BACKGROUND:Data on influenza vaccine effectiveness (VE) against hospitalized severe acute respiratory infection (SARI), particularly in Asia, remain limited for the 2025/26 Northern Hemisphere influenza season. This study aimed to evaluate real-world VE against A(H3N2)-associated SARI hospitalization and provide timely, locally relevant evidence to inform seasonal influenza vaccination policy. METHODS:A test-negative design was used to estimate VE against influenza A(H3N2)-associated SARI hospitalization in Beijing, China, from 10 November 2025 to 18 January 2026. VE was estimated by comparing the odds of influenza vaccination between case-patients (those who tested positive for A(H3N2)) with controls (those who tested negative for influenza). RESULTS:Among 1883 enrolled SARI inpatients, 220 (11.7%) tested positive for influenza A(H3N2). Overall vaccination coverage was 11.4%, with the highest coverage observed among children aged 5-17 years (29.6%). Influenza positivity was higher among rural residents, patients with pneumonia or hypoxemia, and those with symptom onset in November. The adjusted overall VE was 7.5% (95% CI: -45.8% to 43.3%). Moderate VE was observed among children aged 5-17 years (45.4%, 95% CI: -33.6% to 79.5%), although the confidence interval included zero and the estimate was not statistically significant. Negative VE estimates were observed among younger children and older adults. Among patients with underlying respiratory conditions, VE was 75.4% (95% CI: -27.4% to 98.7%), although this estimate was also not statistically significant. CONCLUSIONS:During the 2025/26 influenza season in Beijing, VE against A(H3N2)-associated SARI hospitalization was suboptimal. Moderate protection was observed among children aged 5-17 years, the group with the highest vaccination coverage, but the estimate was not statistically significant. The low overall VE may be attributable to antigenic mismatch between vaccine and circulating strains, as well as low population-level vaccination coverage. These findings highlight the need to improve vaccine formulations and increase vaccination coverage, particularly among adults and older populations.

During the early 2025/26 influenza season, influenza A(H3N2) subclade K rapidly predominated in Beijing, China. Using a test-negative design, we estimated influenza vaccine effectiveness (VE) among influenza-like illness outpatients tested between weeks 40/2025 and 04/2026. Among 10,484 participants, sequencing of 316 randomly selected A(H3N2)-positive samples showed 84.8% were subclade K, and antigenic analysis of 65 viruses indicated antigenic divergence. Despite this, adjusted VE against laboratory-confirmed influenza was 23.5% (95% confidence interval: 11.7–33.7), indicating modest protection during this subclade K-dominated season.

OBJECTIVES:Schools are high-risk settings for influenza transmission. During the 2025-26 Northern Hemisphere season, influenza A(H3N2) subclade K (J.2.4.1) rapidly emerged and became predominant, but vaccine effectiveness (VE) in school-associated outbreaks remains unclear. METHODS:We conducted a field investigation of 93 school-associated febrile outbreaks in Beijing, China, from September to December 2025. Cases were febrile students with laboratory-confirmed influenza, and afebrile classmates served as controls. VE against laboratory-confirmed symptomatic infection was estimated using mixed-effects logistic regression, adjusting for age, sex, school type, and outbreak-level clustering. RESULTS:A total of 3797 students were included, of whom 788 were laboratory-confirmed febrile cases and 3009 were afebrile classmates. Primary schools accounted for 74.2% of outbreaks. Overall vaccination rate was 54.6%, with 50.6% among cases and 56.1% among controls. Unadjusted VE was 19.6% (95% CI: 5.9-31.3). After adjustment, VE against laboratory-confirmed influenza was 30.0% (95% CI: 17.3-40.7). Sensitivity analyses yielded consistent results. CONCLUSION:Influenza vaccination provided modest protection against symptomatic A(H3N2) infection in school-aged children, even under conditions of high-intensity exposure and antigenic drift. These findings underscore the value of vaccination in outbreak-prone school settings and highlight the importance of ongoing vaccine strain optimization and outbreak-based VE surveillance.

