Respiratory viruses circulation during the SARS-CoV-2 pandemic period

Background: Even after the lockdown period (March-June 2020) the Italian public health measures against the spread of SARS-CoV-2, such as social distancing and the mandatory use of face masks, lasted with variable severity depending on the epidemiological data and the local risk. From 28 June 202 these measures have been weakened, and the obligation of wearing face masks outdoors was suspended. This study aims to describe the circulation of respiratory viruses, other than SARS-CoV-2, during the different phases of the COVID-19 pandemic. Methods: The study was performed in a tertiary-care hospital in Parma, Northern Italy, on samples from in- and out-patients including adults and children. The results obtained by different diagnostic methods (viruses antigen detection by immunofluorescence, nucleic acid amplification assays and rapid and conventional cell cultures) for the detection of respiratory viruses on samples collected in 2019 (n=1961), 2020 (n=1377) and during the period January-November 15th 2021 (n=963) were compared. Results: The number of samples collected in 2020 was lesser than 2019 and a lower circulation of viral agents was observed. The prevalence rate of viral respiratory infections (VRI) in 2019 was significantly higher as compared to 2020 (55% vs 37.5%): while in January-February of 2020 the prevalence (65.6%) was similar to that of the same months of 2019 (60%), significant differences have been accounted in the period March-December (24.8% in 2020 vs 51.6% in 2019). The prevalence rate of VRI restart to increase progressively in 2021; starting from September 2021, the respiratory viruses represented 51.1% of all the viruses detected in 2021 with a high frequency of Respiratory Syncytial Virus attested to 21.4% of overall viruses circulating in the period, and observed earlier (October-November) than in previous years (December-January). Conclusions: On the basis of the observed data, the trend of the circulation of respiratory viruses during the COVID-19 pandemic fluctuated in correspondance of and probably in response to the tightening or loosening of containment measures. The significant decreasing of VRI from March 2020 corresponded to the Italian lockdown period and the re-burden of VRI was corresponding and probably subsequent to the weakening of the social distancing measures from July 2021.