## 1 Introduction  
Respiratory tract infections, defined as any of a number of infectious diseases involving the respiratory tract, are the most common infections in humans of all ages, leading to a significant rate of mortality and morbidity worldwide [1]. According to the World Health Organization (WHO), approximately 290,000 to 650,000 deaths annually are caused by influenza virus infection alone [2]. Ominously, in the last 20 years, novel coronaviruses causing acute respiratory syndromes, such as SARS-CoV, MERS-CoV, and, currently, SARS-CoV-2, have emerged and triggered several global pandemics with high case fatality rates [3].  
Among all populations, elderly people are at increased risk of acquiring respiratory infections and of experiencing a more severe disease course following infection due to coexisting chronic diseases, functional impairment, malnutrition, polypharmacy and immunosenescence [4; 5]. The number of hospital admissions for respiratory infections is much higher among people aged \(\geq\)75 years, and it has been shown that the average length of hospital stay increases with age [1; 6]. Moreover, these risks are even greater in older adults who reside in long-term care facilities [7] due to the close proximity of residents in combination with advanced age, multimorbidity and frailty [8]. Therefore, strategies that prevent respiratory infections, ameliorate their severity, or shorten the average duration of the disease will have the greatest benefit in the elderly population, especially older adults residing in long-term care facilities.  
Natural substances and their derivatives have a long history of use for protection against infection and enhancement of the immune response [9]. Indeed, over the years, scientific evidence demonstrated the immunomodulatory and antiviral properties of several of these botanical and fungal extracts used in ancient medicine [9; 10]. In this regard, elderberry (the whole fruit of _Sambucus nigra L_. containing multiple bioactive compounds) extract, which has been traditionally used to address cold and flu symptoms [11], has been shown in two recent meta-analyses to reduce overall symptom duration and severity when it is supplemented at the onset of upper respiratory infections in children and adults [12; 13]. Moreover, preclinical studies have shown that the acidic polysaccharide-rich fraction contained in elderberry extract presented antimicrobial and antiviral effects, specifically against influenza viruses [14]. On the other hand, reishi (_Ganoderma lucidum_), the oldest known mushroom in ancient Chinese medicine [15], has been largely used to promote health and treat a large number of ailments [16]. Reishi has been reported to have a number of pharmacological effects, including immunomodulatory, anti-inflammatory, analgesic, antibacterial, and antiviral properties [17; 18; 19; 20], which has been mainly attributed to its content in _G. lucidum_ polysaccharide (GLPS) [17].  
In addition, it has been suggested that the combination of different plant extracts may have a synergetic effect on the protection and treatment of respiratory infections [10]. On this subject, a drink containing _Echinacea purpurea_ and _Sambucus nigra_ extracts was demonstrated to be as effective as oseltamivir in the treatment of influenza virus infection in patients with early symptomatology [21]. Considering the individual properties of elderberry and reishi extracts, the combination of both compounds may have a promising effect on the protection against respiratory infection in the elderly population.  
Therefore, the objective of the present study is to evaluate the efficacy of daily consumption of a combination of elderberry and reishi extracts on the incidence, severity and duration of respiratory tract infections in a group of healthy elderly volunteers living in a nursing home.