Drivers of the summer influenza epidemic in New South Wales, 2018-19

Objectives To assess the extent to which the 2018-19 New South Wales summer influenza epidemic was associated with overseas or domestic travel and with seasonal influenza vaccination status. Design, setting Unmatched case-control study, based on an online survey distributed from the NSW Notifiable Conditions Information Management System (NCIMS) to people for whom mobile phone numbers were available. Participants A case was defined as a person with notified laboratory-confirmed influenza with onset of illness between 1 December 2018 and 21 March 2019. People with notified pertussis infections (confirmed or probable) were selected as controls. Main outcome measures Notified influenza infection, by travel and contact with unwell overseas travellers in the week before onset of illness and seasonal influenza vaccination status (as the primary exposures). Results Valid survey responses were provided by 648 of 2806 invited people with notified influenza (23%) and 257 of 796 invited people with notified pertussis (32%). The demographic characteristics of the respondents were similar to those of the source population (7251 cases, 2254 controls). During the first two months of the summer of 2018-19, notified influenza was more likely for people who had travelled overseas or had contact with an ill overseas traveller in the week before symptom onset (adjusted OR [aOR], 6.99; 95% CI, 3.59-13.6), but not during the second two months (aOR, 1.63; 95% CI, 0.79-3.35). Influenza vaccination status was not associated with the likelihood of notified influenza. Conclusions Travel-related factors were early drivers of the 2018-19 NSW summer influenza epidemic; local transmission sustained the outbreak despite unfavourable conditions later in summer. Our findings prompted re-evaluation of recommendations for pre-travel vaccination in NSW. The role of travel in out-of-season influenza outbreaks should be considered in other temperate zones.