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Abstract
Is there a role for the screening method in estimating influenza vaccine effectiveness (VE)? The answer is yes, but the simplicity of the method used has raised concerns about its validity, and several cautions should be noted. The screening method provides an approximation of influenza VE by comparing the proportion of cases vaccinated (PCV) with the proportion of persons vaccinated (PPV) in the general population. This method has an important disadvantage: VE estimation could be inaccurate if the values for PCV and PPV are drawn from different populations, but it has an important strength, compared with other observational studies, in providing an early indication of VE in the field. Thus, when an infrastructure, such as routine surveillance, is in place to collect robust PCV values, and PPV can be obtained from routine vaccine uptake monitoring systems, the screening method can provide early estimates of influenza VE in target groups.
Financial & competing interests disclosure
L Minodier, F Leccia, L Varesi and A Falchi have received support from EA 7310, “Bioscope Corse Méditerranée”, University of Corsica. T Blanchon, C Turbelin and C Souty has received support from Inserm, UMR 1136, Institut Pierre Louis d’Epidémiologie et de Santé Publique, Paris, France, Sorbonne Universités, UPMC Univ Paris 06, UMR 1136, Institut Pierre Louis d’Epidémiologie et de Santé Publique, Paris, France. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
Key issues
Evaluation of influenza vaccine effectiveness (VE) is important primarily to inform target groups of complementary public health measures and to assist the WHO in deciding each year on the composition of the annual influenza vaccine for the following season.
How the influenza vaccine performs in target group populations cannot be anticipated because of the ethical impossibility of recruiting ‘high-risk’ subjects to randomized controlled trial.
Clinical and virological surveillance of influenza-like illness can be used to assess population-level impact, but estimating VE – how well the vaccine performs in the field among target groups under real-world circumstances – must be done through observational studies.
Estimating VE in real time is an additional indication in the management of the influenza epidemic and could notably be useful in anticipating hospital bed needs and the decision to administer antiviral drugs to people at risk.
The screening method may provide an early indication of VE in the field compared with case–control and cohort studies.
The screening method can provide early estimates of VE in target groups by using robust sources of data to define proportion of cases vaccinated and proportion of persons vaccinated.
The best reference group to assess proportion of persons vaccinated should be the one representing the vaccination coverage in the population from which cases (proportion of cases vaccinated) were drawn, and it is very important that the vaccination status of the cases should be compared with the vaccination status of the reference group estimated in the period of occurrence of cases.
When an infrastructure such as a routine surveillance is in place to collect robust case data, and data on the reference group should be obtainable from routine vaccine uptake monitoring systems, the screening method can provide early estimates of VE among target groups and could be useful to detect low VE in real time to alert public decision-makers.
