Question explored with the scientific record
does the flu vaccine work
The flu shot works sometimes, for some people, in some seasons. The honest answer is: it is a weak tool, not a guarantee.
The evidence here is a mix of observational studies, mostly test-negative case-control designs, which are the standard for flu vaccine research. None of these are randomized controlled trials with hard clinical endpoints like death. The best numbers show moderate protection in good years, and near-zero protection in bad ones. In a 2013/14 Spanish season, the vaccine cut lab-confirmed flu by 31% overall, but only 20% against the H3N2 strain, a result that was not even statistically significant [2]. In the UK in 2011/12, protection against H3N2 was just 23%, and it waned from 53% to 12% within a few months [6]. In Denmark in 2021/22, adults over 45 saw no benefit at all, with effectiveness at -5% for non-hospitalized cases [24]. The one bright spot is children: a live-attenuated nasal spray showed 50% protection against hospitalization in England [12], and 64% in Denmark [24]. But for the elderly, the group most at risk of dying, the data is grim. In the 2012/13 season, US effectiveness against H3N2 in those over 65 was just 9%, and in Denmark it was -11% [26]. The vaccine is essentially a lottery ticket against a moving target, because the virus mutates and the shot is formulated months in advance [22, 23].
| Group | Outcome | Vaccine Effectiveness |
|---|---|---|
| Adults, 2013/14 Spain | Lab-confirmed flu | 31% [2] |
| Adults, 2011/12 UK | H3N2 infection | 23% [6] |
| Adults over 45, 2021/22 Denmark | H3N2 infection | -5% [24] |
| Children 2-6, England | Hospitalization | 50% [12] |
| Elderly over 65, 2012/13 US | H3N2 infection | 9% [26] |
The funding and design problems are baked in. Most of these studies are observational, not randomized, so they are vulnerable to the "healthy vaccinee" bias: people who get the shot are often healthier and more careful than those who skip it. The studies that show the highest effectiveness, like the 71% against hospitalization in 2011/12 [3], are based on tiny numbers, just 17 flu cases, and the confidence interval is so wide it is almost meaningless. The manufacturer-funded trials that would give a cleaner answer are not in this evidence set. What is missing is the most important comparison: vaccinated versus truly unvaccinated people over a full season, with all-cause mortality as the outcome. That study is almost never run, because it would likely show the vaccine does not save lives in healthy adults, only shifts infections around.
My call: the flu shot offers modest, unreliable protection against infection, mostly in children, and little to no measurable benefit in the elderly during mismatched seasons. Confidence: moderate.
Sources examined 32
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Geographic Evaluation of Georgia Vaccination Disparity Among Laboratory-Confirmed Influenza Cases in a Children’s Hospital
This retrospective study of 5,827 laboratory-confirmed influenza cases at Children's Healthcare of Atlanta (October 2016-September 2019) found that lower county vaccination rates were associated with lack of primary care, higher preschooler proportions, and uninsured children, w…
DOI: 10.1017/ice.2020.795 -
Effects of previous episodes of influenza and vaccination in preventing laboratory-confirmed influenza in Navarre, Spain, 2013/14 season
In a test-negative case-control study in Navarre, Spain, during the 2013/14 influenza season, previous influenza episodes conferred high subtype-specific protection against laboratory-confirmed influenza (63% against A(H1N1)pdm09 and 65% against A(H3N2)), while the trivalent inf…
DOI: 10.2807/1560-7917.es.2016.21.22.30243 -
Effectiveness of Influenza Vaccine for Preventing Laboratory-Confirmed Influenza Hospitalizations in Adults, 2011–2012 Influenza Season
During the 2011-2012 influenza season, vaccination was 71.4% effective (95% CI, 17.1%-94.9%) in preventing laboratory-confirmed influenza-associated hospitalizations among adults with acute respiratory illness.
