Influenza vaccine effectiveness in general practice and in hospital patients in Victoria, 2011-2013


The 9th edition of the Immunisation Handbook sponsored by the National Health and Medical Research Council maintained that influenza vaccines were 70%-90% effective in preventing influenza when the match between vaccine strains and circulating strains was good. 1 Even when published in 2008, this was probably a generous assessment of the evidence. The 10th edition, published in 2013, maintained that influenza vaccines were 59% effective in preventing influenza in healthy adults and at least as effective in children, although in some years there was no evidence of any benefit. 2

Although not explicitly stated in the handbooks, these estimates referred to efficacy in protecting against influenza infections managed in the community, the majority of which are relatively mild. While protection against the mild disease seen in primary care might be modest, it is nevertheless possible that the protection provided against more serious disease, including confirmed influenza infections requiring admission to hospital, might be greater.

In Victoria, two surveillance schemes make it possible to investigate whether there was any major difference in vaccine effectiveness estimates in community and hospital patients. The Victorian Sentinel Practice Influenza Network (VicSPIN) is a group of sentinel general practitioners in Melbourne and regional Victoria, operating since 1997, that has provided estimates of influenza vaccine effectiveness for protection against laboratory-confirmed influenza since 2003. 3 The Influenza Complications and Alert Network (FluCAN) is a national hospital-based sentinel surveillance scheme that has provided estimates of influenza vaccine effectiveness since 2010. 4 About 40% of patients registered by this scheme were reported by Victorian hospitals.

We compared influenza vaccine effectiveness estimates for 3 years in Victoria, basing our analysis on data from these two sentinel surveillance systems. Each scheme has published separate vaccine effectiveness estimates for the three study years. 5- 10

Methods

We reviewed data for the influenza seasons of 2011-2013 from the general practice and hospital-based schemes. Vaccine effectiveness was estimated by comparing the vaccination status of influenza cases (patients with laboratory-confirmed influenza) with that of non-cases (patients for whom influenza test results were negative). The use of test-negative controls is an established variation of the case-control study design. 11

VicSPIN uses a community-based, test-negative design. Sentinel GPs were located in metropolitan Melbourne, Geelong and regional Victoria, and patients were recruited by sentinel GPs when they presented with symptoms consistent with influenza infection. At presentation and before their case status was known, patients were swabbed at the discretion of the GP. Patients with positive influenza test results were defined as cases, and those with negative results as non-cases or controls. Vaccine effectiveness was calculated as 1 − odds ratio (OR), and expressed as a percentage, where the OR compared the odds of vaccination for cases with the odds for controls. Logistic regression was used to adjust estimates for age group (0-17 years, 18-64 years, ≥ 65 years), comorbidity (yes v no), and time within influenza season (number of weeks from peak). Estimates were restricted to patients vaccinated at least 14 days before the onset of symptoms, accepted as the time needed to produce protective …read more

Source:: Daily times

      

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