Comparison of the incidence and risk factors of COVID-19 and influenza associated acute respiratory illnesses: Results of the SHIVERS-II, III, IV prospective community cohort study.

Abstract

Background: While severe outcomes among hospitalised patients with COVID-19 and influenza are well described, comparative studies on community transmission and milder illnesses associated with COVID-19 and influenza are lacking.

Methods: We compared incidence, risk/protective factors, and clinical features among prospective community cohort participants with PCR-confirmed COVID-19-associated and influenza-associated acute respiratory illnesses (ARI) from 7-February to 2-October 2022 in Wellington, New Zealand.

Findings: The crude COVID-19-associated ARI incidence was 59/100 person-years (PY). The adjusted cumulative incidence for COVID-19 [77/100-PY;95%CI,75-80] was 4.5 times higher than for influenza [17/100-PY;95%CI,15-19]. The proportion of children (0-17 years) with COVID-19 of all COVID-19 cases was substantial but smaller than those of influenza [402/1229 (33%) vs 173/255 (68%), p<0.0001]. The highest incidence of COVID-19 was among adolescents (12-17 years) [109/100-PY;95%CI,97-119] and non-Māori/Pacific [83/100-PY;95%CI,80-86] whereas the highest influenza incidence was among children (1-4 years) [49/100-PY;95%CI,40-58] and Māori [35/100-PY;95%CI,28-43].Adolescents (12-17 years) had 2.5 times higher peak COVID-19 incidence (5.9/100) than adults ≥18 years (2.4/100). Adolescents with two doses of the COVID-19 vaccines had 75% greater risk of COVID-19 infection [hazard ratio:1.75,95%CI,1.40-2.20] compared to adults with three doses.Vaccination, age, ethnicity, and household size were independent protective/risk factors for COVID-19 or influenza.Participants with COVID-19, compared with influenza, were less likely to access healthcare or experience febrile/severe illnesses, but more likely to report sore throat, headache, myalgia, and taste/smell loss.

Interpretation: As the world transitions to COVID-19 endemicity, estimating disease burdens in community settings becomes important to understand complete disease pyramids, risk factors and clinical progression for informing countermeasures.