Comparison of the Incidence and Risk Factors of Acute Respiratory Illness Associated With COVID-19 and Influenza: Results of the SHIVERS II, III, and IV Prospective Community Cohort Study.

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

Methods: This study is based on a prospective community cohort in Wellington, New Zealand, consisting of participants with acute respiratory illness associated with COVID-19 and influenza, as confirmed by polymerase chain reaction. From 7 February to 2 October 2022, we compared the incidence, risk/protective factors, and clinical features among them.

Results: The crude incidence of COVID-19-associated acute respiratory illness was 59 per 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). Among all COVID-19 cases, the proportion of children aged 0 to 17 years with COVID-19 was substantial but smaller than those of influenza (402/1229 [33%] vs 173/255 [68%], P < .0001). The highest incidence of COVID-19 was among adolescents aged 12 to 17 years (109/100 PY; 95% CI, 97-119) and individuals who were European and other ethnicity (83/100 PY; 95% CI, 80-86), whereas the highest influenza incidence was among children aged 1 to 4 years (49/100 PY; 95% CI, 40-58) and Māori (35/100 PY; 95% CI, 28-43). Adolescents aged 12 to 17 years had 2.5-times higher peak COVID-19 incidence (5.9/100) than adults aged ≥18 years (2.4/100). Adolescents with 2 doses of the COVID-19 vaccines had 75% greater risk of COVID-19 infection (hazard ratio, 1.75; 95% CI, 1.40-2.20) as compared with adults with 3 doses. Vaccination, age, ethnicity, and household size were independent protective/risk factors for COVID-19 or influenza. Participants with COVID-19, as compared with influenza, were less likely to access health care or experience febrile and severe illnesses but more likely to report sore throat, headache, myalgia, and taste or smell loss.

Conclusions: 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.