Common colds are among the top reasons children miss school and adults miss work. The US-based Centers for Disease Control and Prevention (CDC) recently estimated that there are millions of cases of the common cold in the United States each year [1].

According to the CDC, adults have an average of 2–3 colds per year, whereas children usually have even more. Some authors suggested even higher numbers, with an average of 4–6 common colds in adults per year and up to 8 in children [2]. Although colds are more common in the winter months they may occur at any time throughout the year and are usually self-limiting [2], [3]. Nevertheless, they account for a high economic cost of loss of productivity and are a major reason why employees cannot fulfill their obligations [4].

Arroll and Kenealy defined the common cold as “an acute, self-limited inflammation of the upper respiratory mucosa that may involve any or all of the nose, throat, sinuses, and larynx” [5]. Although many types of pathogens can cause the common cold, viruses - and rhinoviruses in particular - are the most common causes [1]. Bacterial involvement, on the other hand, is rare [6].

Risk factors include smoking, stress, poor sleep efficiency and a few types of social contacts [6], [7], [8], [9]. Notably, there are also some lifestyle factors which exert potentially protective effects including the intake of specific nutrients and regular exercise [10], [11], [12].

Vitamin C (ascorbic acid) intake is one of the many nutrients that received particular attention when it comes to the common cold, as it plays an important role in the normal functioning of the immune system [13]. The essential trace elements zinc and copper also yield immunomodulatory functions, and have been reported to modulate the general risk of infection [14], [15], [16]. Both have an impact on the cell-mediated immune reactions of the innate and acquired immune defense [17], [18]. Moreover, zinc possesses direct antiviral properties, and is required to mount an effective antiviral response [19]. Some studies suggested that zinc may enhance the proliferation of CD8+ cytotoxic T-lymphocytes, which are essential for the immune response [14], [19]. Copper, on the other hand, acts on natural killer cells and can thus play an important role in virus elimination [20].

Based on these findings, some authors proposed that the copper-to-zinc ratio could play an important role in several infectious pathologies [21], [22]. During infections, serum copper levels may rise [23], whereas zinc levels may decrease [17]. As a corollary, the copper-to-zinc ratio, which assesses the reciprocal relation of both trace elements, increases. A Finnish study suggested that an increased copper-to-zinc ratio is associated with an increased risk of incident infections in middle-aged men [21].

Studies in other populations, however, are still scarce. To the best of our knowledge, no large population-based studies assessed potential associations between common colds and the copper-to-zinc ratio in samples that also encompass females. To address this gap in the literature, we used data from the National Health and Nutrition Examination Surveys (NHANES). Our analysis is based on the preliminary hypothesis that higher serum copper levels may increase the odds for head and chest colds, whereas higher serum zinc levels may decrease it.