Thymosin alpha-1: what is this immune-related peptide?

What is the idea behind it?

The immune system is a network of cells and signals that helps respond to infections and other challenges. Thymosin alpha-1 is a short peptide associated with immune signalling; its synthetic version is called thymalfasin. Researchers have explored whether it can modify particular immune responses in specific circumstances. “Immune modulation” means changing an immune response, not necessarily making every part of immunity stronger or proving fewer infections in healthy people. For the wider question of immune support, see what immune claims actually mean.23

What is Zadaxin, and where is it authorised?

Zadaxin is a named thymosin alpha-1 medicine. The Italian product information made available by AIFA in February 2026 describes it as an adjunct to influenza vaccination in immunocompromised people: it is used alongside a vaccine, not instead of one. Its indication and the applicable label must not be assumed for other countries or products. In a December 2024 US advisory meeting, FDA staff said no thymosin alpha-1 product was FDA-approved in the United States at that time; orphan-drug designation is a different process from approval. Check current local records and the exact product. For the vaccine studies and the separate US assessment, see the flu-vaccine guide.12

What did hepatitis B studies find?

Chronic hepatitis B is a long-lasting viral infection that can damage the liver. In a 1998 study, 98 people were randomly assigned to one of two thymosin alpha-1 regimens or no specific treatment. At 18 months, 40.6% and 26.5% in the two treated groups had cleared two viral markers. In the untreated group, 9.4% had cleared them. The first comparison met the study’s statistical threshold, but the second did not. A later 1999 double-blind trial of 97 people found its defined sustained marker response in 7 of 49 receiving thymosin alpha-1 and 2 of 48 receiving placebo; the difference was not statistically significant. Different designs and populations matter, and loss of markers is not automatically a cure. These historical trials do not compare the peptide with today’s standard antiviral care. For the study designs and what the markers mean, see the hepatitis B trials explained.45

What did a larger sepsis trial show?

Sepsis is a life-threatening response to infection requiring emergency hospital care. In a trial conducted from 2016 to 2020 at 22 centres in China and published in 2025, 1,106 adults with sepsis were randomly assigned thymosin alpha-1 or placebo alongside standard care; 1,089 received the study treatment. Within 28 days, 127 of 542 in the thymosin group and 132 of 547 in the placebo group died: 23.4% versus 24.1%. The trial did not show a clear overall reduction in 28-day mortality; it cannot rule out every smaller effect or establish effects in other settings. The journal later corrected some survival analyses, without changing that overall conclusion. The funders included public research programmes and SciClone Pharmaceuticals; the authors reported that funders did not direct the trial’s conduct or interpretation. For trial design, corrected figures and the wider evidence, read the TESTS study explained.67

Does this mean it prevents infections or treats long COVID?

The hepatitis and hospital sepsis studies asked different questions from whether a healthy person gets fewer infections or whether someone with long COVID feels better. Changes to immune cells or laboratory markers are not, by themselves, proof of those outcomes. The Italian vaccine-adjunct label is a specific use for a specific product and population; it does not establish general immune protection for everyone.16

What should I know about risks and other medicines?

The Italian Zadaxin information contraindicates use during pregnancy and breastfeeding, advises caution with a history of hypersensitivity and calls for individual assessment in autoimmune disease. Other products and routes may differ. Someone using medicines that change immune activity, or undergoing cancer treatment or a transplant, should discuss any such product with their care team; these studies do not establish the safety of every combination. Product identity, formulation and sterility matter, especially outside an authorised supply chain.12

For compounded thymosin alpha-1, the US FDA has identified a possible risk of unwanted immune reactions with certain routes, as well as concerns about peptide impurities and how well the active ingredient is characterised. It says available safety information is insufficient to understand the extent of potential risks in the proposed compounded drug. Compounded drugs are not FDA-approved, and FDA does not verify their safety, effectiveness or quality before marketing. Its assessment is distinct from the information for a particular authorised medicine.21112

Is it the same as thymosin beta-4 or TB-500?

No. Thymosin alpha-1 and thymosin beta-4 are different peptides, and TB-500 is a name used for a fragment related to beta-4. A study of one cannot simply be assigned to the others. The thymosin name guide compares their research stories; the TB-500 guide explains the fragment in more detail.310

What if I have hepatitis B or symptoms of serious infection?

A clinician can assess hepatitis B and discuss monitoring or antiviral treatment based on the person’s situation; the World Health Organization describes established antiviral options. Do not replace prescribed care with a peptide or stop treatment independently. Sepsis can worsen quickly: if a person is seriously unwell and sepsis is suspected, seek emergency care. For how a particular product’s identity and quality are documented, see the certificate-of-analysis guide.89