Respiratory tract infections are the second most common cause of mortality and morbidity worldwide [
1] and viruses are the most frequently detected pathogens in acute respiratory illness [
2]. The influenza virus causes seasonal epidemics leading to excess hospitalisations and death mainly in the elderly and in patients with co-morbidity [
3,
4]. Annual seasonal influenza vaccine is recommended in at risk groups [
5‐
7] however vaccine coverage rate is sub-optimal [
8,
9] and high quality evidence for significant protection in the elderly is lacking [
10,
11].
The rate of hospitalisation in adults with influenza has been estimated at 5 to 20 per 100,000 overall [
12,
13] and may be as high as 1,200 per 100,000 in those over 85 years old [
4]. Hospitalisation and death result from the complications of influenza including pneumonia and exacerbation of underlying cardiopulmonary conditions [
14]. In adult patients hospitalised with laboratory confirmed influenza, 10–30% are admitted to critical care units and 3–15% die in hospital [
15‐
17] with outcomes being predicted by co-morbidity [
17,
18]. Estimates of the burden of influenza virus infection in hospitalised adults have traditionally been based on the incidence of the influenza-like-illness syndrome (ILI, defined as fever of >38 °C and new respiratory symptoms) rather than on laboratory confirmed influenza. ILI has poor sensitivity (around 50%) and specificity (0–63%) for the diagnosis of influenza in hospitalised adults even during periods of peak activity [
19‐
22]. Where estimates of disease burden are based on laboratory confirmed influenza, laboratory testing of patients is based on clinical suspicion of influenza and is generally targeted to patients with respiratory symptoms and fever. However, in addition to acute respiratory presentations, influenza may present as decompensated cardiovascular disease, collapse or diabetic emergencies [
23,
24]. For this reason many hospitalised cases of influenza are likely to remain undiagnosed. A recent Canadian study estimated that only around one in 14 ED visits due to influenza virus infection were correctly attributed to influenza [
25]. It is likely, therefore, that the burden of influenza and other respiratory viruses amongst hospitalised adults and its economic impact have been under-estimated. In addition to influenza viruses, other respiratory viruses including rhinovirus, respiratory syncytial virus, parainfluenza viruses, human metapneumovirus and coronaviruses, cause acute exacerbations of COPD and asthma as well as other acute respiratory presentations [
2], which lead to large numbers of hospitalisations every year and significant burdens upon healthcare systems.