Editor’s Note:
Each year, as autumn and winter arrive, influenza viruses return as expected, which poses a threat not only to individual health but also places a significant burden on public health systems. Vaccination remains the most effective and cost-efficient means of preventing influenza and its severe complications.
As the flu season approaches, we will release two special weekly series featuring summaries of scientific evidence, implementation experiences, and intervention practices related to influenza vaccination—primarily based on China’s domestic research and field data. This series aims to provide useful references for disease control and public health authorities in strengthening influenza vaccination programs, health education, and policy advocacy.
Evidence for Different Population Group
1. Children
01
Preplanned Studies: Comparison of 1 Versus 2 Doses of Quadrivalent Influenza Vaccine in 3–8-Year-Old Children with Different Immunological States—Jiangsu Province, China, 2021
For children aged 3 to 8 who have previously received two or more doses of influenza vaccine, one dose of seasonal influenza vaccine per year is sufficient to provide protection. The study recommends that children receiving the influenza vaccine for the first time should receive two doses to achieve optimal immune responses.
This study used an open-label, self-controlled clinical trial design to evaluate the immunogenicity and safety of receiving one or two doses of quadrivalent influenza vaccine (QIV), taking into account each child’s prior influenza vaccination history. The results showed that among children with a history of influenza vaccination (Group A), the geometric mean titers (GMTs) against all four strains (H1N1, BY, BV) increased significantly after the first dose. After the second dose, the GMTs for the H1N1 and BV strains showed slight decreases.
In children receiving the influenza vaccine for the first time, all strains elicited a clear antibody response after the first dose, and GMTs for all four strains rose further after the second dose. No serious adverse events occurred in any group throughout the study.
https://weekly.chinacdc.cn/en/article/doi/10.46234/ccdcw2024.082
02
Immunogenicity and safety of live attenuated influenza vaccine in children aged 3–17 years in China
The live attenuated influenza vaccine (LAIV) demonstrates good immunogenicity and safety among children and adolescents aged 3–17 years in China. Using a phase III, multicenter, randomized, double-blind, placebo-controlled trial design, the study found that 625 participants (41.7%) in the vaccine group and 591 participants (39.4%) in the placebo group experienced adverse reactions. The frequencies of nasal congestion (P = 0.032), headache (P = 0.045), and myalgia (P = 0.047) were significantly higher in the vaccine group than in the placebo group.
Regarding immunogenicity, baseline hemagglutination inhibition antibody titers (HI GMTs) were comparable between the vaccine and placebo groups for all influenza virus types. After vaccination, the GMTs for all virus types were higher in the vaccine group compared with the placebo group. The geometric mean fold increase (gMFI) for vaccine-related virus serotypes was also higher in the vaccine group than in the placebo group.
https://doi.org/10.1016/j.vaccine.2024.126653
03
Effectiveness and safety of a novel intranasal influenza vaccine in Chinese children: A phase IV multi-Center, randomized, double-blind, placebo-controlled clinical trial
Using a multicenter, randomized, double-blind, placebo-controlled phase IV clinical trial design, the study systematically evaluated the protective effectiveness and safety of the domestically produced intranasal trivalent live attenuated influenza vaccine (Ganwu®) in children aged 3–17 years. The overall effectiveness of the vaccine in preventing laboratory-confirmed influenza was 46.8% (95% CI: 25.8%–61.9%). Subtype-specific effectiveness was 30.4% for A/H1N1, 55.1% for A/H3N2, and 84.6% for B/Victoria. The incidence of laboratory-confirmed influenza was significantly lower in the vaccine group compared with the control group (HR = 0.53, 95% CI: 0.38–0.74). In addition, the vaccine provided 15.2% protection against influenza-like illness (ILI) (95% CI: –2.3%–29.7%). The safety profile was favorable.
