Preliminary Safety Assessment of a Quadrivalent Subunit Influenza Vaccine in China: A Phase IV Study

Abstract

Introduction

The aim of this work is to preliminarily evaluate the post-marketing safety of a quadrivalent subunit influenza vaccine in healthy individuals aged 3 years and above through active surveillance of adverse reactions following vaccination.

Methods

During the 2024–2025 influenza season, healthy participants aged 3 years and older were enrolled in Jiangsu Province. All participants received a single dose of the quadrivalent subunit influenza vaccine. Adverse events (AEs) and serious adverse events (SAEs) were monitored within 0–28 days post-vaccination, and information on SAEs occurring within 6 months post-vaccination was also collected. Descriptive analyses were performed to calculate the reporting rates of AEs, adverse drug reactions (ADRs) and SAEs. We compared differences in overall and symptom-specific ADR reporting rates across age groups and sexes.

Results

A total of 3105 participants completed vaccination and were included in the safety analysis set. A total of 282 participants reported 477 AEs, yielding an overall AE reporting rate of 9.08%. A total of 188 participants reported 318 ADRs, with an ADR reporting rate of 6.05%. The most frequently reported ADR symptoms were tenderness (69 episodes, reporting rate 2.22%), cough (57 episodes, 1.83%), and fever (46 episodes, 1.48%). The majority of ADRs (272 episodes, 85.53%) were grade 1 in severity, and most symptoms (253 episodes, 79.56%) resolved within 3 days of onset. No vaccine-related SAEs were reported, and no new serious adverse reactions were identified.

Conclusions

This active surveillance study demonstrated a favorable safety profile for the quadrivalent subunit influenza vaccine, with low ADR reporting rates and predominantly mild, transient reactions. These findings provide reliable evidence supporting the preliminary post-marketing safety assessment of the vaccine in China.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s40121-026-01439-9.

Key Summary Points

Why carry out this study?

Safety data from prelicensure clinical trials are inherently constrained by limited sample sizes and restricted population diversity. Consequently, post-marketing surveillance is essential for ongoing safety monitoring and for detecting unexpected serious adverse events that may not have been identified in premarket studies.

This single-center prospective active surveillance study aimed to evaluate the safety profile of the quadrivalent subunit influenza vaccine in healthy individuals aged ≥3 years, including long-term safety monitoring through the collection of serious adverse events within 6 months post-vaccination.

What was learned from this study?

The vaccine demonstrated a favorable safety profile in healthy individuals aged ≥3 years, with a low adverse drug reaction reporting rate (6.05%) and predominantly mild, transient reactions. No vaccine-related serious adverse events were identified during the 6-month follow-up period, and no new safety signals were detected.

The traditional active surveillance approach combining investigator-led follow-up with participant self-reporting proved feasible for post-marketing vaccine safety monitoring in China.

Introduction

Influenza is an acute respiratory infectious disease caused by influenza viruses that poses a serious threat to human health [1]. Because influenza viruses are highly mutable, antibodies acquired through natural infection, vaccination, or both are often insufficient to neutralize antigenically drifted strains, leaving the general population susceptible to recurrent infections [2]. The incidence rate of symptomatic influenza among unvaccinated individuals of different age groups ranges from 4 to 14% [3]. As a populous country, China faces a substantial influenza-related morbidity and mortality burden. Seasonal influenza was estimated to impose an annual economic burden of approximately RMB 33 billion to 106 billion (including direct medical costs, direct non-medical costs, and indirect costs) in China between 2011 and 2019 [4], underscoring the urgent need for effective influenza control measures. The Technical Guidelines for Influenza Vaccination in China recommend annual influenza vaccination as a cost-effective strategy for the prevention of influenza [5].

In 2023, the Quadrivalent Influenza Subunit Vaccine (QIV-Sub), independently developed by China, was launched on the market. It removes internal viral proteins and retains only highly pure HA and NA antigenic components, offering better safety compared with split vaccines [6]. Pre-licensure randomized controlled trials of QIV-Sub demonstrated that among 921 participants aged 6–35 months and 1619 participants aged 3 years and older who received at least one dose of QIV-Sub, the reported adverse reactions were either comparable to or fewer than those observed with split-virion vaccines [7–9]. Although vaccine safety was evaluated in pre-licensure clinical trials, the relatively limited sample sizes and restricted population diversity of these trials constrain their ability to detect rare or long-term adverse events. Therefore, post-marketing safety evaluation is essential for detecting rare or delayed vaccine-related adverse events, mitigating potential population health risks, and maintaining public confidence in vaccination [10–13].