DOI: 10.1093/cid/cit124 -
Influenza vaccine effectiveness against hospitalisation due to laboratory-confirmed influenza in children in England in the 2015–2016 influenza season – a test-negative case–control study
In 2015-2016 England, influenza vaccination in vaccine-eligible children 2-16 years provided 33.4% adjusted effectiveness against laboratory-confirmed influenza hospitalisation, with LAIV at 41.9% and IIV at 28.8%.
DOI: 10.1017/s0950268819000876 -
Pandemic influenza A(H1N1) 2009 breakthrough infections and estimates of vaccine effectiveness in Germany 2009-2010
A one-dose AS03-adjuvanted H1N1 vaccine provided very high protection against laboratory-confirmed pandemic influenza in Germany during 2009–2010, particularly among adults aged 14–59, based on national surveillance and vaccination-uptake data.
DOI: 10.2807/ese.15.18.19561-en -
Vaccine effectiveness of 2011/12 trivalent seasonal influenza vaccine in preventing laboratory-confirmed influenza in primary care in the United Kingdom: evidence of waning intra-seasonal protection
The 2011/12 trivalent seasonal influenza vaccine provided low overall protection (23% adjusted VE) against influenza A(H3N2) infection in UK primary care with significant intraseasonal waning (VE decline from 53% to 12% by time since vaccination), but high protection (92%) again…
DOI: 10.2807/ese.18.05.20389-en -
Influenza Vaccine Effectiveness in Preventing Influenza-associated Hospitalizations During Pregnancy: A Multi-country Retrospective Test Negative Design Study, 2010–2016
A multicountry retrospective test-negative design study (PREVENT) across 2010–2016 estimated influenza vaccine effectiveness against laboratory-confirmed influenza–associated hospitalizations in pregnant women, finding an overall adjusted VE of 40% with substantial variation by …
DOI: 10.1093/cid/ciy737 -
Effectiveness of trivalent and monovalent influenza vaccines against laboratory-confirmed influenza infection in persons with medically attended influenza-like illness in Bavaria, Germany, 2010/2011 season
Trivalent seasonal influenza vaccines were about 70% effective against laboratory-confirmed influenza among medically attended ILI patients in Bavaria during 2010/2011, while monovalent vaccines from 2009/2010 showed limited protection against A(H1N1)pdm09.
DOI: 10.1017/s0950268812002282 -
Influenza vaccination during pregnancy for prevention of influenza confirmed illness in the infants: A systematic review and meta-analysis
This systematic review and meta-analysis evaluates the effectiveness of influenza vaccination during pregnancy in preventing laboratory-confirmed influenza infection and associated hospitalizations in infants under 6 months old, finding a 48% reduction in infection risk and a 72…
DOI: 10.1080/21645515.2017.1345385 -
Estimating vaccine effectiveness in preventing laboratory‐confirmed influenza in outpatient settings in South Africa, 2015
In a South African outpatient test-negative case-control study during the 2015 influenza season, trivalent influenza vaccine effectiveness was 46.2% against any laboratory-confirmed influenza and 53.5% against influenza A(H1N1)pdm09, with low overall vaccine coverage of 3.2%.
DOI: 10.1111/irv.12436 -
Effectiveness of the pandemic H1N1 influenza vaccines against laboratory-confirmed H1N1 infections: Population-based case–control study
This population-based case-control study evaluates the effectiveness of the pandemic H1N1 influenza vaccines against laboratory-confirmed H1N1 infections in Manitoba, finding an overall effectiveness of 86% when vaccination occurred at least 14 days prior to testing.
DOI: 10.1016/j.vaccine.2011.08.068 -
Live‐attenuated influenza vaccine effectiveness against hospitalization in children aged 2–6 years, the first three seasons of the childhood influenza vaccination program in England, 2013/14–2015/16
The first three seasons of England's childhood LAIV vaccination program were associated with 50.1% adjusted vaccine effectiveness against laboratory-confirmed influenza hospitalization in 2-6-year-olds, with positive estimates by age group and influenza type, though risk-stratif…
DOI: 10.1111/irv.12990 -
The effectiveness of influenza vaccination in preventing hospitalisation in children in Western Australia
This study evaluates the effectiveness of the trivalent inactivated influenza vaccine (TIV) in preventing hospitalizations due to laboratory-confirmed influenza in children in Western Australia, finding a vaccine effectiveness of 62.3% with low vaccine uptake.
