Table 2.

Methods Used for Estimating Rates of Influenza Infection During the 2009 H1N1 Outbreak in Singapore

Method and Data Requirements	Advantages (+) and Disadvantages (−)
Method 1: paired serologic surveysa
Seroconversion data from cohort study	+ Detects subclinical cases
Sensitivity of the serologic test to detect true infection	− Difficulties in timely data collection during an evolving pandemic
Total population size (to determine confidence interval for the estimate)	− No estimate of clinical infection rate
	− Availability of results is dependent on sampling intervals
Method 2: cross-sectional serologic surveysb
Proportion of persons with high pre- and postpandemic titers	+ Relative ease of data collection in comparison with paired serologic surveys
Sensitivity to detect change in titers (proportion of true infections that have high postpandemic and low prepandemic titers using the cutoff titer)	− Risk of underestimation because of persons with high baseline titers
Total population size (to determine confidence interval for the estimate)	− Difficult to generalize to population when using banked samples
Method 3: syndromic surveillance for ILIc
Data on all ILI consultations from sentinel GPs	+ Allows for “real-time” estimation of infection rate
Proportion of influenza cases involving consultation for ILI	+ Data collection is possible with minimal resources
Proportion of ILI consultations due to influenza	− Unable to capture subclinical infections
Market share of GPs surveyed among the total population	− Dependent on clinician reporting
Total population size	− Difficulties in estimating input parameters
	− Large margin of error if given inaccurate data
Method 4: syndromic surveillance for ILI with virologic datad
Data on all ILI consultations from sentinel GPs	+ Margin of error is reduced in comparison with method 3
Market share of GPs surveyed among the total population	+ Allows for “real-time” estimation of infection rate
Proportion of influenza cases involving consultation for ILI	− Additional resources required for laboratory testing
Laboratory proportion of ILI samples that test positive for influenza	− Dependent on sensitivity of laboratory test
Sensitivity of the laboratory test
Total population size

Abbreviations: GP, general practitioner; ILI, influenzalike illness.

^aMethod 1 infection rate = (no. of persons who seroconverted)/[(total no. followed up) × (sensitivity of the serologic test)].

^bMethod 2 infection rate = [(proportion with high postpandemic titers) − (proportion with high prepandemic titers)]/(sensitivity to detect true change in titers).

^cMethod 3 infection rate = (no. of ILI cases)/[(market share of GPs surveyed) × population × (proportion of influenza cases that involved consultation for ILI) × (proportion of ILI consultations due to influenza)].

^dMethod 4 infection rate = (no. of ILI cases)/[(market share of GPs surveyed) × population × (proportion of influenza cases that involved consultation for ILI) × (proportion of ILI samples that tested positive/sensitivity of the laboratory test)].