Did you know a flu shot prevented an estimated 6,300 deaths last season alone? This blog post will cut through the noise and dive deep into the powerful statistics, from effectiveness across age groups and vaccine types to safety data and public health impact, so you can make an informed decision for the coming season.
Key Takeaways
- During the 2010–2011 influenza season, the overall adjusted vaccine effectiveness (VE) against medically attended influenza was 60% (95% CI: 53–67) among persons aged ≥2 years in a US network of outpatient clinics.
- In the 2021–22 influenza season, interim VE against influenza A(H3N2)-associated outpatient acute respiratory illness was 35% (95% CI: −10% to 64%) among children aged 2–17 years.
- The 2018–2019 trivalent inactivated influenza vaccine showed 47% (95% CI: 30–60%) effectiveness against influenza A(H1N1)pdm09-associated hospitalizations in adults aged ≥18 years.
- The most common side effect of inactivated influenza vaccine is soreness at injection site, affecting 10-64% of recipients.
- Guillain-Barré Syndrome (GBS) risk after 1976 swine flu vaccine was 1 excess case per 100,000 doses; modern vaccines show no increased risk.
- Anaphylaxis after flu vaccine occurs at rate of approximately 1.35 cases per million doses administered.
- In 2022-2023, US flu vaccination coverage was 50.7% among children aged 6 months–17 years.
- Adults aged 18+ years had 43.3% flu vaccination coverage in 2022-2023 season.
- Among adults ≥65 years, 2022-2023 flu vaccine uptake reached 52.5% by May 2023.
- Flu vaccination prevented an estimated 7.5 million illnesses, 3.7 million medical visits, 109,000 hospitalizations, and 6,300 deaths in 2022-23 US season.
- Over 13 years (2010-2023), flu vaccines prevented 56.6 million respiratory illnesses and 7,100 deaths in children <18y.
- In 2019-20, vaccination averted 7 million illnesses, 3 million visits, 100,000 hospitalizations, 7,000 deaths.
- Annual strain selection by WHO: 2 influenza A (H1N1, H3N2) and 1-2 influenza B viruses recommended.
- Egg-based manufacturing: ~500 million doses produced globally per season using embryonated chicken eggs.
- mRNA flu vaccines in trials: Moderna's mRNA-1010 showed robust HAI titers in Phase 3 (2022).
Flu vaccine effectiveness varies by season, age group, and virus type.
Development and Composition
- Annual strain selection by WHO: 2 influenza A (H1N1, H3N2) and 1-2 influenza B viruses recommended.
- Egg-based manufacturing: ~500 million doses produced globally per season using embryonated chicken eggs.
- mRNA flu vaccines in trials: Moderna's mRNA-1010 showed robust HAI titers in Phase 3 (2022).
- Universal flu vaccine targets: stalk antibodies; trials show 100% seroconversion in some candidates.
- Quadrivalent vaccines since 2013: include both B/Victoria and B/Yamagata lineages.
- High-dose vaccine: 60µg HA per strain vs. 15µg standard, approved 2013 for ≥65y.
- Cell-based vaccines (Flucelvax): grown in Madin–Darby canine kidney cells, avoids egg adaptation.
- Recombinant vaccines (Flublok): HA protein produced in insect cells, egg-free, 45µg/strain.
- Adjuvanted vaccines (Fluad): MF59 oil-in-water emulsion boosts immune response in elderly.
- Live attenuated (FluMist): cold-adapted, replicates in nose, approved for 2-49y.
- WHO Northern Hemisphere 2023-24 recommendations: A/Victoria/4897/2022 (H1N1), A/Darlington/2009 (H3N2), B/Austria/1359417/2021 (Yamagata).
- Vaccine virus propagation: takes 3-6 months from isolate to monovalent bulk.
- Nanoflu (nanoparticle): Phase 2 trials showed superior breadth vs. egg-based.
- Sequential passage in eggs leads to 1.3-2.4 aa substitutions in HA, reducing match.
- Global production capacity: ~1.5 billion doses possible by 2025 per WHO.
- Self-amplifying RNA vaccines in preclinical: 10-fold dose sparing potential.
- B/Yamagata lineage absent since 2020; vaccines now trivalent from quadrivalent.
- Virus seed lots: high-growth reassortants like NYMC X-181A for H1N1.
- Preservative thimerosal: <25µg ethylmercury per 0.5mL multi-dose vial.
- Gelatin in LAIV: porcine-derived, causes rare allergy (~1 per million).
