The purpose of this study is to evaluate the impact of replacing usual snacks with alternative snack foods on the immune response to influenza vaccination in a population of healthy, middle-aged adults.

Nutrition plays an important role in the immune system by providing energy and metabolites to support the function of immune cells, allowing them to initiate effective immune responses. Diet is therefore a modifiable factor in impacting immune function and is currently a topic of substantial interest in health research. Snack consumption has been shown to account for approximately 20-30% of daily energy intake in adults. Therefore, snack choices have the potential to influence dietary intake and quality, and therefore immune function, both positively and negatively. This study assesses the effect of replacing usual snacks with alternative snack foods on the immune response in a model of viral infection - the seasonal influenza vaccine containing four prevalent influenza virus strains for the 2022/23 or 2023/24 influenza season, as determined by the World Health Organization.

This study is a parallel group, randomised controlled trial that will examine the replacement of usual snack foods with alternative snack foods on the immune response to seasonal influenza vaccination in humans, which will be assessed by measuring rates of seroconversion, and other immunological markers following vaccination. The intervention will be for 8 weeks, and influenza vaccination will be administered at 4-week midpoint. Participants will be followed up 3 months post-vaccination to assess incidence of upper respiratory symptoms.

Inclusion Criteria:

1. Men or women, aged between 40-64 years
2. Body mass index (BMI) of 18.50 - 29.99 kg/m2
3. Individuals who regularly consume snacks (≥2 per day, excluding fruit, vegetable, nut and seed snacks)
4. Fibre intake <30 g/d
5. Willing to avoid receiving any vaccination (except for COVID-19 vaccination) from one month prior to the baseline visit until completion of the 8-week intervention period
6. Willing to avoid receiving any COVID-19 vaccination/booster between week 2 and week 8 of the intervention period
7. Willing to discontinue use of prebiotics and probiotics during the trial
8. Willing to follow the protocol and provide consent

Exclusion Criteria:

1. Allergy or intolerance to any intervention products

2. Dislike of any intervention products

3. Immunodeficiency/immunosuppression due to disease or medication, such as:

   • Chronic inflammatory disease or autoimmune disease (e.g., rheumatoid arthritis, inflammatory bowel disease, psoriasis) or primary or secondary immunodeficiency disease (e.g., HIV infection)
   • Ongoing therapy with immunomodulators or immunosuppressants (e.g., chemotherapy, oral corticosteroids, daily use of inhaled or nasal corticosteroids)
   • Other immunodeficient state (e.g., asplenia).

4. Medical history of any of the following: diabetes, major active psychiatric conditions (e.g. schizophrenia), current eating disorder, alcohol abuse, active treatment for cancer in the last year, severe neurological, endocrine, renal, cardiac or pulmonary disease (or any other chronic medical condition), severe oesophagitis, gastritis or duodenitis, active diverticulitis or intestinal/colonic strictures, Coeliac disease, Crohn's disease or Ulcerative colitis, stem cell or organ transplant, gut resection surgery, bleeding disorder, anaphylaxis or any other major or chronic condition known to impact study outcome measures.

5. Ongoing use of antiviral agents, or any other drugs known to impact study outcome measures

6. Use of immunoglobulins and/or any blood products within the three months prior to vaccination

7. Ongoing use of anticoagulants (e.g., warfarin)

8. Antibiotic treatment in the month prior to the start of the study

9. Consumption of probiotics or prebiotic products within the four weeks prior to the start of the study

10. History of severe adverse reaction and/or allergic reaction associated with the influenza vaccine or any other vaccine

11. Known allergy or hypersensitivity to any component of the vaccine including: sodium chloride, potassium chloride, magnesium chloride hexahydrate, disodium phosphate dihydrate, potassium dihydrogen phosphate; and possible trace residues: beta-propiolactone, cetyltrimethylammonium bromide, and polysorbate 80

12. Suffered from influenza illness in the six months prior to the start of the study

13. For participants recruited on or before 30/06/2023, exclusion criteria is: already vaccinated with any influenza vaccine licensed for the 2022/2023 season; For participants recruited after 30/06/2023, exclusion criteria is: already vaccinated with any influenza vaccine licensed for the 2023/2024 season

14. Received any influenza vaccination within six months prior to the start of the study

15. Received any other vaccinations within one month prior to the start of the study (except for COVID-19 vaccination)

16. Women who are pregnant, lactating or planning pregnancy

17. Ongoing alcohol, drug or medication abuse

18. Unexplained or unintentional weight loss in the past six months