https://doi.org/10.1016/j.vaccine.2025.127268
2. Pregnant Women
01
A Retrospective Real-World Cohort Study on the Effectiveness of Influenza Vaccination During Pregnancy in Preventing Influenza-Like Illness Among Pregnant Women in Gansu Province
Using data from Gansu Province’s electronic medical record system and the immunization information system, this study constructed a retrospective matched cohort of pregnant women who received an influenza vaccine during pregnancy in the 2019–2021 influenza seasons and assessed influenza-like illness (ILI) occurring within six months postpartum. Influenza vaccination during pregnancy effectively reduced the incidence of ILI among pregnant women, with particularly notable protection among those aged 30–45 years. The incidence of ILI was 4.70% (54/1,150) among vaccinated pregnant women and 6.91% (16,990/245,754) among unvaccinated women. The adjusted overall vaccine effectiveness against ILI was 33.55% (95% CI: 12.71%–49.41%).
https://doi.org/10.19914/j.CJVI.2024071
02
Maternal Vaccine Effectiveness Against Influenza-Associated Hospitalizations and Emergency Department Visits in Infants
Using data from seven pediatric institutions in the United States between 2016 and 2020, the study included 3,764 infants (223 influenza-positive), of whom 2,007 mothers (53%) received an influenza vaccine during pregnancy. The overall vaccine effectiveness was 34% (95% CI: 12%–50%), with a 39% reduction in influenza-associated hospitalizations (95% CI: 12%–58%) and a 19% reduction in influenza-related emergency department visits (95% CI: –24%–48%). Among infants younger than 3 months, vaccine effectiveness reached 53% (95% CI: 30%–68%). Maternal vaccination during the third trimester demonstrated 52% effectiveness in preventing infant influenza (95% CI: 30%–68%), compared with 17% effectiveness (95% CI: –15%–40%) when vaccination occurred during the first or second trimester.
International Evidence: https://doi.org/10.1001/jamapediatrics.2023.5639
03
Reliability and validity study of the “5Cs” hesitancy scale for maternal influenza vaccination among pregnant and postpartum women
A cross-sectional survey of 2,035 pregnant and postpartum women was conducted across nine provinces in China between January and March 2024. The scale developed in this study can serve as a standardized tool for assessing influenza vaccine hesitancy among Chinese pregnant women. Results showed that participants scored highest on the “collective responsibility” dimension (2.73 ± 0.63), while scores for “complacency” (2.16 ± 0.69) and “constraints” (2.17 ± 0.69) were the lowest. These findings suggest that although the public generally recognizes the social value of vaccination, awareness of influenza-related risks remains insufficient, and barriers to accessing vaccination services persist.
https://doi.org/10.1186/s40249-025-01295-8
04
Maternal influenza vaccination and associated risk of fetal loss: A claims-based prospective cohort study
Using de-identified national administrative claims data from the Optum Labs Data Warehouse, the study analyzed influenza vaccination patterns and pregnancy outcomes. Results showed that 31.4% of participants had a record of influenza vaccination: 10.0% were vaccinated only before pregnancy, 17.8% only during pregnancy, and 3.6% both before and during pregnancy. Compared with unvaccinated individuals, vaccination during pregnancy was associated with a 39% reduction in the risk of spontaneous abortion (adjusted hazard ratio [aHR] = 0.61; 95% CI: 0.50, 0.74). The risks of medically indicated termination and stillbirth were HR = 0.69 (95% CI: 0.45, 1.03) and aHR = 0.99 (95% CI: 0.76, 1.30), respectively. Similar patterns were observed among women vaccinated both before and during pregnancy. Vaccination prior to pregnancy was not associated with spontaneous abortion, medically indicated termination, or stillbirth.
https://doi.org/10.1016/j.vaccine.2024.126256
05
Knowledge, Attitudes, and Practices Towards the Influenza Vaccine Among Pregnant Women: A Systematic Review of Cross-Sectional Studies
Common barriers to vaccination among pregnant women include concerns about side effects, misconceptions regarding vaccine effectiveness, limited accessibility, time costs, and certain cultural taboos. Although most pregnant women possess basic knowledge about influenza, significant gaps remain in their understanding of transmission routes, risks of pregnancy-related complications, and vaccine safety. For example, a study in Singapore found that while 90% of pregnant women knew influenza is caused by a virus, 46% mistakenly believed that antibiotics are the primary treatment. In China, 86.3% of respondents were worried that vaccination might negatively affect the fetus. Nearly all studies consistently identify healthcare providers’ recommendations as the key factor influencing pregnant women’s vaccination behaviors.