Post-marketing surveillance can be broadly categorized into passive and active surveillance. Passive surveillance relies on spontaneous reporting by vaccine recipients or healthcare providers and is often subject to high underreporting rates and reporting biases. Reports tend to focus on known, severe, or acute adverse reactions, while new-onset, mild, or delayed reactions are substantially underreported [14, 15]. In contrast, active surveillance emphasizes the systematic and proactive collection of adverse event information. The U.S. Food and Drug Administration (FDA) has identified prospective active post-marketing surveillance programs as a common approach for monitoring the occurrence of specific adverse events following product approval [16]. In China, post-marketing active surveillance for vaccines is still in the exploratory stage [17]. As the value of active surveillance in vaccine safety monitoring gains broader recognition, it has progressively become a key direction for post-marketing safety surveillance. In this context, the need for post-marketing active vaccine safety surveillance in China has become increasingly evident and has received due attention from relevant national authorities [11, 18, 19].

Accordingly, this study was conducted in Jiangsu Province during the 2024–2025 influenza season to systematically monitor and analyze adverse reactions following QIV-Sub vaccination in healthy individuals aged 3 years and older over a 6-month period. By combining investigator-led proactive follow-up with participant self-reporting, we aimed to comprehensively collect adverse reaction information, evaluate the safety profile of QIV-Sub in this population, and identify infrequent adverse reactions that may not have been detected in pre-licensure clinical studies.

Methods

Ethical Approval

This study adheres to the ethical principles for medical research involving human participants as outlined in the Declaration of Helsinki. This study was reviewed and approved by the Ethics Committee of Jiangsu Provincial Center for Disease Control and Prevention (approval number JSJK2023-B016-02). The study was registered in the Drug Clinical Trial Registration and Information Disclosure Platform of China (www.chinadrugtrials.org.cn) with registration number CTR20244333, and in ClinicalTrials.gov with registration number NCT07710586.

Participants

This study was uniformly organized and implemented by the Jiangsu Provincial Center for Disease Control and Prevention. Healthy individuals aged 3 years and older who voluntarily received the QIV-Sub vaccination between November 2024 and May 2025 were enrolled. Inclusion criteria were defined as follows: (1) aged ≥ 3 years; (2) the participant (or their guardian/legally authorized representative) voluntarily provided written informed consent and signed the informed consent form, and agreed to comply with the study protocol requirements. Exclusion criteria were defined as follows: (1) known allergy to any component of the study vaccine, including eggs, excipients, formaldehyde, or Triton N-101; (2) the presence of acute disease, severe chronic disease, acute exacerbation of a chronic disease, common cold, or fever; (3) uncontrolled epilepsy or other progressive neurological disorders, or a history of Guillain–Barré syndrome; (4) any other condition deemed by the investigator to be inappropriate for participation in this study.

Sample Size

The sample size was calculated using the standard formula for adverse event reporting rates, as previously described [20]:

$$\beta \text{=}\sum _{i=\text{0}}^{A\text{-1}}\frac{N^i{R}_0^i{e}^{-N{r}_0}}{i\text{!}}$$

where β denotes the probability of type II error, N denotes the sample size, A denotes the number of adverse event reports to be observed, r₀ denotes the background incidence rate, R₀ denotes the overall reporting rate, and D denotes the excess adverse event reporting rate attributable to the vaccine.

Assuming an adverse event reporting rate (i.e., D) of 0.1%, a statistical power of 95% $i.e.,\beta =0.05$, a background incidence rate (r₀) of 0, and an overall reporting rate (R₀) of 0.1%, with at least one event expected to be observed $i.e.,A=1$, the initial estimated sample size was calculated to be 2996 participants. Accounting for an anticipated 3% dropout rate, the target sample size was adjusted to 3086 participants. Accordingly, a minimum of 3086 participants was required to ensure 95% power to detect an adverse reaction with a reporting rate of 0.1%.

Vaccines

The quadrivalent subunit influenza vaccine (lot number: A20240722; specification: 0.5 ml) used in this study was manufactured by Ab&B Biotech Co., Ltd (Jiangsu, China). It was presented as a slightly opalescent liquid and contained 15 μg of hemagglutinin per strain for each of the influenza virus strains recommended by the World Health Organization (WHO) for the 2024–2025 Northern Hemisphere influenza season: A/Victoria/4897/2022 (H1N1) pdm09-like virus, A/Thailand/8/2022(H3N2)-like virus, B/Austria/1359417/2021 (B/Victoria lineage)-like virus, and B/Phuket/3073/2013 (B/Yamagata lineage)-like virus. The vaccine was formulated without antibiotics or preservatives. All lots of the study vaccine passed lot release testing conducted by the National Institutes for Food and Drug Control (NIFDC) of China and were stored and transported under the required cold chain conditions (2–8 °C).