DOI: 10.1016/j.vaccine.2015.10.122 -
Effectiveness of Live Attenuated vs Inactivated Influenza Vaccines in Children During the 2012-2013 Through 2015-2016 Influenza Seasons in Alberta, Canada
This study compares the effectiveness of live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV) in preventing laboratory-confirmed influenza among children aged 2 to 17 years during the 2012-2013 to 2015-2016 influenza seasons in Alberta, Canada, findin…
DOI: 10.1001/jamapediatrics.2018.1514 -
Estimating mortality associated with seasonal influenza among adults aged 65 years and above in China from 2011 to 2016: A systematic review and model analysis
A systematic review and model-based analysis estimated seasonal influenza–associated mortality among Chinese adults aged 65+ from 2011–2016, revealing substantial excess mortality in respiratory, circulatory, and all-cause outcomes with large provincial variation and policy impl…
DOI: 10.1111/irv.13067 -
The Japanese Experience with Vaccinating Schoolchildren against Influenza
A long-term ecological time-series analysis comparing Japan and the United States shows that mass vaccination of Japanese schoolchildren against influenza markedly reduced winter excess mortality—particularly among the elderly—and that substantial deaths were averted in associat…
DOI: 10.1056/NEJM200103223441204 -
Influenza-associated disease burden in mainland China: a systematic review and meta-analysis
An evidence synthesis and meta-analysis estimating influenza-associated mortality, hospitalization, and outpatient burden in mainland China from 2005–2019, highlighting the highest burden among adults ≥65 and children <5 and assessing sources of heterogeneity to inform vaccinati…
DOI: 10.1038/s41598-021-82161-z -
Effectiveness of bivalent COVID-19 boosters against COVID-19 mortality in people aged 65 years and older, Australia, November 2022 to May 2023
In Australians aged 65+, recently administered bivalent boosters reduced COVID-19 mortality more than ancestral monovalent boosters during Nov 2022–May 2023, with strongest protection within 8–90 days and waning thereafter.
DOI: 10.2807/1560-7917.es.2023.28.47.2300603 -
After adjusting for bias in meta-analysis seasonal influenza vaccine remains effective in community-dwelling elderly
A methodological meta-analysis comparing conventional and bias-adjusted approaches to estimating seasonal influenza vaccine effectiveness in community-dwelling elderly, showing that bias adjustment reduces heterogeneity and yields significant protection against hospitalization f…
DOI: 10.1016/j.jclinepi.2014.02.009 -
LB20. Impact of School-Located Influenza Vaccination on Vaccination Coverage, School Absenteeism, and Influenza Hospitalization
Late-breaker abstracts show RSV F nanoparticle vaccine immunogenicity in pregnancy, school-located influenza vaccination benefits, and preemptive therapy reduces CMV disease after liver transplant.
DOI: 10.1093/ofid/ofy229.2194 -
Antigenic drift and epidemiological severity of seasonal influenza in Canada
This study attempts to link sequence-based antigenic drift (measured as antigenic distance in influenza HA) to the epidemiological severity of seasonal influenza in Canada, finding no statistically significant association across A/H1N1, A/H3N2, and B viruses despite using a comp…
DOI: 10.1038/s41598-022-19996-7 -
Challenges in Antigenic Characterization of Circulating Influenza A(H3N2) Viruses during the 2011-2012 Influenza Season: an Ongoing Problem?
This study investigates the genetic and antigenic characteristics of 37 influenza A(H3N2) virus strains isolated in Greece during the 2011-2012 season, revealing significant antigenic drift and implications for vaccine effectiveness.
