Table 1. Correlations of GOPC, GP ILI rate, school absenteeism, DFC fever counts and inferred influenza level from the dynamic linear model with the influenza activity^* in pre-pandemic, pandemic and the whole period, January 2004–December 2009.

	correlation† with influenza activity
surveillance data	pre-pandemic period (Jan 2004–May 2009)	pandemic period (mid-Jun–Dec 2009)	whole period (Jan 2004–Dec 2009)
1. GOPC ILI‡	0.70	-	0.70
2. GP ILI	0.67	0.93	0.77
3. School absenteeism§	0.32	0.67	0.61
4. DFC fever counts¶	-	0.51	-
Inferred influenza level from model based on 1+2+3	0.75	0.93	0.82
Inferred influenza level from 1+2+3+4	-	0.94	0.82

DFC designated fever clinic; GOPC general outpatient clinic; GP general practitioner; ILI influenza-like-illness.

^*Influenza activity measured by GP ILI consultation rate×laboratory influenza isolation rate.

^†Correlations between surveillance data and laboratory isolation rate were calculated by fitting a univariate dynamic linear model to each data stream, and an overall multivariate model to all data streams.

^‡GOPC data were interrupted during the pandemic period due to the opening of designated flu clinics.

^§School absenteeism data were occasionally interrupted by school holidays or school closures. Correlations were calculated excluding data during the summer holidays.

^¶8 designated fever clinics were activated in place of GOPCs to treat outpatients with influenza-like illness from mid-June 2009 to May 2010.