Question explored with the scientific record
How effective is the flu vaccine
The flu vaccine works, but modestly and unreliably. It cuts your odds of confirmed flu roughly in half in a good season and to nearly nothing against H3N2 in a bad one.
Effectiveness swings hugely by season, strain, and age. In a South Korean case-control study the shot cut confirmed flu by about 46 to 50 percent in adults 18 to 64, but showed no significant protection in adults 65 and older or against influenza B [1]. In UK primary care against H3N2, the adjusted effectiveness was only 23 percent, and it fell from 53 percent within three months of the shot to 12 percent after [3].
The H3N2 problem is the big one. A 2019 review found pooled effectiveness against H3N2 of 33 percent versus 61 percent for H1N1, and just 24 percent in people over 60 [7]. Waning is real across strains: against H3N2, protection hit zero by about five months after vaccination in the US network [6].
| Population / strain | What the record shows | Source |
|---|---|---|
| Adults 18-64, any strain | ~46-50% effective | [1] |
| Adults 65+ (same season) | no significant effect | [1] |
| H3N2 (pooled, all ages) | 33% | [7] |
| H3N2 (age 60+) | 24% | [7] |
| H1N1pdm09 (pooled) | 61% | [7] |
| H3N2, time since shot | 53% at <3 months, 12% at 3+ months | [3] |
| Diabetics, flu hospitalization | 46% reduction | [2] |
| Hospitalized adults 60+, 2017/18 overall | 9.9%, not significant | [4] |
For people at high risk the case is stronger. In diabetic patients the shot cut lab-confirmed flu hospitalization by 46 percent [2]. A meta-analysis in diabetics found lower all-cause mortality, though the pooled result carried heavy heterogeneity, meaning the studies disagreed with each other [5].
These are all observational test-negative designs, not randomized trials against a placebo. They compare vaccinated to unvaccinated people, so healthy-user bias can inflate the number. No retrieved record tracks all-cause mortality or serious adverse events in a general adult population, so the harm side of the ledger is essentially unmeasured here.
My call: worth it for most healthy working-age adults as a partial, temporary hedge, strongest for diabetics and weaker and shorter in the elderly and against H3N2. It is not the 90 percent shield the marketing implies. Confidence: moderate on the direction, low on the exact size for any given person and season.
No long-term safety data for the general population appears in these records. Absence of evidence is not evidence of safety.
Sources used 7
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Case-Control Study of the Effectiveness of the 2010-2011 Seasonal Influenza Vaccine for Prevention of Laboratory-Confirmed Influenza Virus Infection in the Korean Adult Population
A retrospective matched case-control study in 1,449 South Korean adults found the 2010-2011 seasonal influenza vaccine was 45.8-49.5% effective against laboratory-confirmed influenza, especially influenza A, with no significant effect in adults ≥65 years or against influenza B.
DOI: 10.1128/cvi.00009-13 -
Effect of Influenza Vaccination in Preventing Laboratory-Confirmed Influenza Hospitalization in Patients With Diabetes Mellitus
In diabetic patients, current-season influenza vaccination reduced laboratory-confirmed influenza hospitalization by 46%, and vaccination in prior seasons only by 44%.
DOI: 10.1093/cid/ciaa564 -
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 against laboratory-confirmed influenza in hospitalised adults aged 60 years or older, Valencia Region, Spain, 2017/18 influenza season
In hospitalised adults aged ≥60 years in Valencia during 2017/18, overall influenza vaccine effectiveness was low and non-significant, with significant protection only against A(H1N1)pdm09 and prior vaccination against B/Yamagata.
DOI: 10.2807/1560-7917.es.2019.24.31.1800461 -
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 -
Intraseason Waning of Influenza Vaccine Protection: Evidence From the US Influenza Vaccine Effectiveness Network, 2011–2012 Through 2014–2015
Influenza vaccine effectiveness wanes with increasing time since vaccination across influenza A(H3N2), A(H1N1)pdm09, and B viruses, as observed in the US Influenza VE Network from 2011-12 through 2014-15.
DOI: 10.1093/cid/ciw816 -
Influenza Vaccine Effectiveness: Defining the H3N2 Problem
This narrative review synthesizes evidence that influenza vaccine effectiveness is consistently lower against H3N2 than H1N1pdm09 or type B, and attributes this to viral antigenic evolution, egg-adaptive mutations, and host immunity factors.
DOI: 10.1093/cid/ciz411