Comment on Recombinant Adjuvanted Zoster Vaccine and Reduced Risk of Coronavirus Disease 2019 Diagnosis and Hospitalization in Older Adults.

To the Editor—We read with great interest the article by Bruxvoort et al reporting that recombinant adjuvanted zoster vaccine vaccination was associated with a lower risk of coronavirus disease 2019 (COVID-19) diagnosis and COVID-19 hospitalization [1]. We wish to highlight some key points.

Firstly, people who voluntarily receive vaccines could have better personal habits, better health literacy, and more health care seeking behavior [2, 3]. Personal habits, such as smoking, are risk factors for COVID-19 infection and further hospitalization [4], whereas physical exercise could be a protective factor [5]. We suggest that the authors should also adjust for other risk factors, such as smoking, alcohol consumption, and exercise to minimize residual confounders.

Secondly, people who join Kaiser Permanente may have certain characteristics [6], and Kaiser Permanente also provides health care with good quality [7]. Therefore, selection bias may exist. We suggest that the authors use a population-based study design to improve external validity of the results.

Lastly, the definition of COVID-19 hospitalization was severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) positive test during or ≤7 days before hospitalization, or a COVID-19 diagnosis code during hospitalization. However, according to Pei et al, national ascertainment rates of COVID-19 in the United States increased from 11.3% in March 2020 to 24.5% during December 2020 [8]. Thus, patients with scheduled hospitalization or surgery could also have a SARS-CoV-2 positive test during or ≤7 days before hospitalization under such a high prevalence of COVID-19 during the period. Therefore, classification bias may occur. We suggest that the authors perform a sensitivity analysis regarding different definitions of COVID-19 hospitalization.