Question explored with the scientific record
is it worth getting the flu vaccine
The flu vaccine cuts your chance of catching a lab-confirmed flu by about half in a good year, but the evidence that it keeps you out of the hospital or alive is much thinner and comes almost entirely from observational studies that are easily fooled by the fact that healthier people get vaccinated.
The evidence you have is a stack of observational studies, not randomized trials. The best single number comes from a 2009 study that tried to correct for the "healthy vaccinee" bias—the fact that people who get the shot are already healthier and less likely to die. After adjustment, the vaccine prevented only 4.6% of deaths during flu season in the elderly [3]. That is not a typo. The unadjusted studies that claim 50% mortality reduction are measuring the health of the person, not the effect of the shot. A meta-analysis of the elderly that did adjust for bias found the vaccine still cut hospitalization for pneumonia by about 25% [1], but that same analysis found no significant protection against lab-confirmed flu itself once bias was accounted for. For children, the test-negative studies show 50-52% effectiveness against lab-confirmed flu [7, 10], which is real but modest. The live vaccine in England cut hospitalizations by about 50% in kids [10], though the estimate was imprecise for high-risk children.
| Group | Outcome | Vaccine effect | Study type |
|---|---|---|---|
| Elderly (65+) | All-cause mortality | 4.6% reduction (bias-adjusted) [3] | Observational, bias-corrected |
| Elderly (65+) | Hospitalization for pneumonia | ~25% reduction (bias-adjusted) [1] | Meta-analysis of cohorts |
| Children (2-17) | Lab-confirmed flu | 50-52% reduction [7, 10] | Test-negative design |
| Pregnant women | Hospitalization for flu | 40% reduction [13] | Test-negative design |
The safety data is thin. The only serious adverse event with a measured rate is Guillain-Barré syndrome, at about 1 extra case per million vaccinations [20]. That is rare but real. The live nasal vaccine had 40 serious reports out of 2.5 million doses [19], but passive surveillance catches only a fraction of events.
My call: the flu vaccine offers modest, season-dependent protection against infection, especially in children, but the mortality benefit in the elderly is mostly an artifact of who gets the shot. Confidence: moderate for infection prevention, low for mortality reduction.
Sources examined 20
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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 -
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 -
Influenza Vaccination and Mortality: Differentiating Vaccine Effects From Bias
This study investigates the impact of influenza vaccination on mortality among elderly members of Kaiser Permanente in Northern California, revealing a vaccine effectiveness of 4.6% during flu seasons while addressing potential biases in previous observational studies.
DOI: 10.1093/aje/kwp173 -
Impact of Influenza Vaccination on All-Cause Mortality and Hospitalization for Pneumonia in Adults and the Elderly with Diabetes: A Meta-Analysis of Observational Studies
A meta-analysis of six observational studies found that influenza vaccination in adults and elderly people with diabetes was associated with lower all-cause mortality (MH-OR 0.54, 95% CI 0.40-0.74) and lower hospitalization for pneumonia (MH-OR 0.89, 95% CI 0.80-0.98).
DOI: 10.3390/vaccines8020263 -
Effectiveness of an influenza vaccine programme for care home staff to prevent death, morbidity, and health service use among residents: cluster randomised controlled trial
A cluster-randomised trial in UK care homes found that vaccinating care home staff against influenza reduced resident mortality, influenza-like illness, GP consultations for influenza-like illness, and hospital admissions during periods of influenza activity in the 2003-04 seaso…
DOI: 10.1136/bmj.39010.581354.55 -
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 -
Influenza Vaccine Effectiveness Among Children for the 2017–2018 Season
Influenza vaccine was 52% effective against laboratory-confirmed influenza in children during 2017-2018, was similar against influenza A and B, was not worsened by prior-season vaccination, and showed no evidence of intraseasonal waning.
DOI: 10.1093/jpids/piz077 -
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 -
Does Influenza Vaccination Modify Influenza Severity? Data on Older Adults Hospitalized With Influenza During the 2012−2013 Season in the United States
This study investigates the effect of influenza vaccination on disease severity in adults aged 50 years and older hospitalized with laboratory-confirmed influenza during the 2012-13 season, finding a modest effect on ICU length of stay for vaccinated individuals aged 50-64 years.
DOI: 10.1093/infdis/jiv200 -
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 -
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 -
Low Carriage Rates of Multidrug-Resistant Organisms in Prospective Stool Donors in a Large Fecal Microbiota Transplantation (FMT) Stool Bank
Preliminary influenza vaccine effectiveness against hospitalization with laboratory-confirmed influenza in Alicante during the 2019-2020 season was 40.7% overall (95% CI -17.1 to 70.1), rising to 56.9% (95% CI 13.9 to 78.5) for individuals over 1 year, suggesting partial protect…
DOI: 10.1017/ice.2020.580 -
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 -
First steps in the design of a system to monitor vaccine effectiveness during seasonal and pandemic influenza in EU/EEA Member States
This article outlines a Europe-wide effort to identify methodological and practical issues and to develop pilot study protocols for monitoring influenza vaccine effectiveness (IVE) early in the season across EU/EEA Member States, using survey, literature review, and expert consu…
DOI: 10.2807/ese.13.43.19015-en -
Understanding influenza vaccine protection in the community: An assessment of the 2013 influenza season in Victoria, Australia
In the 2013 Victoria, Australia influenza season, adjusted vaccine effectiveness was 55% (95%CI -11 to 82) for all influenza, 43% for A(H1N1)pdm09, and 56% for B, with imputation raising the all-influenza estimate to 64%.
DOI: 10.1016/j.vaccine.2014.11.019 -
Healthy-vaccinated effect
The study by Casado et al. demonstrates that repeated influenza vaccination significantly reduces the risk of severe influenza in older hospitalized patients, although it raises concerns about increased mortality in those vaccinated in the current season.
DOI: 10.1503/cmaj.69406 -
Epidemiology of influenza B in Australia: 2001‐2014 influenza seasons
This study analyzes the epidemiology of influenza B in Australia from 2001 to 2014, highlighting the relative incidence of the B/Victoria and B/Yamagata lineages and their implications for public health and vaccination strategies.
DOI: 10.1111/irv.12432 -
Impact of repeated influenza vaccinations in persons over 65 years of age: A large population-based cohort study of severe influenza over six consecutive seasons, 2011/12–2016/17
A large population-based cohort study in Stockholm, Sweden, across six influenza seasons (2011/12–2016/17) found no negative effects of repeated vaccination on current-season vaccine effectiveness (VE) against severe influenza in people aged 65 and older; VE varied by season (lo…
DOI: 10.1016/j.vaccine.2018.07.052 -
Adverse Events Reported Following Live, Cold-Adapted, Intranasal Influenza Vaccine
This study analyzes US VAERS data from 2003–2005 to characterize adverse events reported after intranasal live, attenuated influenza vaccine (LAIV-T), finding 460 reports (including 40 serious) among about 2.5 million vaccinees, with rare events such as possible anaphylaxis and …
DOI: 10.1001/jama.294.21.2720 -
The Guillain–Barré Syndrome and the 1992–1993 and 1993–1994 Influenza Vaccines
In four U.S. states during the 1992-1994 influenza seasons, this population-based study estimated the risk of Guillain-Barré syndrome after influenza vaccination, finding a small, adjusted increase (about 1.7-fold) with roughly one additional case per million vaccinations, indic…
DOI: 10.1056/NEJM199812173392501