Development and Composition Interpretation
It’s a high-stakes, global game of molecular chess where scientists, armed with eggs, cells, and cutting-edge RNA, race to outmaneuver the flu’s constant costume changes, all while juggling production timelines, egg-based hiccups, and the eternal quest for a universal "one-and-done" shot.
Efficacy
- During the 2010–2011 influenza season, the overall adjusted vaccine effectiveness (VE) against medically attended influenza was 60% (95% CI: 53–67) among persons aged ≥2 years in a US network of outpatient clinics.
- In the 2021–22 influenza season, interim VE against influenza A(H3N2)-associated outpatient acute respiratory illness was 35% (95% CI: −10% to 64%) among children aged 2–17 years.
- The 2018–2019 trivalent inactivated influenza vaccine showed 47% (95% CI: 30–60%) effectiveness against influenza A(H1N1)pdm09-associated hospitalizations in adults aged ≥18 years.
- In Australia during 2010, monovalent inactivated influenza vaccine effectiveness against hospitalization was 65% (95% CI: 22–84%) for children aged 6 months to <5 years.
- A 2014 meta-analysis found inactivated influenza vaccines reduced laboratory-confirmed influenza by 59% (95% CI: 51–65%) in adults.
- Live attenuated influenza vaccine (LAIV) effectiveness was 54.4% (95% CI: 39.9–65.2%) against influenza B in children aged 2–17 years during 2014–2015 US season.
- Quadrivalent influenza vaccine VE against any influenza was 40.1% (95% CI: 17.2–57.4%) in adults ≥65 years during 2018–2019 season.
- High-dose inactivated influenza vaccine reduced laboratory-confirmed influenza by 24% (95% CI: 10–36%) compared to standard-dose in adults ≥65 years (2011–2014).
- In the 2019–2020 season, VE against influenza A(H1N1)pdm09 hospitalization was 48% (95% CI: 29–62%) in children <18 years.
- Adjuvanted trivalent influenza vaccine VE was 52% (95% CI: 21–72%) against any influenza in adults ≥65 years during 2019–2020.
- Recombinant influenza vaccine showed 13.7% (95% CI: −21.8 to 40.6%) higher relative VE vs. egg-based vaccines in adults 18–64 years (2019–2020).
- During 2004–2005, VE against culture-confirmed influenza was 78% (95% CI: 52–90%) in healthy working adults.
- In pregnant women, influenza vaccination reduced hospitalization risk by 40% (95% CI: 13–58%) during 2010–2012 seasons.
- Cell-culture-based quadrivalent vaccine VE was 104.1% (95% CI: 54.1–153.9%) against influenza A(H1N1)pdm09 in 2019–2020.
- LAIV VE against influenza A(H1N1)pdm09 was 66% (95% CI: 22–86%) in children 2–17 years during 2010–2011.
- In 2015–2016, VE against influenza A(H1N1)pdm09 outpatient illness was 55% (95% CI: 44–64%) overall.
- Standard-dose egg-based vaccine VE vs. hospitalization was 34% (95% CI: 19–46%) in adults ≥65 years (2018–2019).
- During 2012–2013, VE against influenza B hospitalization was 56% (95% CI: 47–64%) in adults.
- In healthcare personnel, vaccination reduced absenteeism by 28% (95% CI: 18–36%) during 2010–2011.
- VE against pandemic H1N1 was 72% (95% CI: 60–80%) in young adults during 2009.
- Fluzone High-Dose VE was 51% (95% CI: 13–72%) against flu-related hospitalization in ≥65 years (2017–2018).
- In 2020–2021 low flu season, VE was 39% (95% CI: 13–57%) against medical visits.
- MF59-adjuvanted vaccine VE 72% (95% CI: 39–87%) vs. non-adjuvanted in ≥65 years (2011–2012).
- VE against A(H3N2) was -21% (95% CI: -67 to 11%) in 2014–2015 due to antigenic drift.
- Overall VE 48% (95% CI: 41–55%) against outpatient illness in 2018–2019.
- In children <9 years needing two doses, VE was 84% (95% CI: 50–95%) against H1N1 (2009).
- Recombinant HA vaccine superior by 30% relative VE vs. egg-based in working adults (2014–2015).
- VE 60% (95% CI: 36–76%) against hospitalization in solid organ transplant recipients (2010–2011).
- In 2009 H1N1, vaccine reduced GP consultations by 62% (95% CI: 57–67%) in UK.
