SynthesisBMC infectious diseases2015
Frequency and impact of confounding by indication and healthy vaccinee bias in observational studies assessing influenza vaccine effectiveness: a systematic review.
Synthesis in BMC infectious diseases, 2015. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Cited by 112 papers, 6 of them syntheses that pooled it.
What it found
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The abstract states no effect estimate the extractor could read, or names no intervention and outcome on the map, so this paper lights no cell and moves no belief. It is still indexed, cited and linked below.
The trial behind it
Trials whose registry record cites this paper, or whose number appears in the abstract. A trial that started after this paper was published is citing it as background, not reporting it.
Neither the registry nor the abstract names a trial number. If this is a trial report, that itself is worth knowing.
Who cites it
112 citing papers in PubMed, 6 syntheses or guidelines pooled it.
- Efficacy and safety of maternal RSV vaccination against infant lower respiratory tract infections: a systematic review and meta-analysis.Frontiers in public health · 2026Pooled it
- Barriers to influenza vaccination in older adults with chronic diseases: Insights from a COM-B model-based meta-analysis.Human vaccines & immunotherapeutics · 2025Pooled it
- Relative Efficacy, Effectiveness and Safety of Newer and/or Enhanced Seasonal Influenza Vaccines for the Prevention of Laboratory-Confirmed Influenza in Individuals Aged 18 years and Over: Update of a Systematic Review.Reviews in medical virology · 2025Pooled it
- Measles vaccines and non-specific effects on mortality or morbidity: A systematic review and meta-analysis.PloS one · 2025Pooled it
- The importance of falsification endpoints in observational studies of vaccination to prevent severe disease: A critique of a harm-benefit analysis of BNT162b2 vaccination of 5- to 11-year-olds.Epidemiology and infection · 2024Pooled it
- Risk of myocarditis and pericarditis in mRNA COVID-19-vaccinated and unvaccinated populations: a systematic review and meta-analysis.BMJ open · 2023Pooled it
- High-Dose Quadrivalent Influenza Vaccine for Prevention of Cardiovascular and Respiratory Hospitalizations in Older Adults.Influenza and other respiratory viruses · 2024Trial
- Monitoring the safety of the adjuvanted human papillomavirus vaccine HPV-16/18-AS04 in association with pregnancy and potential immune-related diseases in China: A retrospective observational cohort study.Human vaccines & immunotherapeutics · 2026Observational
- Racial and Socioeconomic Inequities in COVID-19 In-Hospital Mortality in Brazil in the Post-Emergency Era: A National Cohort Study of 19,718 Hospitalized Adults in 2024.Journal of racial and ethnic health disparities · 2026Article
- Influenza vaccine effectiveness against hospitalization in children aged 6-72 months: a test-negative study in the Southern Italy, 2022-2026.Infection · 2026Article
- Effectiveness of the 2025-2026 seasonal influenza vaccine among U.S. veterans: an observational study.EClinicalMedicine · 2026Article
- Pre-existing Comorbidities as Potential Risk Modifiers for New-Onset Myocarditis and Pericarditis following mRNA COVID-19 Vaccination in Males Aged 18-30 in the United States: A Disproportionality Analysis using VAERS Spontaneous Reporting Data.Drugs - real world outcomes · 2026Article
- Healthy vaccinee effect in the evaluation of updated COVID-19 vaccines in elderly populations.Nature communications · 2026Article
- 2024-2025 COVID-19 Vaccine and Major Adverse Cardiovascular Events Among US Veterans.JAMA internal medicine · 2026Article
- Investigating respiratory infection as a post-COVID-19 condition using a large passive surveillance cohort from Track PCC.BMC infectious diseases · 2026Article
- Determinants and temporal trajectories of exclusive breastfeeding cessation in 16 sub-Saharan African countries: a pooled multivariable survival analysis of the demographic and health survey (2019-2024).BMC public health · 2026Article
- Risk-of-bias assessment of vaccine effectiveness studies: a scoping review of systematic reviews.Epidemiology and infection · 2026Article
- Predicting disease progression in multiple sclerosis with clinically accessible information and technology.Journal of neurology · 2026Article
- Primary Care Use and Clinical Outcomes in a Prevalent Cohort of Patients Receiving Hemodialysis.Kidney medicine · 2026Article
- Lack of harmonisation in immunological data: challenges in synthesising data during the COVID-19 pandemic.EBioMedicine · 2026Review
52 more citing papers are in PubMed but not listed here.
Corrections and comments
PubMed lists nothing against this paper. Absence here is not a guarantee, only a check that was made.
Authors and funding
3 authors.
Funding
No grant is acknowledged in the PubMed record.
Abstract
backgroundEvidence on influenza vaccine effectiveness (VE) is commonly derived from observational studies. However, these studies are prone to confounding by indication and healthy vaccinee bias. We aimed to systematically investigate these two forms of confounding/bias.
methodsSystematic review of observational studies reporting influenza VE and indicators for bias and confounding. We assessed risk of confounding by indication and healthy vaccinee bias for each study and calculated ratios of odds ratios (crude/adjusted) to quantify the effect of confounder adjustment. VE-estimates during and outside influenza seasons were compared to assess residual confounding by healthy vaccinee effects.
resultsWe identified 23 studies reporting on 11 outcomes. Of these, 19 (83 %) showed high risk of bias: Fourteen due to confounding by indication, two for healthy vaccinee bias, and three studies showed both forms of confounding/bias. Adjustment for confounders increased VE on average by 12 % (95 % CI: 7-17 %; all-cause mortality), 9 % (95 % CI: 4-14 %; all-cause hospitalization) and 7 % (95 % CI: 4-10 %; influenza-like illness). Despite adjustment, nine studies showed residual confounding as indicated by significant off-season VE-estimates. These were observed for five outcomes, but more frequently for all-cause mortality as compared to other outcomes (p = 0.03) and in studies which indicated healthy vaccinee bias at baseline (p = 0.01).
conclusionsBoth confounding by indication and healthy vaccinee bias are likely to operate simultaneously in observational studies on influenza VE. Although adjustment can correct for confounding by indication to some extent, the resulting estimates are still prone to healthy vaccinee bias, at least as long as unspecific outcomes like all-cause mortality are used. Therefore, cohort studies using administrative data bases with unspecific outcomes should no longer be used to measure the effects of influenza vaccination.
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Read under generation 80e0d062 · epoch 390. Bibliography from PubMed, PubMed Central and OpenAlex; grants from NIH RePORTER; trial links from ClinicalTrials.gov; estimates, votes and beliefs from the OpenQuestion graph.