Primary care sentinel surveillance of influenza-like illness and laboratory detections of respiratory viral infections in Denmark, week 40 2021 to week 39 2023

BACKGROUNDThe Danish respiratory virus sentinel surveillance system has monitored influenza-like illness and influenza virus activity for over 30 years. During the last decade, additional virus groups were added. In 2021, the system was updated to include SARS-CoV-2, collect detailed symptomatic data, and transitioned to year-round surveillance.AIMTo explore the first two seasons of year-round respiratory virus surveillance and the first symptomatic data collected in the Danish primary care sentinel surveillance system.METHODSIn 2021/22 and 2022/23 seasons, 156 and 147 participating general practitioners, respectively, reported influenza-like illness consultation rates, collected symptomatic data and swabbed patient volunteers. Swabs were sent to Statens Serum Institut for multiplex PCR analysis, with additional characterisation using high-throughput sequencing or type-specific PCR assays for viruses such as influenza, SARS-CoV-2 and respiratory syncytial virus (RSV).RESULTSDuring the two seasons, 4,391 and 6,034 swabs, respectively, were collected and analysed. Year-round surveillance detected an unusually early wave of RSV during 2022/23. While present in nearly all weeks, SARS-CoV-2 showed waves with increased detection. Year-round surveillance also highlighted consistent patterns, such as continuous presence of entero-/rhinoviruses and endemic coronaviruses, as well as parainfluenza virus appearing after influenza virus. Symptom data showed differences by both sex and virus type, e.g. headaches were more commonly reported by women with RSV.CONCLUSIONSOur findings highlight the value of year-round respiratory virus surveillance in identifying both atypical virus activity and consistent patterns outside the winter season. Symptom data suggest the need for further research into sex-specific symptom patterns.