Supplementary Material for: The incidence of necrotizing enterocolitis and late-onset sepsis during the COVID-19 pandemic in Sweden: A population-based cohort study

Introduction: The effect of the pandemic restrictions in the NICUs is not well studied. Necrotizing enterocolitis (NEC) is characterized by intestinal inflammation and bacterial invasion. This study aimed to investigate whether the incidence of NEC has changed during the COVID-19 pandemic in Sweden and whether it was associated with a change in the frequency of extremely preterm births. Methods: Data were retrieved from the Swedish Neonatal Quality Register (SNQ) for infants registered between January 2017 and December 2021 born below a gestational age of 35 weeks. The registry completeness is 98-99%. The diagnosis of NEC was the primary outcome. Generalized linear model analysis was used to calculate the risk ratio for NEC. Results: Totally 13239 infants were included. 235 (1.8%) infants developed NEC, out of which 91 required surgical treatment. 8967 infants were born before COVID-19 pandemic and 4272 during. Median gestational age at birth was 32.8 weeks in both periods. The incidence of NEC was significantly lower during COVID-19 pandemic compared to the prior period (1.43% vs 1.94% p 0.037), but not the incidence of surgical NEC. The crude Risk ratio of developing NEC during COVID-19 pandemic was 0.74 (95% CI(0.55-0.98)). The incidence of Late onset sepsis with positive culture was also declined during COVID-19 (3.21% vs 4.15%, p value 0.008). Conclusion: While we found significant reduction in the incidence of NEC and culture-positive late-onset sepsis during the COVID-19 pandemic, the number of extremely preterm births was unchanged.