The trend analysis for age in the multivariable analysis yielded a P value of 0.11, with advanced age associated with a higher risk of adverse outcome. When the timing of treatment with neuraminidase inhibitors after the onset of influenza was analysed, the benefit was confined to administration within the first 48 h after the onset of symptoms. Table 3 shows the variables associated with length of hospital stay. The use of neuraminidase inhibitors within the first 2 days after the onset of influenza reduced hospital stay by a mean of 1.9 days (from 6.6 to 4.7, P,0.001), whereas delayed administration was associated with an increase in hospital stay. Pneumonia diagnosed at admission was clearly associated with longer hospital stay, as were comorbidities (COPD, neurological impairment and cardiovascular disease) and some therapies (proton pump inhibitors).