RSV circulation during SARS-CoV-2 pandemic era: what happened in Italy

Background: In the northern hemisphere, Respiratory Syncytial Virus (RSV) is more frequently detected from December to February. In Italy, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) presented a peak in incidence from the end of December 2021 to February 2022. Aim(s): To evaluate how SARS-CoV-2 pandemic has influenced RSV circulation. Method(s): We evaluated 389 children, aged 0-18 years, admitted for respiratory tract infections from September 2021 to January 2022 throughout Italy, from the north to the south. Children underwent nasal washing from 1 to 3 days after hospitalization. A (RT)-PCR was developed for detecting 15 respiratory viruses, including RSV, influenza virus A and B, human coronavirus OC43, 229E, NL-63 and HUK1, adenovirus, rhinovirus, parainfluenza virus 1-3, human bocavirus and human metapneumovirus. Result(s): We detected a virus in 338 children (86.9%): RSV was found in 267 (68.7%), other viruses in 71 (18.3%). 51 children (13.1%) resulted negative. Dividing our observational period in two-week timeframes, we found that RSV showed an early peak from October to the first half of December 2021 compared to its usual seasonality. In a previous study, we have demonstrated that RSV circulation was incredibly low from September 2020 to January 2021, in contrast with what we found in the same period in 2021-2022. Comparing RSV and SARS-CoV-2 incidences, we found that these two viruses spread in opposite ways: when SARS-CoV-2 present an incidence peak, RSV circulation reduced and viceversa. Conclusion(s): The relationship between RSV and SARS-CoV-2 showed that viral interference plays a crucial role in their epidemiology.