Pre-existing and breadth of immunity to seasonal influenza viruses among community-dwelling older adults in Thailand

Older adults are at high risk for influenza-associated morbidity and mortality. Monitoring influenza immunity in this population is crucial for disease prevention and control. However, data on immunity to seasonal influenza viruses among elderly populations remain limited. This study aimed to characterize the quantity, functional quality, and breadth of antibody responses to circulating influenza viruses among community-dwelling elderly in Thailand. A total of 442 plasma samples from elderly individuals aged ≥ 60 years residing in high-density urban communities in central Thailand, collected during 2022–2023, were analyzed. Antibody responses to four representative circulating A/H1N1, A/H3N2, and type B (Victoria and Yamagata lineages) influenza viruses were assessed using hemagglutination inhibition (HAI) and microneutralization (MNT) assays. In accordance with World Health Organization guidance on influenza vaccination, seroprotection is defined as HAI titers equal to or exceeding 40. Most elderly individuals exhibited pre-existing antibodies to all circulating influenza viruses. HAI titers and seroprotection rates were higher against influenza B viruses than influenza A viruses, with the weakest responses observed against A/H3N2. In contrast, neutralizing (NT) antibodies were universally detectable, with NT titers consistently exceeding HAI titers across all strains. Despite exhibiting the highest HAI titers against B/Victoria lineage, this virus showed the highest HAI to MNT ratio, indicating reduced functional antibody quality. Older age (≥ 65 years) and female were associated with higher HAI titers against influenza A viruses; however, these factors did not consistently translate into increased NT activity. Only 15.6% of the older adults exhibited broad antibodies against all four circulating strains, indicating limited cross-strain breadth. The elderly population exhibits pre-existing immunity to seasonal influenza A and B viruses; however, these observational serological findings are heterogeneous, strongly strain-specific, and limited in breadth, with notable gaps against A/H3N2. Functional immunity assessed by MNT may differ from estimates based on HAI, underscoring that incorporating assessment of antibody functional quality could provide complementary information for influenza surveillance. The data provide a comprehensive serosurveillance and support the use of enhanced vaccination strategies to mitigate influenza-associated risk in ageing populations.