How well do flu vaccines work?

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1. 1

   Vaccines for preventing influenza in the elderly

   Cochrane Database of Systematic Reviews (Demicheli et al.)Published Feb 1, 2018Checked Sep 30, 2026

   “AUTHORS' CONCLUSIONS: Older adults receiving the influenza vaccine may have a lower risk of influenza (from 6% to 2.4%), and probably have a lower risk of ILI compared with those who do not receive a vaccination over the course of a single influenza season (from 6% to 3.5%). We are uncertain how big a difference these vaccines will make across different seasons. Very few deaths occurred, and no data on hospitalisation were reported. No cases of pneumonia occurred in one study that reported this outcome. We do not have enough information to assess harms relating to fever and nausea in this population.The evidence for a lower risk of influenza and ILI with vaccination is limited by biases in the design or conduct of the studies. Lack of detail regarding the methods used to confirm the diagnosis of influenza limits the applicability of this result. The available evidence relating to complications is of poor quality, insufficient, or old and provides no clear guidance for public health regarding the safety, efficacy, or effectiveness of influenza vaccines for people aged 65 years or older. Society should invest in research on a new generation of influenza vaccines for the elderly.”

2. 2

   Effectiveness of Influenza Vaccines in Asthma: A Systematic Review and Meta-Analysis

   Clinical Infectious Diseases (Vasileiou et al.)Published Jun 6, 2017Checked Sep 30, 2026

   “There is uncertainty about the effectiveness of influenza vaccination in persons with asthma and its impact on asthma outcomes, which may contribute to the suboptimal vaccination rates in persons with asthma. This systematic review and meta-analysis involved searching 12 international databases for randomized controlled trials (RCTs) and high-quality quasi-experimental and epidemiological studies (1970-2016). The risk of bias was low for 3 included RCTs. The quality of 3 included observational studies was moderate. The quality of evidence was very low for all study outcomes. Pooled vaccine effectiveness in 1825 persons with asthma from 2 test-negative design case-control studies was 45% (95% confidence interval [CI], 31%-56%) for laboratory-confirmed influenza. Pooled efficacy of live vaccines in reducing influenza was 81% (95% CI, 33%- 94%). Live vaccine reduced febrile illness by 72% (95% CI, 20%-90%). Influenza vaccine prevented 59%-78% of asthma attacks leading to emergency visits and/or hospitalizations. For persons with asthma, influenza vaccination may be effective in both reducing influenza infection and asthma attacks.”

3. 3

   Indirect Influenza Vaccine Effectiveness Under Randomized Conditions: A Systematic Review and Meta-Analysis.

   Clinical infectious diseases : an official publication of the Infectious Diseases Society of America (Gilmore et al.)Published Sep 1, 2026Checked Oct 4, 2026

   “Risk of bias was low. Across trials conducted between 2008 and 2013, estimates of direct vaccine effectiveness (DVE) were moderate, whereas IVE estimates were lower and often centered near no effect, with the CIs for 9 of 12 trials including zero. The pooled DVE was 44.1% (95% CI: 29.6%, 55.6%), and the pooled IVE was 13.7% (95% CI: 1.5%, 24.5%). Sensitivity analyses excluding pandemic outcomes yielded minimally higher point estimates and did not materially change the findings. Between-study heterogeneity was moderate. There was no evidence of publication bias. Certainty of evidence was rated as moderate.ConclusionsThis review provides a methodologically rigorous benchmark of IVE measurable under randomized conditions. While pooled DVE was moderate, pooled IVE was lower, and most individual trials did not demonstrate statistically significant indirect effects. The findings indicate that individual vaccination is the most reliable means of preventing influenza, rather than depending on indirect or community-level protection.”

4. 4

   Influenza vaccine (Wikipedia)

   WikipediaPublished Sep 29, 2026Checked Sep 30, 2026

   “Influenza vaccines, colloquially known as flu shots or flu jab, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their effectiveness varies from year to year, most provide modest to high protection against influenza. Vaccination against influenza began in the 1930s, with large-scale availability in the United States beginning in 1945. Both the World Health Organization and the US Centers for Disease Control and Prevention (CDC) recommend yearly vaccination for nearly all people over the age of six months, especially those at high risk, and the influenza vaccine is on the World Health Organization's List of Essential Medicines. The European Centre for Disease Prevention and Control (ECDC) also recommends yearly vaccination of high-risk groups, particularly pregnant women, the elderly, children between six months and five years, and those with certain health problems. The vaccines are generally safe, including for people who have severe egg allergies. A common side effect is soreness near the site of injection.”

