Table 1. Summary of system components of the IMS and PBS during the influenza seasons 2011–2012 and 2012–2013.

Component	Season 1 (2011–2012)		Season 2 (2012–2013)
	IMS	PBS	IMS	PBS
Invited sample	–	14,022	2,511	14,558
Invited participants (response proportion %)	–	2,580 (18)	166 (7)	2,236 (15)
Self-recruited participants	2,552	–	2,486	–
Geographical area	Sweden	Stockholm	Sweden	Sweden
Calendar weeks operating	46–20	38–20	47–21	44–20