Immunization Schedule

All participants received a single 0.5-ml dose of QIV-Sub administered intramuscularly into the deltoid muscle of the upper arm.

Surveillance Content and Methods

This single-center, single-arm, open-label study was conducted in accordance with the Guiding Principles for Quality Management of Vaccine Clinical Trials (Trial Version) [21]. Vaccination and safety follow-ups were performed by trained medical personnel. Demographic characteristics, including age and sex, were collected from all participants. After vaccination, participants received 30 min of on-site observation by investigators to assess immediate AEs. From 30 min post-vaccination through day 28 (inclusive), participants self-monitored and recorded any AEs, concomitant medications, and concomitant vaccinations using paper diary cards. Investigators conducted centralized in-person follow-up visits on day 7 and day 28 to verify diary entries and collect the cards. From day 29 through 6 months post-vaccination, SAEs were monitored through spontaneous reporting. Investigators additionally conducted a telephone follow-up at 6 months post-vaccination to confirm any potential SAEs. In this study, ADRs were categorized into solicited and unsolicited events, as well as local and systemic events, according to standard vaccine safety assessment criteria. Solicited ADRs were defined as pre-specified local and systemic reactions actively ascertained by investigators via scheduled follow-up and paper-based diary cards. Unsolicited ADRs included all adverse events spontaneously reported by participants that were not on the pre-defined symptom checklist. Local ADRs corresponded to reactions occurring at or near the injection site, whereas systemic ADRs represented generalized symptoms unrelated to the injection site.

The severity of adverse events was graded on a five-point scale according to the Guiding Principles for Classification Standards of Adverse Events in Preventive Vaccine Clinical Trials (2019 Edition) issued by the National Medical Products Administration of China. Grade 1 (mild) was defined as transient (< 48 h) or mild discomfort that did not interfere with daily activities and required no treatment. Grade 2 (moderate) was defined as mild to moderate limitation of activity that might require medical consultation, with no or only mild treatment needed. Grade 3 (severe) was defined as marked limitation of activity requiring medical consultation and therapeutic intervention, with possible hospitalization. Grade 4 (life-threatening) was defined as potentially life-threatening conditions with severe limitation of activity requiring intensive medical monitoring and treatment. Grade 5 was death.

Statistical Analysis

Participants with at least one safety follow-up visit were included in the safety set (SS). The number of participants reporting a specific AE or ADR was used for statistical analysis, while the total number of reported events for a specific AE or ADR was expressed as episodes. Reporting rate (%) was calculated as the number of participants reporting a specific AE or ADR divided by the total number of vaccinated participants × 100%.

Descriptive analyses were performed to evaluate reporting rates of AEs, ADRs, and SAEs, overall and stratified by age group and sex. All analyses were conducted on a per-participant basis, meaning that each event was counted only once per participant rather than by the number of occurrences. For recurrent events of the same type occurring in the same participant, the event was summarized using the most severe grade, the most plausible causal relationship to vaccination, and the earliest onset time.

Between-group differences in the overall and symptom-specific ADR reporting rates were assessed using the Pearson chi-square test, Fisher's exact test (for 2 × 2 tables when chi-square assumptions were not met) when the assumptions for the chi-square test were not met and the table was 22), or the Monte Carlo exact test (for higher-dimensional tables when chi-square assumptions were not met). For comparisons showing statistically significant overall differences, post-hoc pairwise comparisons were performed using the Bonferroni correction, and adjusted P values are reported. A two-sided P value of < 0.05 was considered statistically significant for all tests. All data management and statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA).

Results

Participants and Demographic Characteristics

A total of 3108 participants were screened and enrolled, of whom 3105 completed vaccination. Among the vaccinated participants, six completed vaccination and attended visits 1 through 3 but were lost to follow-up at visit 4, while 3099 completed the full study protocol. All 3105 participants who completed vaccination were included in the SS (Fig. 1).

Fig. 1.

