Outbreaks with viruses occur continuously, and novel viruses or new variants of existing viruses can surface after a zoonotic event or human-to-human transmission. This project proposal is designed to detect early circulation of (novel) respiratory viruses in both symptomatic and asymptomatic participants, either by direct detection of the virus or changes in local or systemic immunity. Additionally, SENTINEL will provide information on infectivity and (protective) immune responses in viral outbreaks or vaccination campaigns.