International Evidence: https://doi.org/10.3390/healthcare13111290
3. Older Adults
01
Influenza Vaccination and Short-Term Risk of Stroke Among Elderly Patients With Chronic Comorbidities in a Population-Based Cohort Study
A population-based cohort study conducted in four districts of Shanghai found a significant association between influenza vaccination and reduced stroke risk. Multivariable-adjusted Cox regression analysis showed that the adjusted hazard ratio (aHR) was 0.27 (95% CI: 0.10–0.73) for the 2017–2018 influenza season and 0.46 (95% CI: 0.21–1.02) for the 2018–2019 season. Results from a Poisson regression model (RR = 0.26, 95% CI: 0.10–0.70) were consistent with those of the Cox model. In addition, influenza vaccination was associated with a 54%–73% reduction in one-year stroke risk among patients with chronic diseases.
https://doi.org/10.1111/jch.70044
02
The immunogenicity and safety of an inactivated quadrivalent influenza vaccine and a 23-valent pneumococcal polysaccharide vaccine in individuals with chronic diseases
The study adopted a multicenter, randomized, controlled, open-label, non-inferiority design, randomly assigning 480 participants aged ≥60 years to receive either coadministration of the inactivated quadrivalent influenza vaccine (IIV4) and the 23-valent pneumococcal polysaccharide vaccine (PPSV23) or administration of each vaccine alone. Immunogenicity analyses showed that 28 days after vaccination, the geometric mean titer (GMT) ratios (chronic disease group/healthy group) for influenza antibodies ranged from 1.04 to 1.37 in the overall population (with the lower limit of the 95% CI > 0.5), and from 1.02 to 1.39 in the coadministration group. The geometric mean concentration (GMC) ratios for pneumococcal antibodies ranged from 0.87 to 1.12 in the overall population and from 0.97 to 1.33 in the coadministration group, all meeting non-inferiority criteria. No vaccine-related serious adverse events were reported in either the chronic disease or healthy groups.
https://doi.org/10.3389/fimmu.2025.1624095
03
Reduced risks of influenza-associated hospitalization and complications following vaccination among over 2 million older individuals: a nationwide study using target trial emulation framework
Using data from over 2 million individuals in China Taiwan’s Health Insurance Research Database (NHIRD), the study found that influenza vaccination provides substantial real-world clinical protection for older adults. The vaccine reduced the risk of influenza-related hospitalization by 14% (95% CI: 10%–18%), with consistent effectiveness across different age groups, frailty levels, and high-risk populations. It also effectively lowered the incidence of severe influenza events (such as hospitalization) and related complications. However, vaccine effectiveness varied across subpopulations. Protection was reduced among adults aged ≥75 years and those with frailty, and it declined progressively throughout the influenza season after vaccination, underscoring the importance of regular annual influenza immunization.
https://doi.org/10.1186/s12916-025-03955-w
4. Populations with Special Health Conditions
01
Influenza vaccination to improve outcomes for patients with acute heart failure (PANDA II): a multiregional, seasonal, hospital-based, cluster-randomised, controlled trial in China
Based on a multi-regional, seasonal, hospital-based cluster randomized controlled trial conducted in China, the study found that administering seasonal influenza vaccination to patients hospitalized for acute heart failure before discharge significantly reduced the risk of all-cause death or readmission within 12 months, with a favorable safety profile. In the intervention group, an “in-hospital influenza vaccination” strategy was implemented, whereas the usual-care group received standard advice to obtain vaccination at community clinics at their own expense. Vaccination coverage reached 94.4% in the intervention group compared with 0.5% in the control group. The incidence of the primary composite outcome (all-cause death plus all-cause readmission) was 41.2% (1378/3342) in the vaccinated group and 47.0% (1843/3919) in the usual-care group, yielding an odds ratio (OR) of 0.83 (95% CI: 0.72–0.97; p=0.019), demonstrating a statistically significant reduction in risk.