5. 5

   The impact of repeated vaccination on influenza vaccine effectiveness: a systematic review and meta-analysis

   BMC Medicine (Ramsay et al.)Published Jan 8, 2019Checked Sep 30, 2026

   “We identified 3435 unique articles, reviewed the full text of 634, and included 20 for meta-analysis. Compared to prior season vaccination only, vaccination in both seasons was associated with greater protection against influenza H1N1 (∆VE = 25%; 95% CI 14%, 35%) and B (∆VE = 18%; 95% CI 3%, 33%), but not H3N2 (∆VE = 7%; 95% CI - 7%, 21%). Compared to no vaccination for either season, individuals who received the current season's vaccine had greater protection against H1N1 (∆VE = 62%; 95% CI 51%, 70%), H3N2 (∆VE = 45%; 95% CI 35%, 53%), and B (∆VE = 64%; 95% CI 57%, 71%). We observed no differences in VE between vaccination in both seasons and the current season only for H1N1 (∆VE = 3%; 95% CI - 8%, 13%), but less protection against influenza H3N2 (∆VE = - 20%; 95% CI - 36%, - 4%), and B (∆VE = - 11%; 95% CI - 20%, - 2%). Our results support current season vaccination regardless of prior season vaccination because VE for vaccination in the current season only is higher compared to no vaccination in either season for all types/subtypes, and for H1N1 and influenza B, vaccination in both seasons provides better VE than vaccination in the prior season only.”

6. 6

   Effectiveness of Influenza Vaccines and Duration of Protection Against Hospitalisation During the 2024/25 Season in Northern Ireland, UK.

   Influenza and other respiratory viruses (Bucholc et al.)Published Aug 1, 2026Checked Oct 4, 2026

   “VE against laboratory-confirmed influenza infection was 46.0% (95% CI: 39.7% to 51.8%), with higher VE in children aged 2-17 years (60.8%; 95% CI: 48.3% to 70.5%) than in adults aged 18-64 years (40.5%; 95% CI: 24.9% to 53.2%) and ≥ 65 years (42.3%; 95% CI: 33.2% to 50.1%). VE against influenza A across all ages was 41.4% (95% CI: 34.2% to 47.9%). Vaccination reduced the odds of hospitalisation due to influenza A(H1) by 44.3% (95% CI: 33.2% to 53.7%) and A(H3) by 49.9% (95% CI: 20.3% to 69.3%). VE against influenza B was higher at 76.4% (95% CI: 64.9% to 84.7%). For influenza A, VE was highest 2-8 weeks after vaccination at 51.9% (95% CI: 42.1% to 60.1%) and declined with time since vaccination to 44.6% (95% CI: 35.6% to 52.5%) at 9-16 weeks and 41.4% (95% CI: 15.4% to 60.1%) at ≥ 16 weeks. VE against influenza B remained high throughout the season. No statistically significant differences in VE by vaccine type were found.ConclusionsInfluenza vaccination reduced the risk of hospitalisation with laboratory-confirmed influenza during the 2024/25 season, offering meaningful protection at individual and population levels, with the greatest benefit observed in children.”

7. 7

   A systematic meta-analytic comparative evaluation of seasonal influenza vaccine effectiveness from test-negative design studies in the Northern Hemisphere pre/post COVID-19 pandemic.

   Vaccine (Okoli et al.)Published Jul 22, 2026Checked Oct 4, 2026

   “Pooled VE against A(H1N1)pdm09, A(H3N2), influenza B, and B/Victoria was calculated for three age groups (6 months-17 years, 18-64 years, ≥65 years) using inverse-variance random-effects meta-analysis. Heterogeneity was quantified with the I2 statistic, and differences between pooled VE estimates pre- and post-pandemic periods were tested with the χ2 statistic.ResultsEighty-five publications were included. Post-pandemic VE against A(H3N2) was significantly higher than pre-pandemic VE in 6-month-17-year-olds (53% [42-64%] vs 37% [30-45%]) and 18-64-year-olds (35% [25-45%] vs 21% [13-29%]). Similarly, VE against influenza B was higher post-pandemic in both age groups (85% [78-93%] vs 52% [45-60%] and 73% [59-87%] vs 49% [41-57%], respectively). Conversely, VE against A(H1N1)pdm09 was lower post-pandemic in 18-64-year-olds (37% [27-46%] vs 55% [47-62%]). No significant differences were observed for ≥65-year-olds, and data for B/Victoria were limited.ConclusionsThe evidence suggests slightly higher VE against seasonal influenza after the COVID-19 pandemic although there may be factors other than population immunity that may explain our observations.”

