Immune Aging: Immunosenescence

Labels used on this page: established, promising but limited, mixed, insufficient evidence and preclinical (cells/animals only).

What changes with age

• A slower response. MedlinePlus says that with age the immune system becomes slower to respond, which raises the risk of getting sick, and that flu shots or other vaccines may not work as well or protect for as long as expected. [2] Page reviewed July 2024.
• Healing and cell defects. The same page says the body may heal more slowly because there are fewer immune cells, and that the immune system's ability to detect and correct cell defects declines, which can raise cancer risk. [2]
• Cells, signals and organs. A 2018 review covers how aging affects immune cells (mostly adaptive immunity), soluble signaling molecules and the lymphoid organs that coordinate immune responses. It also discusses the metagenome and exposome as modifiers. [1]
• The thymus. The thymus is the organ where T cells mature. A modeling study reports it shrinks from infancy and that human T-cell production falls exponentially with a half-life of about 16 years. [3] The authors used that model to explain patterns of rising cancer and infection rates with age. [3] It is a mathematical model, so label it promising but limited; it does not prove that thymus decline is the cause of those diseases, and it offers no treatment.

Relation to inflammation

Chronic inflammation is also part of the aging research picture. The 2023 update of the hallmarks of aging lists chronic inflammation as one of twelve proposed hallmarks. [4] See inflammaging and chronic inflammation and the hallmarks of aging.

What trials show about vaccines in older adults

Because older immune systems can respond less strongly, one question tested in trials is whether a higher-antigen influenza vaccine works better. These are vaccines used under medical programs, and decisions belong to a clinician or public health program.

• A 2025 pooled analysis of two randomized trials (Denmark and Spain) included 466,320 adults aged 65 or older. Hospitalization for influenza or pneumonia occurred in 0.56% of the high-dose group and 0.62% of the standard-dose group. All-cause mortality was similar. [5] The analysis was funded by Sanofi. [5]
• A 2026 meta-analysis of 14 randomized trials (581,845 participants) found high-dose vaccine reduced hospitalization for influenza compared with standard dose (relative risk 0.61, high certainty), but probably made little or no difference to all-cause mortality (moderate certainty). [6] The authors concluded the evidence does not support routine preferential use across all older adults irrespective of context. [6]

Label for vaccines overall: the pooled trial results are established for the outcomes listed, with modest absolute benefits. Nothing here shows that vaccines slow aging.

No source we read shows that a supplement, hormone or "thymus" product restores a youthful immune system in people. Peptide products marketed for the thymus are covered at thymic peptides and immune signaling. Do not stop or change any medicine, including immune-related medicines, without talking with your prescriber.

• Frequent infections, fevers, unexplained weight loss, or a new, severe illness need medical care. See frequent illness and low immunity.
• For which vaccines are recommended and for whom, see preventive care, vaccines and screening in older adults and vaccines and infection prevention. General immune biology is in how the immune system works.