Disguised aspects of IgG4.

Dear Editor,

We read with great interest a recently published study by Della-Torre, Lanzillota et al. regarding IgG4 level in COVID-19 patients [1].

The conclusion which was drawn from the study that IgG4 predicts COVID-19 related mortality was quite far-reaching. Especially taking into account the number of enrolled patients (128 participants) and unique properties of IgG4 protein.

It is known that IgG4 levels may vary significantly in healthy individuals [2]. Additionally increased levels may be observed in various diseases such as allergies, cancer, vasculitis, rheumatoid arthritis, and many others [3]. Except for the history of diabetes and hypertension we do not know if any history regarding autoimmune diseases was collected from the patients. We also do not know the baseline level of IgG subclasses in enrolled patients before SARS-CoV-2 infection. It is possible that patients’ IgG4 level was elevated due to another reason. We should not exclude the possibility that patients with elevated baseline IgG4 are more susceptible to a severe course of COVID-19.

A low number of study participants was noticed by the authors in the limitations. When the prevalence of a disease is as high as SARS-CoV-2 infection, studies should enroll more patients to increase the power of the study and make the conclusions more reliable. Although statistically significant, the results may still be clinically irrelevant.

Financial disclosure

None reported.

Ethical approval was not necessary for the preparation of this article.

Conflict of interests

The authors declare they have no conflict of interest.