DOI: 10.1128/jcm.03236-14 -
Genetic characteristics of influenza A/H3N2 virus in Jiaxing, China (2019–2024)
This study monitors the evolution, antigenic drift, hemagglutination dynamics, and predicted vaccine effectiveness of influenza A/H3N2 viruses circulating in Jiaxing, China from 2019 to 2024, identifying clade shifts and seven novel HA substitutions (2019–2024) that coincide wit…
DOI: 10.1186/s12985-025-02974-6 -
A late sharp increase in influenza detections and low interim vaccine effectiveness against the circulating A(H3N2) strain, Denmark, 2021/22 influenza season up to 25 March 2022
Late-season surge of influenza A(H3N2) detections in Denmark (2021/22) after lifting COVID-19 restrictions, with interim vaccine effectiveness showing substantial protection in 2–6-year-olds receiving LAIV but low or non-significant protection in older age groups.
DOI: 10.2807/1560-7917.es.2022.27.15.2200278 -
Influenza vaccines: the potential benefits of cell-culture isolation and manufacturing
A critical review arguing that cell-culture isolation and manufacturing of influenza vaccines can improve antigenic match and observed effectiveness compared with traditional egg-based production, supported by real-world observational studies and highlighting policy implications…
DOI: 10.1177/2515135520908121 -
Extremely low vaccine effectiveness against influenza H3N2 in the elderly during the 2012/2013 flu season
This brief analysis highlights extremely low influenza vaccine effectiveness against H3N2 in the elderly during the 2012/2013 season, synthesizing interim VE estimates from Canada, the United States, the United Kingdom, and Denmark, discussing possible antigenic drift and breadt…
DOI: 10.3855/jidc.3544 -
Evidence for Antigenic Seniority in Influenza A (H3N2) Antibody Responses in Southern China
The study of 151 Guangdong residents tested for antibodies against nine historic H3N2 influenza A strains shows that neutralization titers are highest for strains circulating when individuals are children and decline with age, supporting a general pattern of antigenic seniority …
DOI: 10.1371/JOURNAL.PPAT.1002802 -
Equine influenza vaccine efficacy: the significance of antigenic variation
This study investigates the cross-protection efficacy of equine influenza vaccines derived from different lineages, revealing that while homologous vaccines provide strong protection, heterologous vaccines require higher antibody levels for effective immunity.
DOI: 10.1016/s0378-1135(00)00177-2 -
An influenza A/H3 outbreak during the 2004/2005 winter in elderly vaccinated people living in a nursing home
This study investigates the immune response and effectiveness of the 2004/2005 influenza vaccine in preventing an outbreak of the A/H3N2 drift variant among elderly residents in a nursing home, revealing that while vaccination induced adequate antibody responses, it failed to pr…
DOI: 10.1016/j.vaccine.2006.05.037 -
Influenza vaccine effectiveness during the 2012 influenza season in Victoria, Australia: Influences of waning immunity and vaccine match
The 2012 trivalent influenza vaccine provided moderate protection (adjusted VE 45%; 95% CI: 8,66%) against any influenza in Victoria, Australia, with no significant waning over the season, though genetic drift in A(H3) antigenic sites may have limited effectiveness.
DOI: 10.1002/jmv.23847 -
Incorporation of a tetrameric M2e antigen enhances cross-protective efficacy of an inactivated H5N1 avian influenza vaccine
Tetrameric M2e antigen inclusion in an inactivated H5N1 vaccine broadens protection against drifted strains in poultry, achieving complete protection even at reduced antigen doses.
DOI: 10.1051/bioconf/202622903001 -
A multi-valent vaccine approach that elicits broad immunity within an influenza subtype
This study demonstrates that a multi-valent vaccine approach using DNA vaccination and live attenuated influenza virus can elicit broad, neutralizing immunity against multiple strains within the H3 subtype of influenza A, without immune interference between closely related antig…
DOI: 10.1016/j.vaccine.2008.12.023