Evaluating the lockdown-induced effect on respiratory viruses in blood donors during two COVID-19 waves: a retrospective cross-sectional observational study in Denmark

OBJECTIVES: The impact of nationwide lockdowns on respiratory viruses other than SARS-CoV-2, influenza virus, and respiratory syncytial virus in healthy adults remains unclear. This study examined how Denmark's national lockdowns affected the prevalence of common respiratory viruses in asymptomatic blood donors, starting from the month following lockdown implementation, to inform future public health strategies.

METHODS: This retrospective cross-sectional observational study included 2,811 participants aged 18-69 years from January 2018 to May 2021. Nasal swabs collected between October and May were tested for 22 respiratory viruses by RT-PCR. Virus detection risk was assessed using logistic regression, defining the three-month periods following each lockdown (initiated on March 11 and December 9, 2020) as case months, and comparing them with the corresponding months in other years.

RESULTS: Viruses were detected in 171 participants (6.1%), with human rhinoviruses (75%) being the predominant pathogen. SARS-CoV-2 was identified in only three samples. Co-detection was observed in 24% of cases. Viral detection fluctuated throughout the observation period. Following the implementation of lockdown in March 2020, it dropped in April 2020 (OR: 0.31, CI: 0.11-0.75) and reached zero cases in May 2020. Virus detection also decreased from December 2020 to January 2021 (OR: 0.24, CI: 0.06-0.72). Prevalence rebounded in March 2021 after reopening in February 2021 (12% in March 2021 vs. 2.1% in January 2021, p=0.003).

CONCLUSION: Lockdowns significantly reduced the prevalence and detection risk of respiratory viruses, with a rapid rebound upon reopening. Only three asymptomatic SARS-CoV-2 cases were detected during the first two COVID-19 waves, highlighting the effectiveness of national lockdowns.