Participant disposition flowchart. SS safety set, SAE serious adverse event. Of 3108 screened participants, three declined vaccination, and 3105 received the vaccine and were included in the safety set (SS). All 3105 completed the 28-day post-vaccination safety monitoring (visits 1–3). Subsequently, 3099 completed visit 4 (6-month SAE follow-up), while six were lost to follow-up

The mean age of participants was 32.47 years (SD 22.83), ranging from 3 to 89 years. The largest proportion of participants was in the 3–17 years age group (1499 participants, 48.28%), while the smallest proportion was in the ≥60 years age group (489 participants, 15.75%). The male-to-female ratio was 1:1.39, and this difference was statistically significant (P < 0.001) (Table 1). Standardized residual analysis of sex distribution across age groups revealed that male participants in the 3–17 years group (standardized residual = + 3.8) and female participants in the 18–59 years group (standardized residual = +4.3) deviated from expected frequencies, whereas the sex distribution in the ≥ 60 years group was not statistically significant (P > 0.05). Specifically, the proportion of male participants in the 3–17 years group was significantly higher than that of female participants (55.67% vs. 42.96%, P < 0.05), while the proportion of female participants in the 18–59 years group was significantly higher than that of male participants (42.04% vs. 27.53%, P < 0.005).

Table 1.

Baseline demographic characteristics of participants stratified by sex and age group

Sex	Age group			χ2value	P value
	3–17 years [n (%)]	18–59 years [n (%)]	≥60 years [n (%)]
Male (n = 1297)	722 (55.67)	357 (27.53)	218 (16.80)	70.986	<0.001*
Female (n = 1808)	777 (42.96)	760 (42.04)	271 (14.99)
Total (n = 3105)	1499 (48.28)	1117 (35.97)	489 (15.75)

Pearson’s chi-squared test was used to compare the age composition distribution between male and female participants, and standardized residual analysis was performed to evaluate sex proportion differences across distinct age groups

Active Surveillance Safety Analysis

Overall Adverse Events and Adverse Drug Reactions

Among the 3105 participants, 282 participants (9.08%) reported 477 AEs, and 188 participants (6.05%) reported 318 ADRs (Table 2).

Table 2.

Overall reporting profile of adverse events and adverse reactions

Category	Episodes	Participants	Reporting rate (%)
Adverse events (AEs)	477	282	9.08
Adverse drug reaction (ADRs)	318	188	6.05
Solicited	295	172	5.54
Solicited local	155	92	2.96
Solicited systemic	140	97	3.12
Unsolicited	23	22	0.71
Stratified by severity grade
Grade 1	272	175	5.64
Grade 2	45	31	1.00
Grade 3	1	1	0.03
Grade 4	0	0	0.00
Serious adverse events (SAEs)	23	22	0.71
Days 0–28 post-vaccination	2	2	0.06
Day 29 to 6 months post-vaccination	21	20	0.64
Serious adverse reactions (SARs)	0	0	0.00

AE adverse event, ADR adverse drug reaction. Reporting rates were calculated as the number of participants reporting an event divided by the total number of vaccinated participants (N = 3105) × 100%. Episodes refer to the total number of reported events, while participants refer to the number of individuals reporting at least one event

Within 28 days post-vaccination, two SAEs (0.06%) were reported in two participants. One participant was hospitalized with fever on day 3 post-vaccination and was clinically diagnosed with acute lower respiratory tract infection. The participant received intravenous cloxacillin sodium for 5 days and improved before discharge. Based on the clinical diagnosis and the use of antibiotic therapy, the investigator determined the event to be coincidental and unrelated to vaccination. The other participant was hospitalized with respiratory tract infection symptoms on day 15 post-vaccination and was clinically diagnosed with tracheitis. The participant received intravenous piperacillin sodium for 7 days and improved before discharge. Considering the relatively long interval between symptom onset and vaccination, along with the clinical diagnosis and antibiotic treatment, the investigator determined the event to be unrelated to vaccination. From day 29 through 6 months post-vaccination, 20 participants reported 21 SAEs, yielding a reporting rate of 0.64%. All SAEs were determined to be unrelated to vaccination (Supplementary Table 1).

Severity of Adverse Drug Reactions

Most ADRs were grade 1 (272 episodes, reported by 5.64% of participants) or grade 2 (45 episodes, reported by 1.00% of participants) in severity. Only one grade 3 ADR (0.03%) was reported, which presented as fever; the participant recovered without treatment.

The reporting rates and severity grades of specific solicited ADR symptoms are presented in Fig. 2. The reporting rate for solicited local ADRs was 2.96% (92 participants), with tenderness being the most frequently reported symptom (69 participants, 2.22%), followed by redness (32 participants, 1.03%). The reporting rate for solicited systemic ADRs was 3.12% (97 participants), with fever being the most frequently reported symptom (41 participants, 1.32%), followed by headache (24 participants, 0.77%).

Fig. 2.

