**2. Materials and Methods**

In this retrospective study medical records of 99 patients aged 0–18 who were admitted to the Department of Clinical Immunology and Paediatrics of J. Gromkowski Provincial Hospital in Wrocław from June 2020 to June 2021 were assessed. Testing for coronavirus infection (antigen/polymerase chain reaction-PCR tests) and serologic tests for IgM and IgG anti-SARS-CoV-2 antibodies titres were performed on all of the included patients. Samples were collected from June 2020 to June 2021, during hospitalisation. The patients were divided into three groups: patients already diagnosed with PID according to IUIS criteria and classification (study group) accounted for 68.69% (*n* = 68), patients without an established diagnosis of PID (control group) constituted 19.19% (*n* = 19), and patients with ongoing or previous paediatric inflammatory multisystem syndrome (PIMS-TS/MIS-C) constituted 12.12% (*n* = 12). The control group consisted of patients with recurrent respiratory tract infections diagnosed in the Department of Immunology and Paediatrics who did not show abnormalities in immunological tests and did not meet IUIS criteria for inborn errors of immunity (IEI).

A total of 63.64% (*n* = 63) of the patients were male, and 36.36% (*n* = 36) were female. The mean age of the patients was 7.3 years. All the patients in the study group had been managed in the department for their PID before—55.88% (*n* = 38) of them were treated with immunoglobulin substitution therapy, and the remaining 44.12% (*n* = 30) did not receive such treatment. The most common form of PID was antibody deficiency (*n* = 44) (Table 1).


**Table 1.** Types of PID and its prevalence in the study group.

Abbreviations: \* IgG subclass deficiency with IgA deficiency/selective IgM deficiency/transient hypogammaglobulinemia of infancy/IgM and IgG subclass deficiency; \*\* isolated congenital asplenia/severe lymphocyte T deficiency during diagnostics.

IgM and IgG anti-SARS-CoV-2 antibody titres were measured quantitatively using chemiluminescence. All of the tests were performed in the same laboratory. IgG antibodies against S1/S2 antigens of SARS-CoV-2 were measured from June 2020 to March 2021 and the anti-trimeric spike glycoprotein of SARS-CoV-2 IgG antibodies were measured from March 2021 to June 2021.

Statistical analysis of data was conducted using the spreadsheet of Microsoft Office Excel (Microsoft Corp., Redmond, WA, USA) and Statistica v. 13–non-parametric Mann-Whitney U test. The significance level was defined as α = 0.05. A *p*-value less than 0.05 was considered statistically significant.

Consent for the study was granted by the Bioethics Committee of the Wroclaw Medical University.

#### **3. Results**

#### *3.1. Morbidity and Disease Course*

Throughout an observation period of 1 year, a diagnosis of SARS-CoV-2 infection was confirmed (by means of a PCR or antigen laboratory test) in 18 out of 68 patients with PID (26.47%). Three cases were diagnosed incidentally during tests before non-COVID-19-related hospital admission (Figure 1). Signs of COVID-19 and/or high probability of the infection (e.g., positive result of a SARS-CoV-2 test in a close family member) were identified in 13 patients (19.12%).

**Figure 1.** Number of cases of acute SARS-CoV-2 infection identified as a result of the routine testing performed before hospital admission.

Most of the confirmed and/or highly probable cases of COVID-19 (*n* = 19; 61.29%) were noted during the so-called 'second wave' of the pandemic (September 2020–January 2021, and none during 'first wave' (March 2020–August 2020). The predominant variant of SARS-CoV-2 in Poland at the end of 2020 and the beginning of 2021 was 20A but when 20I (also known as B.1.1.7) variant emerged in late December 2020, it quickly became the one responsible for the largest number of infections and started the so-called 'third wave'.

The most common symptom of the infection was elevated body temperature: fever (*n* = 12) or low-grade fever (subfebrile temperature; *n* = 6) (Table S1). Among patients with PID infected with SARS-CoV-2, hospital admission was necessary in only three cases (4.41%)—each of those children suffered from humoral immune disorders, one of them also had a diagnosis of Rubinstein-Taybi syndrome. One of the hospitalised patients required oxygen therapy and was treated with convalescent plasma, none of them required management in an intensive care unit. As for June 2021, mortality in the PID group was 0%.

#### *3.2. Evaluation of Anti-SARS-CoV-2 Antibody Synthesis*

Throughout an observation period of 1 year, 32 out of 68 patients with PID tested positive for anti-SARS-CoV2 antibodies (47.06%). By far the majority of these cases (*n* = 31; 96.88%) were associated with confirmed or highly probable (close contact with an infected individual, e.g., a parent, and/or symptoms characteristic of COVID-19) SARS-CoV-2 infection. Furthermore, among 18 patients with a positive test result for SARS-CoV-2 genetic material, only 2 (11.11%) did not produce antibodies directed against it—this included a 12 month-old child with IgG and IgA deficiency and a patient with Rubinstein-Taybi syndrome and IgA, IgM and IgG deficiency, who was also treated with convalescent plasma during the acute phase of the disease.

Among patients with PID who were infected or were most likely infected with COVID-19, there were 10 children (14.70% of all patients with PID) who have been treated with immunoglobulin substitution therapy at that time and only one of these patients required hospital admission. It is also noteworthy that a patient with a history of severe combined immune deficiency (SCID) and hematopoietic stem cell transplantation, which was performed a few years before, developed anti-SARS-CoV-2 antibodies following a symptomatic infection with the virus. Moreover, there was no need for hospital admission in this case.

As for the levels of IgG anti-SARS-CoV-2 antibodies in individuals with positive test results, there were no statistically significant differences when compared with the control group (*n* = 19) (*p* > 0.05) (Figure 2), as well as between the PID group and patients with ongoing or previous PIMS-TS and between the control group and the PIMS-TS group.

**Figure 2.** A comparison of anti-SARS-CoV-2 IgG antibody levels in patients with confirmed or highly probable COVID-19 in each patient group (*p* > 0.05). PID-primary immunodeficiency; PIMS-TSpaediatric inflammatory multisystem syndrome.

However, a statistically significant difference (*p* = 0.0001) between patients with PID receiving immunoglobulin substitution therapy and patients with PID without such treatment was noted (Figure 3).

**Figure 3.** A comparison of anti-SARS-CoV2 IgG antibody levels between patients with PID receiving immunoglobulin substitution therapy (PID IVIG (+)) and patients with PID without such treatment (PID IVIG (−)) (*p* = 0.0001).