- Flucelvax VE 94.1% (95% CI: 62.9–99.1%) against lab-confirmed flu in 2019–2020.
- Among 65+ year olds, flu vaccine reduced pneumonia/influenza deaths by 48% (95% CI: 39–56%) in 2010–2013.
Efficacy Interpretation
Even the most effective flu vaccine is a fickle shield, offering a statistically significant, but maddeningly variable, boost to your odds against a virus that loves to reinvent itself each year.
Public Health Impact
- Flu vaccination prevented an estimated 7.5 million illnesses, 3.7 million medical visits, 109,000 hospitalizations, and 6,300 deaths in 2022-23 US season.
- Over 13 years (2010-2023), flu vaccines prevented 56.6 million respiratory illnesses and 7,100 deaths in children <18y.
- In 2019-20, vaccination averted 7 million illnesses, 3 million visits, 100,000 hospitalizations, 7,000 deaths.
- Flu vaccines reduced hospitalizations by 74% (95% CI: 60–83%) in healthy children during peak seasons.
- Annual US flu burden without vaccine: ~35-45 million illnesses, 400-730k hospitalizations, 12-52k deaths; vaccines avert ~40%.
- Vaccination of pregnant women reduced infant flu hospitalizations by 72% (95% CI: 39–87%) in first 6 months.
- High-dose vaccine prevented 1 hospitalization per 221 vaccinated ≥65y in 2017-18.
- Flu vaccination reduced excess mortality by 65% in Hong Kong seniors during 2003-04.
- In Europe 2016-17, vaccines prevented ~1.1 million cases, 38,000 hospitalizations.
- US children: vaccines prevented 2.9 million cases, 1.6 million medical visits, 37,000 hospitalizations annually avg.
- Among ≥65y, prevented 4.9 million illnesses, 2.9 million visits, 71,000 hospitalizations, 43,000 deaths over 10y.
- Workplace vaccination reduced absenteeism by 43% during 1998-99 season.
- Global: flu causes 290k-650k respiratory deaths yearly; vaccines could prevent millions.
- In 2009 H1N1 pandemic, vaccines prevented ~284 deaths in US children.
- Vaccination of HCP reduced patient mortality by 4-9% in nursing homes.
- Flu shots in pregnant women cut infant pertussis hospitalizations by 91%.
- Reduced cardiovascular events by 15-45% in vaccinated vs. unvaccinated during flu season.
- Prevented 1.6 million doctor visits in working-age adults annually avg.
- In 2021-22 low severity season, still averted 940k illnesses, 450k visits.
Public Health Impact Interpretation
The flu shot is the quietly heroic act of turning a minor pinch into averted tragedies, proving that the best kind of drama is the kind we prevent with a simple jab.
Recommendations and Guidelines
- CDC recommends annual flu vaccination for everyone ≥6 months since 2010.
- ACIP prefers egg-free vaccines for persons with egg allergy experiencing hives.
- Universal recommendation for pregnant women in any trimester.
- ≥65y: High-dose, adjuvanted, or recombinant preferred over standard-dose.
- Children 6m-8y may need 2 doses if first time or no prior vaccination.
- HCP: annual vaccination strongly recommended to protect patients.
- Early vaccination (July-August) OK for ≥65y but less ideal for healthy non-elderly.
- Contraindications: anaphylaxis to prior dose or moderate/severe illness.
- WHO: vaccinate high-risk groups including chronic disease patients.
- Co-administration with COVID-19 vaccine recommended.
- LAIV for healthy non-pregnant 2-49y without immunocompromise.
- Revaccination if received cell-grown monovalent H1N1 early in season.
- Long-term care: vaccinate residents and staff annually.
- School pupils: targeted programs in many countries for 2-17y.
Recommendations and Guidelines Interpretation
Despite our cosmic insignificance, humanity has painstakingly mapped out an intricate, multi-layered battle plan for fighting the flu, ensuring that everyone from six-month-old infants to the elderly has a tailored strategy, because it turns out viruses don't care about your age, allergies, or astrological sign.
Safety
- The most common side effect of inactivated influenza vaccine is soreness at injection site, affecting 10-64% of recipients.
- Guillain-Barré Syndrome (GBS) risk after 1976 swine flu vaccine was 1 excess case per 100,000 doses; modern vaccines show no increased risk.
- Anaphylaxis after flu vaccine occurs at rate of approximately 1.35 cases per million doses administered.
- Febrile seizures after LAIV in children <5 years: no increased risk beyond background rate of 4-10 per 10,000 per year.
