The purpose of this study is to determine whether probiotic treatment improves the protection against respiratory infections after influenza vaccination in elderly living in nursing homes.

Influenza virus is a member of the orthomyxovirus family and causes an acute viral disease of the respiratory tract. The illness is usually self-limiting. Hospitalization and deaths mainly occur in high-risk groups (elderly, chronically ill). Response to vaccination is subject to high levels of variability due to age, stress, nutritional stage, etc.

Lactic acid bacteria are naturally commensal bacteria in the small and large intestines. These bacteria protect the host against potential pathogens by competitive exclusion and also by the production of antibacterial agents known as bacteriocins. It has only been recently shown that another mechanism whereby probiotic bacteria may provide a health benefit is by modulating immune responses.

Lactobacillus casei strain Shirota (LcS) is widely consumed in fermented milk products. Several studies, in animals as wells as in humans, report on the immunomodulatory functions of LcS.

The aim of this multicentre, randomized, placebo controlled, double blind study (RCT) is to assess if probiotic treatment with Lactobacillus casei Shirota improves the protection against influenza(-like) infections after vaccination with trivalent influenza vaccine in elderly residing in nursing homes.

Trial endpoints:

1. Clinical outcome: difference in incidence in upper respiratory tract infections (influenza-like illness)
2. Serological outcome: difference in influenza vaccine specific serum IgG between probiotic and control treatment

Inclusion Criteria:

• healthy male and female volunteers of 65 years or older
• volunteers reside in residential homes
• volunteers must be willing to swallow Yakult/placebo 2x/day for 3 weeks prevaccination and 5 months follow-up
• participants must be able to comply with requirements of study (e.g. assessment of respiratory symptoms, Yakult/placebo administration)
• participants must read and sign written Informed Consent Form after the nature of the study has been fully explained

Exclusion Criteria:

elderly with:

• any medical or practical condition which make the volunteer not suitable for participating in this study at discretion of the investigator
• any current relevant infectious disease
• any current known disorder having negative repercussions on the volunteer's immune system, such as auto-immune diseases, COPD requiring oxigen, cancer, chronic inflammatory disease
• allergy to influenza vaccine, eggs, neomycin, amphotericin B, erythromycin, amantadine
• ongoing treatment with immunosuppressive drugs, chemotherapeutics or other antineoplastic medication
• current use of antibiotics or use 6 weeks prior to study entry
• use of any investigative drug (other drugs who are also under investigation) within 90 days prior to study entry
• with markedly abnormal results in any of the screening laboratory tests