8. 8

   Effectiveness of MF59-adjuvanted seasonal influenza vaccine in the elderly: A systematic review and meta-analysis

   Vaccine (Domnich et al.)Published Dec 23, 2016Checked Sep 30, 2026

   “Of the 1993 papers screened, 11 (6 case-control, 3 cohort and 2 prospective case-control) studies were identified. Hospitalization due to pneumonia/influenza and laboratory-confirmed influenza were reported in more than one study, while other outcomes (influenza-like illness, cardio- and cerebrovascular accidents) were investigated only by one study each. Pooled analysis of four case-control studies showed an adjusted MF59-TIV effectiveness of 51% (95% CI: 39-61%) against hospitalizations for pneumonia/influenza among community-dwelling seniors. Pooled results of the adjusted vaccine effectiveness against laboratory-confirmed influenza were also high (60.1%), although the 95% CI passed through zero (-1.3 to 84.3%). Other single community-based studies showed very high effectiveness of MF59-TIV in preventing hospitalizations for acute coronary [87% (95% CI: 35-97%)] and cerebrovascular [93% (95% CI: 52-99%)] events. Our results suggest that MF59-TIV is effective in reducing several influenza-related outcomes among the elderly, especially hospitalizations due to influenza-related complications.”

9. 9

   Influenza vaccine effectiveness according to frailty status in adults aged ≥ 50 years: a systematic review and meta-analysis.

   BMC infectious diseases (Pott et al.)Published Jul 11, 2026Checked Oct 4, 2026

   “Effect sizes were reported as odds ratios (ORs) with 95% confidence intervals (CIs).ResultsEight studies with 262,952 pooled participants were included. The prevalence of frailty ranged from 14.3% to 36.4%, with a pooled estimate of 24.3% (95% CI, 19.0%-30.4%; I²=99.8%). Four test-negative studies were pooled for LCI cases. Vaccination was associated with reduced odds of LCI (pooled OR, 0.53; 95% CI, 0.37-0.76; I²=4.0%), with a 95% prediction interval of 0.30-0.96. Stratified analyses showed pooled ORs of 0.43 (95% CI, 0.24-0.75) and 0.66 (95% CI, 0.35-1.25) in non-frail and frail individuals, respectively; no statistically significant effect modification was detected (p = 0.10). Hospitalization and mortality outcomes were not pooled because of heterogeneity. The certainty of evidence was low for LCI and very low for other outcomes.ConclusionsInfluenza vaccination protected against LCI across frailty strata, though study heterogeneity requires cautious interpretation, and estimates were less precise for frail individuals. The elevated risk of adverse clinical outcomes in this age group supports immunization, and current data do not justify withholding vaccination.”

10. 10

    Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices—United States, 2018–19 Influenza Season

    MMWR Recommendations and Reports (Grohskopf et al.)Published Aug 23, 2018Checked Sep 30, 2026

    “For ccIIV4, history of a severe allergic reaction (e.g., anaphylaxis) to any ccIIV of any valency or any component of ccIIV4 is a contraindication to future use of ccIIV4. For RIV4, history of a severe allergic reaction (e.g., anaphylaxis) to any RIV of any valency or any component of RIV4 is a contraindication to future use of RIV4. This report focuses on recommendations for the use of vaccines for the prevention and control of seasonal influenza during the 2021-22 influenza season in the United States. A brief summary of the recommendations and a link to the most recent Background Document containing additional information are available at https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/flu.html. These recommendations apply to U.S.-licensed influenza vaccines used according to Food and Drug Administration-licensed indications. Updates and other information are available from CDC's influenza website (https://www.cdc.gov/flu); vaccination and health care providers should check this site periodically for additional information.”