Table 1 Characteristics of influenza laboratory confirmed cases (n = 331) and test-negative controls (n = 995) in the surveillance-based study^a, season 2009-2010, Spain

Mean age (SD^c)

23.4 (17.8)

25.4 (21.4)

0.125

Sex: male

162/330 (49.1)

492/984 (50.0)

0.775

Pandemic influenza vaccination

9/331 (2.7)

41/995 (4.1)

0.234

Seasonal influenza vaccination

37/330 (11.2)

104/988 (10.5)

0.727

Symptoms:

   ▪ sudden onset

211/241 (87.5)

562/817 (68.8)

< 0.0001

   ▪ fever

321/328 (97.9)

890/991 (89.8)

< 0.0001

   ▪ malaise

268/315 (85.1)

713/967 (73.7)

< 0.0001

   ▪ headache

251/320 (78.4)

588/964 (61.0)

< 0.0001

   ▪ myalgia

213/322 (66.2)

589/978 (60.2)

0.057

   ▪ cough

295/329 (89.7)

775/985 (78.7)

< 0.0001

   ▪ sore throat

224/322 (69.6)

601/970 (61.9)

0.013

   ▪ shortness of breath

27/305 (8.9)

84/948 (8.9)

0.996

At least one chronic condition

39/247 (15.8)

120/826 (14.5)

0.624

Pregnancy

5/329 (1.5)

7/976 (0.7)

0.187

Complying with EU case definition

195/331 (58.9)

506/995 (50.8)

0.011

Delay onset-swabbing less than four days

301/331 (90.9)

874/995 (87.8)

0.124