Burden and risk factors of influenza-associated acute respiratory infections among older adults in India: a multicentric community-based surveillance (2018–2023)

Background: Community-level estimates of influenza burden among older adults are important to inform prevention strategies, but limited data exist in India. This study determines the incidence and associated risk factors of influenza-associated acute respiratory infection (ARI) among older adults (age ≥ 60 years) using surveillance data across four community-based sites in India during 2018-2023. Methods: Acute upper respiratory (AURI) and lower-respiratory infections (ALRI) were identified through weekly domiciliary surveillance by trained healthcare workers among older adults residing in four community-based surveillance sites in India. Nasal and throat swabs were collected from all individuals with ALRI episodes and from a subset of individuals with AURI episodes for influenza RT-PCR testing. Predominant clades and subclades were detected by whole genome sequencing. Incidence rates per 1000 person-years of influenza-associated AURI and ALRI were calculated for pre-COVID-19 (July 2018-March 2020) and during COVID-19 pandemic (September 2020-March 2023) periods. Risk factors for influenza-associated ALRI were identified using Poisson regression. Findings: During the 19,914.2 person-years of follow-up, 23,576 AURI and 1952 ALRI episodes were identified. Influenza A accounted for 190 (73.1%) AURI and 140 (82.8%) ALRI episodes. Influenza A(H3N2), particularly subclade 3C.2a1b.2a predominated overall, though circulation patterns varied across sites and time periods. Incidence rates of influenza-associated AURI and ALRI were 39.2 and 8.5/1000 person-years respectively. AURI and ALRI incidence rates were higher in pre-COVID-19 (60.6 and 12.4/1000 person-years) than during the COVID-19 (25.1 and 5.9/1000 person-years) period. Factors associated with influenza ALRI included pre-COVID-19 and latter part of COVID-19 pandemic (2022-2023) periods, Chennai and Ballabgarh surveillance sites, age group 65-69 and greater than equal to 75 years, female sex, chronic respiratory disease, and presence of any disability. Interpretation: These findings highlight the burden of influenza-associated respiratory illness among older adults and has elucidated individual and household-level factors associated with severe disease. The results provide potential evidence to inform future prevention strategies including vaccination programmes prioritising older adults. Funding: This study was supported by National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention (CDC) through the cooperative agreement (U01IP001074) with AIIMS New Delhi.