Severity grading of solicited adverse drug reactions within 28 days post-vaccination. ADR adverse drug reaction. The numbers in each column represent the reporting rate (%) of ADRs across all severity levels for that symptom. Left (yellow), middle (green), and right (blue) represent the reporting rates of grade 1, 2, and 3 ADRs, respectively

The reporting rate for unsolicited ADRs was 0.71% (22 participants), among which the most frequently reported symptoms were pharyngalgia, rhinorrhea, and dizziness, each reported by four participants (0.13%), with all events being mild to moderate in severity (Supplementary Table 2).

Time to Onset, Outcomes, and Management of Adverse Drug Reactions

ADR onset-time distribution, clinical outcomes, and case-management information are summarized in Table 3. Within 30 min post-vaccination, 19 participants reported 24 immediate ADRs. The most frequently reported immediate ADRs were fever (seven participants, 0.23%) and tenderness (five participants, 0.16%). All ADRs were reported within 7 days post-vaccination.

Table 3.

Time to onset, outcomes, and management of adverse drug reactions (SS)

Category	Episodes [n (%)]	Participants [n (%)]	Reporting rate (%)
By time of onset
Within 30 min	24 (7.55)	19 (10.11)	0.61
>30 min to 7 days	294 (92.45)	169 (89.89)	5.44
8–28 days	0 (0.00)	0 (0.00)	0.00
29 days to 6 months	0 (0.00)	0 (0.00)	0.00
By outcome
Resolved	316 (99.37)	186(98.94)	5.99
Not resolved/persistent	2 (0.63)	2 (1.06)	0.06
Improved	0 (0.00)	0 (0.00)	0.00
Resolved with sequelae	0 (0.00)	0 (0.00)	0.00
Death	0 (0.00)	0 (0.00)	0.00
Unknown	0 (0.00)	0 (0.00)	0.00
By time to resolution
Within 3 days	253 (79.56)	161 (85.64)	0.61
4–7 days	48 (15.09)	30 (15.96)	6.05
>7 days	15 (4.72)	12 (6.38)	0.00
By management
No treatment	284 (89.31)	172 (90.96)	5.51
Self-medication	26 (8.18)	21 (11.17)	0.67
Outpatient medication	5 (1.57)	5 (2.66)	0.16
Outpatient without medication	0 (0.00)	0 (0.00)	0.00
Hospitalization	0 (0.00)	0 (0.00)	0.00
Total	318 (100.00)	188 (100.00)	6.05

SS safety set, ADR adverse drug reaction, AE adverse event

The majority of ADRs were transient and resolved without medical intervention. Two ADR episodes (0.63%) remained unresolved/persistent, involving two participants (1.06%); the remaining 316 ADR episodes all resolved completely. Analysis of symptom onset and resolution times showed that 79.56% (253 episodes) of ADRs resolved within 3 days. Regarding management of ADRs, 89.31% (284 episodes) resolved without treatment.

Comparison of ADR Reporting Rates Across Age Groups

The overall, local, and systemic ADR reporting rates across different age groups are presented in Table 4. The overall ADR reporting rates for the 3–17 years, 18–59 years, and ≥60 years age groups were 5.67% (85 participants), 7.88% (88 participants), and 3.07% (15 participants), respectively, with a statistically significant difference among the groups. Post-hoc pairwise comparisons using the Bonferroni correction revealed that the overall ADR reporting rate in the 18–59 years group was significantly higher than that in the ≥60 years group (adjusted P < 0.001). No statistically significant differences were observed between the 3–17 years group and the 18–59 years group (P = 0.075), or between the 3–17 years group and the ≥60 years group (P = 0.066).

Table 4.

Comparison of ADR reporting rates within 28 days post-vaccination by age group and sex

Category	Total ADRs			Local ADRs			Systemic ADRs
	Participants [n (%)]	χ2	P	Participants [n (%)]	χ2	P	Participants [n (%)]	χ2	P
Age group (years)
3–17	85 (5.67)	14.590	0.001*	46 (3.07)	14.085	0.001*	49 (3.27)	4.822	0.090
18–69	88 (7.88)			46 (4.12)			50 (4.48)
≥60	15 (3.07)			3 (0.61)			12 (2.45)
Sex
Male	67 (5.17)	3.095	0.079	32 (2.47)	2.635	0.105	42 (3.24)	0.732	0.392
Female	121 (6.09)			63 (3.48)			69 (3.82)

ADR adverse drug reaction. P values were derived from Pearson's chi-square test. *Statistically significant differences after Bonferroni correction for multiple comparisons

