The Impact of Influenza in Older Age Groups in Portugal, 2015-2018

OBJECTIVES: To quantify and compare the impact of influenza in older age-groups in Portugal, from 2015 to 1018.

METHODS: The BARI study analyzed all discharge records from public hospitals in mainland Portugal and all death certificates per age and cause, regardless of place of death, from 2008 to 2018. Influenza cases were defined as those codified with ICD9/10 J09, J10 or J11. Excess respiratory or cardiovascular hospitalizations and all-cause deaths attributable to influenza were estimated based on time series ecological models, using weekly incidence rates of influenza-like-illness as primary predictor. This post-hoc analysis examined data from 2015-2018 for two age groups: 65-74 and ≥75 years old. Results are reported as mean annualized rates per 100,000 people for seasons 2015/2016, 2016/2017 and 2017/2018.

RESULTS: The annual hospitalization rate for influenza was 2.6 times higher in people aged ≥75 years compared to those aged 65-74 years (80.0 versus 30.8 per 100,000 people, respectively). The annual rate of influenza-associated excess hospitalization in people aged ≥75 years was 4.7 times higher than in those aged 65-74 (389.3 [95% CI: 328.4; 452]) versus 82.2 [95% CI: 61.5; 102.1]). The greatest difference across groups was observed in mortality, with a 12.8 times higher annual rate of influenza-associated all-cause death in population ≥75 years old (192.6 [95% CI: 90.8; 354.6]) versus 15.0 [95% CI: 5.0; 25.6]). Thus, for each hospitalization coded with an influenza-specific diagnosis, 2.7 additional influenza-associated hospitalizations were estimated in the 65-74 age-group and 4.9 in the ≥75 years age-group.

CONCLUSIONS: The BARI study results support aging as a risk factor for influenza-associated complications, with individuals aged 75 and above facing higher risks than those aged 65-74. Results also suggest a greater under-detection of influenza in older individuals. Specific preventive strategies may be required to reduce the influenza burden in this more vulnerable group.