Characterization of respiratory illness surge (April-June 2021) post-COVID-19 lockdown

Introduction Following the opening of lockdown and relaxation of public health restrictions, an unusual surge of respiratory illness occurred during the spring/summer (Q2) 2021 season. We aimed to characterize this surge compared to previous years. Methods Children discharged from a single tertiary hospital for respiratory illness or selected noncommunicable diseases (NCDs) between 2015 and 2021 were included, and hospitalization rates were compared. To evaluate whether the current respiratory surge resembles a typical winter, characteristics of the second quarter (Q2, April-June) of 2021 were compared to parallel seasons and to autumn/winter (Q4, October-December) of the previous years. Results Respiratory illness showed a seasonal peak, while the pattern of NCD was consistent throughout the study period. Respiratory illnesses showed a delayed surge in Q2 2021 (389 patients), with a missing seasonal peak in Q2 and Q4 2020 (135 and 119 patients, respectively). There were more cases of croup in Q2 2021 compared to Q2 and Q4 2015-2019 (26.2 vs. 12.5% and 15.8%, p < 0.001), fewer cases of pneumonia (5.9 vs. 15.9% and 11.7%, p < 0.001), lower admission rates (22.4 vs. 36.3% and 31.5%, p < 0.001), and fewer hypoxemia during hospitalization (6.3 vs. 11.7% and 10%, p < 0.001). Additionally, lower use of antibiotics (6.2 vs. 12.3% and 10.3%, p = 0.001 and 0.012, respectively) and higher use of steroids (33.7 vs. 24.8% and 28.3%, p < 0.001 and 0.034, respectively). Conclusions The postponed increase in respiratory morbidity after lockdown had unique characteristics, with upper respiratory illness predominating and fewer hospitalizations. Follow-up is needed to assess the altered epidemiological patterns of respiratory diseases in the post-COVID era.