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The precision of vaccine effectiveness (VE) estimates is dependent on sample size and sampling methods. In Victoria, participating general practitioners (GPs) are not limited by the number of influenza-like illness (ILI) patients they collect respiratory samples (swabs) from in sentinel surveillance. However, in the context of scarce resources it is of interest to determine the minimum sample size needed for reliable estimates.Following the test-negative design, patients with ILI were recruited by GPs and tested for influenza. Descriptive analyses were conducted to assess possible selection bias introduced by GPs. VE was calculated by logistic regression as [1 – odds ratio] x 100% and adjusted for week of presentation and age. Random 20% and 50% samples were selected without replacement to estimate the effect of swab rates on VE estimates.GPs swabbed a smaller proportion of patients aged ≥65 years (45.9%, n=238) than those5 (75.6%, n=288), 5–17 (67.9%, n=547) and 18–64 (75.6%, n=2662) years. Decreasing the swab rate did not alter VE point estimates significantly. However, it reduced the precision of estimates and in some instances resulted in too small a sample size to estimate VE.Imposing a 20% or 50% swabbing rate produces less robust VE estimates. The number of swabs required per year to produce precise estimates should be dictated by seasonal severity, rather than an arbitrary rate. It would be beneficial for GPs to swab patients systematically by age group to ensure there are sufficient data to investigate VE against a particular subtype in a given age group.

The 2008 influenza season in Victoria was distinctive because of the increased proportion of influenza-like illness (ILI) cases due to influenza B infection and the lateness of the season compared with preceding years. Influenza activity fell within the bounds of normal seasonal activity thresholds. The average rate of ILI reported by general practitioners participating in sentinel surveillance was 5.5 cases per 1,000 consultations, peaking at 13.4 cases per 1,000 consultations. The average ILI rate reported by the Melbourne Medical Deputising Service was 5.1 cases per 1,000 consultations over the season peaking at 16.2 cases per 1,000 consultations at the same time as peak rates were reported by rural geneal practitioners (GPs), with a secondary peak observed 2 weeks later (10.9 cases per 1,000 consultations). Metro GP rates peaked in week 35 (week beginning 25 August) at 15.2 cases per 1,000 consultations. Influenza B cases notified directly to the Victorian Department of Human Services (DHS) from other sources peaked in the 1st week of September with peak numbers of influenza A notifications occurring the following week. Overall 56% of notifications of laboratory confirmed influenza to DHS and 56% of influenza positive swabs from sentinel surveillance were influenza type B.