Usefulness of Clinical Definitions of Influenza for Public Health Surveillance Purposes

Domínguez, Àngela; Soldevila, Núria; Torner, Núria; Martínez, Ana; Godoy, Pere; Rius, Cristina; Jané, Mireia

During the study period, 10,830 samples were collected, of which 438 were excluded because of coinfection, and 25 because they were caused by influenza C virus. Therefore, 10,367 samples were analyzed, of which 3241 (31.3%) were positive for influenza (2035 influenza A only and 1196 influenza B only), 50.3% were female, 51.7% were aged <15 years, and 10.9% presented comorbidities. The most frequent clinical manifestations were fever (86.3%) and cough (80.9%; Table 1 ). The WHO definition of ILI showed the highest sensitivity (82%) and the lowest specificity (37%), whereas the ECDC ILI case definition showed the lowest sensitivity (58%) and the highest specificity (52%). The most predictive case definition for laboratory-confirmed influenza was the WHO case definition for ILI, in both all weeks (aDOR 2.69; 95% CI 2.42-2.99) and in epidemic weeks (aDOR 2.20; 95% CI 1.90-2.54). The ECDC acute respiratory infection (ARI) definition had the worst performance (aDOR 1.45; 95% CI 1.33-1.58 for all weeks and 1.36; 95% CI 1.21-1.54 for epidemic weeks). The clinical manifestations that were significant positive predictors of laboratory-confirmed influenza were fever, cough, myalgia, headache, malaise, and a sudden onset of symptoms. Fever had the highest aDOR in all weeks (4.03; 95% CI 3.38-4.80) and in epidemic weeks (2.78; 95% CI 2.21-3.50; Table 2 ).