**5. Conclusions**

We report a patient diagnosed with CVID due to his phenotypic and immunological features in which, among other findings, we found a positive cellular response to the complete COVID-19 vaccination schedule with a very low humoral response. In summary, vaccination against SARS-CoV-2 activates the adaptive immune response by inducing both the humoral response (specific antibodies against SARS-CoV-2) and the cellular response (specific T cells against SARS-CoV-2). However, in our patient, as well as in other immunosuppressed or vulnerable patients, we observed a low and less sustained immune response to the COVID-19 vaccines. Although there are not many studies on the effects of vaccines against SARS-CoV-2 in vulnerable patients (CVID, elderly, immunosuppression), it seems advisable to routinely include serological and/or cellular tests of response to vaccination, since this is usually suboptimal, and it would be necessary to implement additional booster doses (as many as necessary) in these patients with low or no response. The vaccine-induced T-cell response has a greater effect than the humoral response mediated by B cells, in addition to having a protective effect even in the absence of a humoral response by limiting the viral replication and supporting immunological memory in the timing of long-term vaccination. What is clear is that these patients require more research on the effects of vaccination in the short and medium term and closer monitoring to try to keep them adequately treated and protected.

Targeted sequencing using a panel of genes associated with primary antibody immunodeficiency was performed on our patient to elucidate the underlying cause of his disease. Our analysis revealed an R162Q variant, a heterozygous IKZF1 mutation, which was not present in his parents. Importantly, this is the first time this variant is described as a de novo mutation. The p.Arg162Gln variant has a deleterious effect on the IKAROS transcription factor, which in heterozygosity, generates a reduction of the binding affinity for DNA, altering the regulation of the targeted genes involved in lymphocyte differentiation, which supports the CVID phenotype of the patient. Despite all the studies carried out to date, it is still not fully understood how mutations in the *IKZF1* gene influence the etiopathogenesis of CVID. It is necessary to further investigate possible factors that could influence

the clinical–immunological heterogeneity between patients, particularly in individuals with de novo mutations, exemplified by our patient, and asymptomatic patients. Genetic studies of these patients and their relatives must be included in the diagnostic algorithm for these pathologies in order to better understand the development of CVID, to find new pathological variants in *IKZF1* or other genes, or even in novel genes not yet described that could produce CVID and, perhaps in the future, to find a more effective and targeted treatment than the basic intravenous administration of immunoglobulins.

**Author Contributions:** Conceptualization, I.D.-A. and M.Á.L.-N.; Data curation, J.M.G.-A. and C.G.-G.; Investigation, I.D.-A. and A.E.-O.; Methodology, J.M.G.-A., C.G.-G., M.A.-F., A.S. and P.J.; Resources, M.Á.L.-N. and P.J.; Supervision, M.Á.L.-N. and P.J.; Visualization, I.D.-A.; Writing original draft, I.D.-A. and A.E.-O.; Writing—review and editing, M.Á.L.-N. and P.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was partially financed by Palex Medical S.A.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and approved by Portal de Ética de la Investigación Biomédica. Junta de Andalucía (Code: 0297-N-21).

**Informed Consent Statement:** Informed consent was obtained from all family members.

**Acknowledgments:** We thank Per Anderson for the English revision. This study is part of the doctoral thesis of Irene Díaz Alberola, within the program of Biomedicine, conducted at the University of Granada, Spain.

**Conflicts of Interest:** José María García-Aznar and Christian Ganoza-Gallardo are employees of Health in Code S. L. All authors declare no conflict of interest.
