COVID-19　continues　to　threaten　human　life　worldwide.　We　explored　how　human　behaviours　have　been　influenced　by　the　COVID-19　pandemic　in　Hong　Kong,　and　how　the　transmission　of　other　respiratory　diseases　(e.g.　influenza)　has　been　influenced　by　human　behaviour.We　focused　on　the　spread　of　COVID-19　and　influenza　infections　based　on　reported　COVID-19　cases　and　influenza　surveillance　data,　and　investigated　the　changes　in　human　behaviour　due　to　COVID-19　based　on　mass　transit　railway　data　and　the　data　from　a　telephone　survey.　We　did　the　simulation　based　on　SEIR　model　to　assess　the　risk　reduction　of　influenza　transmission　caused　by　the　changes　in　human　behaviour.During　the　COVID-19　pandemic,　the　number　of　passengers　fell　by　52.0%　compared　with　the　same　period　in　2019.　Residents　spent　32.2%　more　time　at　home.　Each　person　on　average　came　into　close　contact　with　17.6　and　7.1　people　per　day　during　the　normal　and　pandemic　periods,　respectively.　Students,　workers,　and　older　people　reduced　their　daily　number　of　close　contacts　by　83.0%,　48.1%,　and　40.3%,　respectively.　The　close　contact　rates　in　residences,　workplaces,　places　of　study,　restaurants,　shopping　centres,　markets,　and　public　transport　decreased　by　8.3%,　30.8%,　66.0%,　38.5%,　48.6%,　41.0%,　and　36.1%,　respectively.　Based　on　the　simulation,　these　changes　in　human　behaviours　reduced　the　effective　reproduction　number　of　influenza　by　63.1%.Human　behaviours　were　significantly　influenced　by　the　COVID-19　pandemic　in　Hong　Kong.　Close　contact　control　contributed　more　than　47%　to　the　reduction　in　infection　risk　of　COVID-19.