The local ADR reporting rates for the 3–17 years, 18–59 years, and ≥60 years age groups were 3.07% (46 participants), 4.12% (46 participants), and 0.61% (three participants), respectively, with a statistically significant difference among the groups. Post-hoc pairwise comparisons using the Bonferroni correction revealed that the local ADR reporting rate in the ≥ 60 years group was significantly lower than that in the other two age groups (18–59 years vs. ≥60 years, adjusted P < 0.001; 3–17 years vs. ≥ 60 years, adjusted P = 0.007), while no statistically significant difference was observed between the 3–17 years and 18–59 years groups (adjusted P = 0.447). Further analysis of specific symptoms showed that, with the exception of injection-site pain, the reporting rates of other local symptoms did not differ significantly across the three age groups (Supplementary Table 3). Pairwise comparisons of injection-site pain reporting rates across the three age groups demonstrated that the reporting rate in the ≥ 60 years group was significantly lower than that in the other two groups (18–59 years vs. ≥60 years, adjusted P = 0.002; 3–17 years vs. ≥60 years, adjusted P = 0.033), whereas no statistically significant difference was observed between the 3–17 years and 18–59 years groups (adjusted P = 0.330).

The systemic ADR reporting rates for the 3–17 years, 18–59 years, and ≥60 years age groups were 3.27% (49 participants), 4.48% (50 participants), and 2.45% (12 participants), respectively, with no statistically significant difference among the groups (P = 0.090). Analysis of specific symptoms showed that, with the exception of fatigue/malaise, the reporting rates of other systemic symptoms did not differ significantly across the three age groups (Supplementary Table 3). Pairwise comparisons of fatigue/malaise reporting rates across the three age groups revealed that the reporting rate in the 3–17 years group was significantly lower than that in the 18–59 years group (adjusted P < 0.001), while no statistically significant differences were observed between the 18–59 years and ≥60 years groups (adjusted P = 0.561), or between the 3–17 years and ≥60 years groups (adjusted P = 0.270).

Comparison of ADR Reporting Rates by Sex

No statistically significant differences were observed in the overall, local, or systemic ADR reporting rates between male and female participants (Table 4). Similarly, no statistically significant differences were found in the reporting rates of individual ADR symptoms by sex (Supplementary Table 4).

Discussion

Influenza remains a global public health concern, and its prevention and control have long attracted widespread attention. Annual influenza vaccination is a key measure to interrupt virus transmission and reduce the morbidity and severe disease burden associated with influenza.

The safety of subunit influenza vaccines has been well documented in multiple studies conducted outside China [22–25], and this has been further corroborated by pre-licensure clinical trials of QIV-Sub [7–9]. The QIV-Sub manufactured by Ab&B Biotech Co., Ltd (Jiangsu, China) represents the first quadrivalent subunit influenza vaccine licensed in China, underscoring the clinical relevance of evaluating its safety in a large-scale vaccinated population.

The Technical Guideline for Clinical Trials of Influenza Virus Vaccines (Draft for Comment), issued by the Center for Drug Evaluation of the National Medical Products Administration (NMPA) in 2025, recommends stratifying participants into at least three age groups (6–35 months, 3–59 years, and ≥60 years) based on accumulated research data [26]. At the time this study was conducted, the study vaccine was licensed for use in individuals aged ≥3 years, which defined the lower age limit of our study population. Considering the differences in immune responses between minors and adults [27], the age groups in this study were defined as 3–17 years, 18–59 years, and ≥60 years.

In this study, the overall ADR reporting rates across age groups, in descending order, were 7.88% (88 participants) in the 18–59 years group, 5.67% (85 participants) in the 3–17 years group, and 3.07% (15 participants) in the ≥60 years group. The reporting rate in the ≥60 years group was significantly lower than the 18–59 years group (adjusted P < 0.001). For local ADRs, the reporting rate in the ≥60 years group was significantly lower than that in the other two groups. Notably, pain—the most frequently reported local ADR across all three groups—exhibited a similar trend, with significant differences between the ≥60 years group and the other two groups. This observed age-related pattern may be attributable to the fact that post-vaccination adverse reactions are associated with mild and transient inflammatory responses induced by the vaccine [27]. Immunosenescence in older adults, affecting both the innate and adaptive immune systems [28, 29], suggests that vaccination elicits weaker inflammatory responses in this population, potentially leading to higher tolerance to adverse reactions [30]. Additionally, older adults may be more susceptible to recall bias than younger age groups, making them more likely to underreport adverse events. In addition to immunological factors, differential health-seeking behaviors and healthcare access across age groups may have contributed to the observed differences in ADR reporting rates. Older adults may have different patterns of healthcare utilization and symptom perception compared with younger individuals, which could influence both the recognition and reporting of adverse reactions. These behavioral and access-related factors should be considered alongside the biological explanations when interpreting the age-stratified findings. For systemic ADRs, no statistically significant differences were observed among the three age groups. Analysis of specific symptoms revealed that the reporting rate of fatigue/malaise in the 3–17 years group was significantly lower than that in the 18–59 years group (0.07% vs. 1.07%, adjusted P < 0.001), whereas no statistically significant differences were found in the reporting rates of other systemic symptoms between any of the remaining groups. A possible explanation is that the immune system of minors is not yet fully mature compared with that of adults [27], which may result in milder systemic inflammatory responses. Furthermore, as a subjective symptom, fatigue/malaise may be more prone to underreporting in minors due to their limited cognitive awareness of such sensations.

