COVID‐19 seroprevalence and clinical picture in Swedish pediatric oncology and hematology patients

BACKGROUND: Children develop symptomatic coronavirus disease 2019 (COVID‐19) more rarely than adults upon infection with severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). Pediatric oncology and hematology patients may be at increased risk of severe COVID‐19 due to their underlying disease or treatment. We investigated COVID‐19 and seroprevalence of anti‐SARS‐CoV‐2 antibodies, respectively, in a Swedish cohort of pediatric oncology and hematology patients. PROCEDURE: Patients (n = 136) were recruited between June 2020 and September 2021 at Uppsala University Children's Hospital, Sweden. Up to six consecutive blood samples per patient were analyzed for wild‐type anti‐S1 IgM and IgG antibodies (including after vaccination, n = 4). Clinical data on COVID‐19 (including polymerase chain reaction [PCR] test results) were collected from electronic medical records. A questionnaire was completed at recruitment. RESULTS: A cumulative seroprevalence (IgM and IgG) of 33% (45/136 patients, 95% confidence interval: 25%–41%) was observed in this patient cohort, of whom 66% (90/136 patients) were under severe immunosuppressive treatment during the study period. Increasing patient age (p = .037) and PCR test results (p < .002) were associated with seropositivity in nonvaccinated cases. Most seropositive, nonvaccinated cases (32/43, 74%) were never PCR‐verified for SARS‐CoV‐2 infection. Of the 13 patients with PCR‐verified infection, nine (69%) reported mild disease. A majority (63%) reported continued school attendance during the pandemic. CONCLUSIONS: Swedish pediatric oncology and hematology patients developed antibodies against SARS‐CoV‐2, despite their diagnosis and/or treatment, and the observed seroprevalence was similar to that in national pediatric outpatients. PCR‐verified cases underestimate the true incidence of COVID‐19 in this patient cohort.