This is a follow-up study of a previously completed randomized controlled trial (NCT06827873) that investigated the effects of oral supplements on influenza vaccine response in adults aged 60-70 years. The original study was completed in April 2025, with participants receiving either TUDCA (Tauro Ursodesoxy Cholic Acid) supplementation, fatty acid supplementation, or placebo during influenza vaccination.

The primary objectives of this follow-up study are to:

1. Evaluate the durability of vaccine-induced antibody responses approximately 8 months post-vaccination
2. Assess the persistence of immune memory cells, particularly long-lived plasma cells and memory B cells
3. Compare long-term immune responses between the TUDCA supplementation group and placebo group

This observational follow-up involves a single visit where participants will:

1. Provide one blood sample for antibody and immune cell analysis
2. No intervention or vaccination will be administered

The study will specifically focus on B cell subsets through flow cytometry analysis, including total B cells, memory B cells, plasma cells, and long-lived plasma cells. This research aims to determine whether TUDCA supplementation can enhance the durability of vaccine-induced immunity in older adults.

Influenza virus infection presents a significant global health challenge, particularly threatening the elderly population due to immunosenescence. The immune response to influenza vaccination involves a complex series of events: after vaccination, hemagglutination inhibition antibody titers peak around day 14, accompanied by the production of neutralizing antibodies and other specific antibodies. This immune response gradually stabilizes to a post-response baseline level as immune memory establishes.

The age-related decline in immune function manifests through multiple mechanisms, including: reduced production of naive T cells; decreased diversity of T cell repertoire; compromised B cell function; and altered cytokine production profiles, which all diminish vaccine response efficacy. Recent advances in immunometabolism have revealed the crucial role of specific fatty acids and bile acids in immune system modulation. Our preliminary explorations found that fatty acid intervention could significantly reduce the time required for antibody production and enhance its levels following rabies vaccination. We also noticed that serum Tauroursodeoxycholic Acid (TUDCA) was elevated in the intervention group. However, the related mechanisms remain unclear.

Besides, evidence from influenza vaccine studies reveals critical temporal dynamics: antibody titers peak at 1-1.3 months post-vaccination, decline by 3 months, yet remain elevated above baseline at 6 months. Vaccine effectiveness demonstrates time-dependent reduction from 80% at 14 days to 37% at 128 days and 46% at 180 days post-vaccination. Crucially, antibody persistence correlates with long-lived plasma cells (LLPCs), which maintain continuous antibody secretion, while memory B cells enable rapid secondary responses upon antigen re-exposure by differentiating into plasma cells and LLPCs.

Building on these findings, we hypothesized that TUDCA itself may play an active immunoregulatory role during vaccination, influencing B-cell differentiation and antibody persistence. To test this hypothesis and further characterize its potential benefits in elderly populations, we designed an extended follow-up study.

The original study (February-April 2025) demonstrated that TUDCA supplementation significantly enhanced memory B cell responses compared to placebo and fatty acid supplementation groups during the acute phase post-vaccination (Day 0-24). In this extended follow-up, conducted approximately 12 months after vaccination, participants will return for a single fasting venous blood draw (approximately 10 mL) to assess antibody titers and characterize immune-cell subsets. No further intervention or vaccination will be performed. This timepoint represents a critical window for assessing immune durability in the elderly. By quantifying long-term antibody titers and characterizing B cell subsets-particularly plasma cells and long-lived plasma cells-we will determine whether TUDCA supplementation enhances the persistence of vaccine-induced humoral and cellular immunity. This investigation addresses whether nutritional strategies can extend vaccine protection duration in older adults, where immunosenescence substantially limits both vaccine efficacy and protection longevity.

Inclusion Criteria:

1. Originally assigned to either the TUDCA supplementation group or placebo control group
2. No influenza vaccination or other vaccines received between April 2025 and December 2025
3. Willing to participate in this follow-up study and sign supplementary informed consent
4. Stable health condition

Exclusion Criteria:

1. Started using immunosuppressants or hormonal medications after the original study
2. Experienced serious illness or hospitalization within the past month, or planning surgery soon
3. Experienced fever, cold, severe diarrhea, or taken influenza antiviral medications within the past month
4. Unable or unwilling to participate in blood collection
5. Lost to follow-up or unable to contact

This extended follow-up study includes participants from the original randomized controlled trial (Protocol ID: THU01-20240201; NCT06827873) conducted between February and April 2025. Thirty older adults (aged 60-70 years) were previously randomized to either the TUDCA group (n=15) or placebo group (n=15). All completed the 25-day intervention, received quadrivalent influenza vaccine on Day 3, and finished all Day 24 assessments. Participants were recruited from community health centers affiliated with the Second Affiliated Hospital of Bengbu Medical College, Anhui, China, and provided written informed consent for both the original and follow-up studies.