing literature provide valuable context. Previous studies on CRF changes post-COVID-19 have predominantly focused on hospitalized patients or those with prolonged symptoms, making it challenging to generalize findings to a healthy, occupationally active population. Athlete-focused studies reported similar trends, with a 5.2% decrease in CRF among collegiate soccer players and a 5.9% decrease in professional and amateur endurance-trained athletes. Notably, our firefighter cohort exhibited a slightly higher decline, emphasizing the unique challenges faced by this occupational group.

Healthcare Worker Comparison

In contrast, a study on healthcare workers, albeit over a more extended duration, reported a 13.2% decrease in peak VO2, surpassing the reduction observed in our firefighter cohort. The variance in the magnitude of CRF decline could be influenced by differences in demographics, severity of illness, and occupational demands. Interestingly, firefighters, despite a shorter post-COVID-19 duration, demonstrated a milder reduction, potentially indicating a more favorable recovery trajectory.

Mechanisms and Clinical Implications

Utilizing cardiopulmonary exercise testing (CPET), considered the gold standard for assessing CRF, our study delves into potential mechanisms behind reduced fitness post-COVID-19. While respiratory function minimally contributed to decreased CRF, abnormal ventilatory efficiency and impaired cardiac function were notable factors. Autonomic dysfunction, endothelial dysfunction, myocarditis, and mitochondrial dysfunction were implicated, underscoring the multifaceted impact of COVID-19 on the cardiovascular and respiratory systems.

Heterogeneity and Individual Variations

The study recognizes the heterogeneity in CRF changes, with some firefighters experiencing substantial declines, while others exhibited improvements. Factors such as personal motivation and increased time for exercise during pandemic restrictions may contribute to positive outcomes. However, the overall decline in CRF, not attributed to age-related changes, remains a concern, particularly as higher BMI was associated with larger reductions. This association with BMI aligns with existing literature linking obesity to worse COVID-19 outcomes.

Time-Dependent Recovery

A critical aspect of our findings lies in the time course of recovery. While the overall CRF reduction following COVID-19 was approximately 7%, the study reveals a beneficial effect with increasing time from infection. By 300 days post-COVID-19, the predicted reduction in peak VO2 approached zero, highlighting the potential for recovery over an extended period. These insights contribute to the understanding of the dynamic nature of CRF post-COVID-19.

Implications for Occupational Health

The implications of reduced CRF extend beyond individual health, particularly in professions requiring strenuous physical exertion, such as firefighting. The data underscores the need for vigilant monitoring by occupational physicians and healthcare providers, as individuals may not fully appreciate their diminished fitness. This has implications for “Fitness for Duty” assessments in public safety professions, urging instructors and commanders to adapt expectations and workloads based on potential post-COVID-19 fitness limitations.


In conclusion, this study offers comprehensive insights into the longitudinal changes in CRF among firefighters following mild to moderate COVID-19 infection. The findings contribute significantly to the evolving understanding of the broader health implications of the pandemic on specific occupational groups, emphasizing the importance of ongoing monitoring, support, and adaptive strategies in safeguarding the well-being and performance of essential frontline workers.

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