https://doi.org/10.1016/S0140-6736(25)01485-0
02
Adverse events affecting recovery from seasonal influenza vaccination in the hypertensive population: A population-based pharmacovigilance analysis
Among individuals with hypertension, influenza vaccination is generally safe, with most adverse events (AEs) being mild; however, some experience short-term delays in recovery due primarily to injection-site reactions. Using data from the U.S. Vaccine Adverse Event Reporting System (VAERS), the study analyzed 6,380 vaccine-related AEs reported among 4,647 hypertensive patients from January 2013 to June 2023, categorizing cases into a “recovered” and a “not recovered” group. Results showed that non-serious AEs accounted for 90.3% of all reports, with most occurring within 48 hours after vaccination. Injection-site reactions were identified as the leading contributors to delayed recovery in hypertensive individuals.
International Evidence: https://doi.org/10.1371/journal.pone.0310474
03
Outcomes following diabetes admission in patients who had influenza vaccination: A population-based cohort study
A retrospective cohort study based on China Taiwan’s health insurance data from 2008 to 2013 showed that seasonal influenza vaccination significantly reduces the risk of complications and mortality among hospitalized patients with diabetes. The health benefits were particularly pronounced in patients with poor glycemic control, a history of prior diabetes-related hospitalizations, or coexisting diabetes-related complications. Specifically, vaccinated patients experienced a lower risk of in-hospital death within 30 days (OR 0.75, 95% CI: 0.66–0.84), as well as reduced risks of pneumonia (OR 0.92, 95% CI: 0.87–0.97), sepsis (OR 0.83, 95% CI: 0.79–0.88), urinary tract infections (OR 0.94, 95% CI: 0.90–0.97), and intensive care unit admission (OR 0.29, 95% CI: 0.27–0.30).
https://doi.org/10.1016/j.diabres.2022.109930
5. Population-wide Strategies
01
Influenza vaccine effectiveness against medically-attended influenza infection in 2023/24 season in Hangzhou, China
During the 2023/2024 influenza season in Hangzhou, influenza vaccines provided moderate protection. Notably, the trivalent inactivated vaccine (IIV3) demonstrated superior effectiveness in preventing influenza B infections. Among the 157,291 influenza-like illness (ILI) cases included in the study, 56,704 (36%) tested positive for influenza. The adjusted overall vaccine effectiveness (VE) against laboratory-confirmed influenza was 48% (95% CI: 46%–51%). By vaccine type, IIV3 showed the highest overall VE at 59% (95% CI: 50%–66%), followed by the trivalent live-attenuated influenza vaccine (LAIV3) at 53% (95% CI: 42%–62%) and the quadrivalent inactivated vaccine (IIV4) at 47% (95% CI: 45%–50%). Importantly, IIV3 provided significantly greater protection against influenza B virus infection compared with IIV4.
https://doi.org/10.1080/21645515.2024.2435156
02
The optimal strategy for seasonal influenza vaccination to prevent high-intensity level of influenza epidemics in Zhejiang, China: an integrated transmission-dynamic and health-economic modeling analysis
This study used weekly influenza surveillance data to estimate the influenza incidence in Zhejiang Province, China, from 2018 to 2023, and constructed a “Susceptible–Vaccinated–Infected–Recovered–Susceptible (SVIRS)” transmission dynamics model to simulate influenza spread. A decision tree model was applied to evaluate the preventive impact of seven vaccination strategies during high-intensity influenza seasons. Simulation results indicated that increasing the overall population vaccination coverage to 36.17% could effectively prevent the outbreak from developing into a high-intensity epidemic. Among various supplementary vaccine allocation strategies, prioritizing coverage for children aged 0–14 years and adults aged 60 years and older was most effective. This approach could reduce the weekly peak incidence from the observed 18.283 per 100,000 to a predicted 4.228 per 100,000, save 23.45 CNY per capita, and achieve a per capita health gain of 0.0102 quality-adjusted life days (QALDs).
https://doi.org/10.1186/s12889-025-23648-6
Content Editor: Tianyi Deng
Page Editor: Ruitong Li