In this study, the overall ADR reporting rate was 6.05%. Compared with the phase III trial results of the same vaccine [8] (10.67%), the local (2.96% vs. 5.87%) and systemic (3.12% vs. 5.80%) ADR reporting rates showed consistent trends. The ADR spectrum was also highly consistent between the two studies. In the phase III trial, the most frequently reported local ADR was injection-site pain (4.60%), and the most frequently reported systemic ADR was fever (2.13%), while in the present study, the reporting rates for injection-site pain and fever were 2.22% and 1.48%, respectively, indicating that injection-site pain and fever were the most frequently reported symptoms in both studies. No new safety signals were identified, confirming that the safety profile of the vaccine remains consistent across different study phases. Further analysis across age groups revealed that the differences were mainly concentrated in children and older. In the phase III study, the ADR reporting rates in the 3–8 years and 9–17 years groups were 15.50% and 13.75%, respectively, both higher than the 5.67% observed in the 3–17 years group in the present study. The ADR reporting rate in the ≥65 years group was 7%, also higher than the 3.07% observed in the ≥ 60 years group in this study. In contrast, the ADR reporting rate in the 18–64 years adult group was 6.29%, comparable to the 7.88% observed in the 18–59 years group. It is worth noting that antigenic drift between vaccine strains and currently circulating influenza viruses may confound the interpretation of influenza-related observations, warranting further evaluation in dedicated virological and epidemiological studies.

The slightly lower overall, local, and systemic ADR reporting rates observed in this study, compared with the phase III trial, may be attributed to several factors. Phase III trials are typically conducted in well-controlled clinical settings with more intensive follow-up to ensure complete and accurate data collection. Although this study also employed an active surveillance approach, the real-world setting—including reduced participant compliance (e.g., incomplete diary card completion) and less frequent investigator follow-up compared with phase III trials—may have contributed to the lower reporting rates. In addition, the phase III trial was conducted before vaccine licensure, when participants might have been more vigilant about post-vaccination events. In contrast, this study was conducted two years after the vaccine was licensed, and participants may have placed greater trust in the vaccine, perceiving its safety as already established, which could have led to greater underreporting of mild and transient adverse reactions.

Compared with the safety results of other phase IV active surveillance studies of non-adjuvanted quadrivalent influenza vaccines [31–34] (Supplementary Table 5), the AE and ADR reporting rates observed in this study were comparable to or lower than those reported in these studies. Across-study differences may be attributable to variations in age composition and regional differences in study settings. For example, the study by Kou et al.^[32]exclusively enrolled older adults aged ≥60 years, a population less sensitive to adverse reactions [30], which may explain its lowest ADR reporting rate. In contrast, although the study by Jain et al.^[34]also focused on older adults, the AE reporting rate was 28.30%, which may be associated with regional differences in adverse event reporting practices and healthcare cultural norms in India.

In contrast, data from the Adverse Drug Reaction Direct Reporting System for Marketing Authorization Holders, maintained by the National Center for Adverse Drug Reaction Monitoring of China, reflect post-marketing passive surveillance for QIV-Sub. During the 2024–2025 influenza season in Jiangsu Province, the reported incidence of adverse events following immunization (AEFI)—defined as any adverse reaction or event occurring after vaccination that may have a causal association with vaccination—was 36.17 per 100,000 doses (21 reports per 58,062 doses) for the study vaccine. This rate is comparable to the AEFI reporting rate of 23.77 per 100,000 doses reported in a retrospective study by Xia et al. [35] but differs by an order of magnitude from the AE reporting rate (9.08%) observed in the present active surveillance study. This marked difference underscores that active surveillance can substantially enhance the detection of adverse reactions compared with passive surveillance, and is valuable for comprehensively capturing post-vaccination safety information and generating reliable adverse reaction reporting rates in the post-marketing setting. This finding provides empirical support for the theoretical framework outlined in the Introduction, confirming that passive surveillance is subject to substantial underreporting whereas active surveillance enables more systematic case ascertainment. Taken together, these findings reinforce the complementary roles of both approaches in post-marketing vaccine safety monitoring.