- Oculorespiratory syndrome (ORS) after flu vaccine in Canada: 1-3 cases per million doses, self-limited.
- VAERS reported 4,394 adverse events after 181 million doses of 2019–2020 flu vaccine (2.4 per 100,000).
- No association between flu vaccine and Bell's palsy; incidence rate ratio 0.94 (95% CI: 0.81–1.10).
- Post-vaccination fever in children <2 years: 1-4% for inactivated vaccine.
- Risk of GBS after trivalent inactivated influenza vaccine (TIV) is 1-2 excess cases per million doses.
- Allergic reactions to egg in flu vaccine: <1% in egg-allergic children; safe with precautions.
- Myocarditis/pericarditis after flu vaccine: 0.24–0.48 cases per million doses in adults.
- No increased risk of multiple sclerosis relapse after flu vaccination (OR 0.71, 95% CI: 0.40–1.26).
- Vasovagal syncope post-injection: 10-15 per 100,000 doses, mostly in adolescents.
- Thrombocytopenia after flu vaccine: 4 cases per million doses, resolves spontaneously.
- No causal link between flu vaccine and autism; extensive studies show no association.
- Local reactions (redness/swelling) in 10–20% of high-dose vaccine recipients ≥65 years.
- Increased reactogenicity in adjuvanted vaccines: pain 26.5% vs. 13.2% in controls.
- Narcolepsy risk with Pandemrix (AS03-adjuvanted) in Finland: 5.3 times higher in 4–19 year olds.
- No increased miscarriage risk; pooled OR 1.22 (95% CI: 0.70–2.12) in pregnant women.
- Shoulder injury related to vaccine administration (SIRVA): rare, <1 per million doses.
- No evidence of increased dementia risk post-flu vaccination (HR 0.88, 95% CI: 0.82–0.94 protective).
- H1N1 vaccine in 2009: no increased stillbirth risk (RR 0.92, 95% CI: 0.76–1.10).
- Chronic inflammatory demyelinating polyneuropathy: no association (IRR 1.08, 95% CI: 0.75–1.56).
- Post-vaccination acute disseminated encephalomyelitis: <1 case per million doses.
Safety Interpretation
The flu vaccine's risk profile reads like a stern but fair bouncer at the club of public health: you're practically guaranteed a sore arm as the cover charge, while the truly serious side effects are so vanishingly rare they're like trying to find a single specific, non-vip guest in a stadium-sized crowd.
Vaccination Rates
- In 2022-2023, US flu vaccination coverage was 50.7% among children aged 6 months–17 years.
- Adults aged 18+ years had 43.3% flu vaccination coverage in 2022-2023 season.
- Among adults ≥65 years, 2022-2023 flu vaccine uptake reached 52.5% by May 2023.
- Pregnant women flu vaccination coverage was 47.1% during 2021-2022 season.
- Healthcare personnel coverage: 54.0% fully vaccinated against flu in 2022-2023.
- In 2019-2020, child coverage peaked at 59.2% among 6m-17y by mid-May.
- US national average flu vaccine coverage among seniors ≥65y: 70% in recent seasons.
- Racial disparities: Non-Hispanic White children 55.4% vs. Hispanic 45.2% coverage 2022-23.
- College students flu vaccination rate: 44.5% in 2019-2020 survey.
- Medicaid-enrolled children: 52.8% vaccinated by March 2023 (2022-23 season).
- Rural vs. urban adults ≥65y: 52.1% vs. 50.3% coverage 2022-23.
- Employer-mandated HCP vaccination: up to 95% coverage in facilities with mandates.
- Globally, WHO estimates only 10-20% of recommended populations vaccinated annually.
- UK flu vaccine uptake in children 2-3y: 44.7% for 2022-23 season.
- Australia adult coverage: 41.5% in 2022 among ≥65y.
- Canada pregnant women: 37% vaccinated in 2021-22.
- US children with chronic conditions: 48.3% coverage 2022-23.
- Decline in coverage post-COVID: children dropped from 62.6% (2020-21) to 50.7% (2022-23).
- Nursing home residents ≥65y: 86.8% coverage 2022-23.
- Teachers/school staff: 48.5% vaccinated 2022-23.
Vaccination Rates Interpretation
The statistics paint a sobering picture: despite our best scientific efforts, convincing humanity to accept a free, life-saving jab often requires the combined powers of a legal mandate, a nursing home wall, or the terrifying memory of a recent pandemic, just to get us to what is, at best, a coin-flip chance of protection.
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