We read with great interest the article by Green et al. published in the European Journal of General Practice [Citation1]. Highlighting the importance of physical activities that most primary care physicians are of interest in their practice is noteworthy. Combined with sufficient sample size, this study will provide meaningful information about the practice of COVID-19 in primary care. Although this study is very informative, we can suggest insights about this research question based on the study design.
First, we would like to comment on measuring the exposure variables. In this study, the authors collected the exposure variables, i.e. the presence of physical activity (PA), from the frequency of PA from family physicians’ evaluation. However, as the authors stated in the Discussion, the volume of physical activity can be measured by the types of activities with a specific energy requirement (often expressed as METs) and time performed, both of which were not obtained in this study. Although directly measuring PA is a challenge for prospective epidemiological studies, some validated questionnaires, e.g. the international physical activity questionnaire (IPAQ) [Citation2], has been developed and used elsewhere [Citation3]. If the use of self-reported PA measures, like IPAQ, is considered, the future study will become more valid than the current study.
Second, we would like to comment on the selection of adjustment variables in the multivariable model. In this study, age, gender, smoking status, obesity, vitamin D level, diabetes, hypertension, and attention-deficit/hyperactivity disorder (ADHD) were adjusted as confounders. Although these factors are evidence-based and clinically meaningful, we consider the COVID-19 protection acts should be included. Evidence shows that hand hygiene [Citation4], wearing masks [Citation4], and stay-at-home measures are associated with SARS-CoV-2 infection [Citation4]. These behaviours are based on individual health consciousness. Furthermore, those with high health consciousness are likely to have high PA, suggesting the COVID-19 protection acts may be unadjusted residual confounders. Considering these factors in the analysis, we believe the results might have been more valid.
Third, we would like to comment on the excluded subjects in this study. The study excluded patients who would not be eligible for examining physical activity, such as those with congestive heart failure, dementia, hepatic, lung and renal diseases, and cancer. However, only 15.8% of the participants (113,075 out of 715,000) were included in this study. Although it is essential to define a target population that can normally exercise, such a substantial reduction of the samples will make readers challenging to interpret the external validity of this study. It would have been informative if the authors had provided the excluded participants’ numbers and reasons.
In this letter, we focused on the importance of measurement of the exposure variables, selection of confounders, and detailed presentation of the excluded population, all of which were essential to confirm internal and external validity of the observational studies. Because of this study’s excellence, the statistical significance of this study will not change even after considering the issues as mentioned above. However, our comment will provide future researchers with insights to develop a more reliable and valid study design. Also, we believe that our comments will help readers interpret this study’s results accurately and precisely.
Author contributions
All authors conceptualised and designed the paper. TF and MI wrote the original draft, and MK, SF, KM, and TY critically reviewed and revised the manuscript. All authors reviewed and approved the submission of the final version of the manuscript.
Disclosure statement
The authors report no conflicts of interest. The authors are responsible for the content and writing of the paper.
References
- Ilan G, Eugene M, Shlomo V, et al. A higher frequency of physical activity is associated with reduced rates of SARS-CoV-2 infection. Eur J Gen Pract. 2022:1–7. DOI:10.1080/13814788.2022.2138855
- Lee PH, Macfarlane DJ, Lam TH, et al. Validity of the international physical activity questionnaire short form (IPAQ-SF): a systematic review. Int J Behav Nutr Phys Act. 2011;8:115.
- Nichani V, Vena JE, Friedenreich CM, et al. A population-based study of the associations between neighbourhood walkability and different types of physical activity in Canadian men and women. Prev Med. 2019;129:105864.
- Talic S, Shah S, Wild H, et al. Effectiveness of public health measures in reducing the incidence of covid-19, SARS-CoV-2 transmission, and covid-19 mortality: systematic review and meta-analysis. Br Med J. 2021;375:e068302.