Nasopharyngeal swabs
Nasopharyngeal swabs were obtained by investigators using nylon-flocked swabs (Biocomma; SW01E, flexible minitip, Biocomma, Shenzen, China). Following sampling, swabs were placed into 3 mL Virus Transport Medium (VTM, Biocomma) and delivered to the laboratory as quickly as possible. If delivery took place within 24 h after sampling, samples were to be stored at < 25 °C, if storage period was greater than 24 h (e.g., on Sundays), samples had to be stored and shipped at 2–8 °C. Samples were processed on the day of receipt at the central processing laboratory (Institute of Virology, University Hospital Cologne, Cologne, Germany) by vortexing and aliquoting the viral transport medium and stored at − 80 °C until analysis.