The strengths of this study include its large sample size and broad age range, which encompassed children, young adults, and older adults, thereby enhancing the representativeness and generalizability of the findings. The prospective study design with active surveillance, combined with systematic follow-up monitoring, enabled the comprehensive and accurate collection of adverse reaction data and minimized the potential biases inherent in retrospective studies.

Nevertheless, several limitations should be acknowledged. First, the study was retrospectively registered, which may reduce transparency and introduce reporting bias—a limitation to consider when interpreting the findings. Second, age and sex are important factors influencing AE reporting rates [36]; in this study, the age and sex distribution was not balanced, and thus the interpretation of our findings should be approached with caution. Third, socioeconomic variables such as occupation and educational level were not collected, which may have influenced individuals' ability to recognize adverse reactions and their willingness to report them, potentially introducing bias into the safety assessment. Fourth, this study was conducted solely in Jiangsu Province, and regional differences in lifestyle habits and healthcare accessibility may exist [37]. To enable a more comprehensive evaluation of vaccine safety, a multicenter, large-scale study using electronic diary cards has been initiated for this vaccine, and the relevant results have been reported elsewhere [38]. Fifth, selection bias arose from recruiting only healthy volunteers, resulting in an underestimation of real-world ADR rates, particularly in older adults. Voluntary participation could have dual effects. Health-conscious individuals might over-report mild symptoms, yet their favorable baseline health status lowers actual risks. Comparisons with studies employing broader enrollment criteria suggest that underestimation likely prevails. This bias mainly affects mild ADRs and does not alter our conclusion that no new safety concerns were identified. Future studies should enroll special populations to improve generalizability.

Conclusions

In summary, the quadrivalent subunit influenza vaccine demonstrated a favorable safety profile in a large population of healthy individuals aged 3 years and older, with low ADR reporting rates and most adverse reactions being mild and transient in nature. No new safety signals were identified. These findings support the widespread use of this vaccine in the population aged 3 years and above.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contributions

The conceptualization of the study was carried out by Hongxing Pan, Hang Tao, and Hui Jin. The methodology was developed by Liuyang Kang, Mingwei Wei, Hui Jin, Hongxing Pan and Hang Tao. Funding acquisition was secured by Youcai An and Hang Tao. Protocol implementation and data acquisition, including their supervision and participation, were undertaken by Hui Jiang, Xia Zang, Jing Yang and Siyu Tian. The data analysis plan was designed and data analysis was performed by Liuyang Kang, Mingwei Wei and Yu Pan. Data verification was conducted by Mingwei Wei, Hui Jiang and Xia Zang. Resource coordination and data management were handled by Liuyang Kang and Mingwei Wei. The initial draft of the manuscript was written by Liuyang Kang, Mingwei Wei, Yu Pan, Jing Yang and Siyu Tian. Visualizations were created by Liuyang Kang and Mingwei Wei. Critical review was undertaken by Hui Jin, Hongxing Pan and Hang Tao. All authors have read and approved the final version of the manuscript.

Funding

This study was supported by the Center for Drug Reevaluation, National Medical Products Administration, NMPA (Grant Number CDRZ20234001-2) and Ab&B Biotech Co., Ltd., Jiangsu, China. The Center for Drug Reevaluation, NMPA had no role in the study design, data collection and analysis, and writing of the study report. Ab&B Biotech Co., Ltd., Jiangsu, China, was involved in the study design. The journal’s Rapid Service Fee was funded by Ab&B Biotech Co., Ltd., Jiangsu, China.

Data Availability

The datasets generated and analyzed in this study are available from the corresponding author upon reasonable request.

Declarations

Conflict of interest

All authors have declared no conflicts of interest. Mingwei Wei, Hui Jiang, Xia Zang, Yu Pan, Hongxing Pan, and Hui Jin have nothing to disclose. Youcai An, Jing Yang, Siyu Tian, Liuyang Kang, and Hang Tao are employees of Ab&B Biotech Co., Ltd.

Ethical Approval

This study adheres to the ethical principles for medical research involving human participants as outlined in the Declaration of Helsinki. This study was reviewed and approved by the Ethics Committee of Jiangsu Provincial Center for Disease Control and Prevention (approval number JSJK2023-B016-02). The study was registered in the Drug Clinical Trial Registration and Information Disclosure Platform of China (www.chinadrugtrials.org.cn) with registration number CTR20244333, and in ClinicalTrials.gov with registration number NCT07710586.