**2. SARS-CoV-2 infection in pediatric oncology**

The incidence of COVID-19 among cancer patients varies %1 to 7% [13–15]. In SARS-CoV-2 Infection, cancer patients' hospitalization rate is four times more than the healthy population [16]. Madhusoodhan et al. reported that mortality and morbidity rates in COVID-19 positive children with cancer were higher than the average population. The most common underlying malignancy was acute lymphoblastic leukemia (ALL) (53%). Severe infection and critical support need rates are also higher. Among hospitalized patients with cancer, oxygen support and intensive care unit (ICU) admission rates were significantly higher than the non-cancer group. Sixty-seven percent of positive cases' chemotherapy courses were interrupted between 2 and 78 days. Forty-six percent delays in surgery, thirty percent delays in transplant were noted. The mortality rate was 4.1%, not solely associated with the COVID-19 disease [17].

#### **2.1 Risk of SARS-CoV2 infection in children with cancer**

Cancer patients are immunocompromised due to tumor growth and treatment. Chemotherapy reduces immunoglobulin levels and causes qualitative and quantitative T cell dysfunction. Immunocompromised patients have a higher risk of developing severe disease. Therefore, the leading practices are basic hygiene rules, avoiding crowded places, and possible infection and handwashing situations [18]. Patients with cancer have a higher risk of symptomatic or severe COVID-19 disease. Chemotherapy, surgery in the last month, and immunotherapy administration increase COVID-19 disease severity and associated deaths. However, radiotherapy was not associated with adverse outcomes. Developing symptoms are rapidly, and hospitalization rates and duration were higher in cancer patients. Cancer survivors' signs are more extreme than the average because immune recovery is not completed [19]. In one study, male sex, older age; obesity rates were slightly higher in severe COVID-19 cases with cancer [17]. The United States Centers for Disease Control and Prevention published the risk factors for the severity of COVID-19. Medical complexity, genetic, chronic health conditions, and immunosuppression are presented as possible severity risk factors [18].

### **2.2 Variants of COVID-19**

The mutations in the SARS-CoV2 genome may change its phenotype (transmissibility, virulence). Alfa (B.1.1.7 lineage) variant (20I/501Y.V1) has increased transmission compared with previous strains. Some studies suggest this variant is also associated with severity. Delta (B.1.617.2 lineage), first identified in India, is more transmissible and has more hospitalization rates than the alfa variant. Vaccine effectiveness is also altered in this variant but is high in preventing hospitalization and severe disease. Beta (B.1.351 variant) was identified first in South Africa; vaccine effectiveness may be reduced with this mutation. Gama (P.1 lineage) variant may increase transmissibility. Epsilon variants (B.1.427 and B..1.429) are associated with higher viral mRNA levels on nasal swabs [20].