During the early 2025/26 influenza season, influenza A(H3N2) subclade K rapidly predominated in Beijing, China. Using a test-negative design, we estimated influenza vaccine effectiveness (VE) among influenza-like illness outpatients tested between September and December 2025. Among 9,579 participants, sequencing of 316 randomly selected A(H3N2)-positive samples showed 84.8% were subclade K, and antigenic analysis of 65 viruses indicated antigenic divergence. Despite this, adjusted VE against laboratory-confirmed influenza was 41.3% (95% CI: 29.2 to 51.3), indicating moderate protection during this subclade K-dominated season.

BACKGROUND:In the 2024-2025 season, influenza activity returned to pre-COVID-19 pandemic levels, with A(H1N1)pdm09 as the predominant subtype. Reassessment vaccine effectiveness (VE) in hospitalized patients is critical to inform potential adjustments to vaccination policy. METHODS:Using a test-negative design, we evaluated the VE of the seasonal influenza vaccine during the 2024-2025 season against A(H1N1)pdm09-associated hospitalizations with severe acute respiratory infections (SARI) in Beijing, China, from December 9, 2024 to February 9, 2025. VE was estimated by comparing the odds of influenza vaccination between case-patients (those testing positive for A(H1N1)pdm09) and controls (influenza test-negative patients), applying inverse-propensity-to-be-vaccinated weights. RESULTS:The analysis included 1608 hospitalized SARI patients (17.4% tested positive for A(H1N1)pdm09; 13.8% were vaccinated overall). The adjusted VE against A(H1N1)pdm09-associated hospitalizations was 18.8% (95% CI: 0.9% to 33.6%) overall. Specifically, the VE was 45.4% (95% CI: 10.0% to 67.1%) in children aged 0.5-17 years, 43.8% (95% CI: -20.8% to 76.1%) in adults aged 18-59 years, and 8.6% (95% CI: -15.8% to 27.9%) in older adults ≥ 60 years. Among individuals with chronic conditions, the VE was 18.5% (95% CI: -2.3% to 35.2%) and was higher among those with chronic respiratory conditions (40.6%, 95% CI: -1.7% to 65.7%). CONCLUSIONS:During the 2024-2025 season in Beijing, the overall VE against A(H1N1)pdm09-associated SARI hospitalizations was suboptimal. However, moderate VE was observed among children and younger adults, while limited effectiveness was seen in the elderly. Optimizing influenza vaccination strategies, including the introduction of high-dose or adjuvanted vaccines to enhance immune response in older adults, is crucial to alleviate influenza-associated hospitalizations.

We estimated early influenza vaccine effectiveness (VE) for the 2024/25 season in outpatients, in Beijing using a test-negative design. A(H1N1)pdm09 dominated (99.3%), all sequenced strains (n = 38) clustered in clade 6B.1A.5a.2a, and 37 of 38 antigenically similar to the vaccine strain. VE against any influenza virus infection was 48.5% (95% CI: 34.8–59.5) and 48.7% (95% CI: 35.1–59.7) against A(H1N1)pdm09. Vaccination in the current or previous season against any influenza showed a VE of 52.5% to 54.9%, compared to no vaccination in both seasons.

Introduction: The objective of our study was to estimate the influenza vaccine effectiveness for 2023/24 epidemic of co-circulating influenza A(H3N2) and B(Victoria) viruses in Beijing, China. Methods: The surveillance-based study included all swabbed patients through influenza virological surveillance in Beijing, between October 2023 and March 2024. A Test-Negative Design(TND) was used to estimate influenza vaccine effectiveness(VE) against medically- attended laboratory-confirmed influenza in outpatient settings, also calculated the influenza vaccination rate(IVR). Cases were influenza-like illness (ILI) patients who tested positive for influenza, and controls were ILI who influenza negative patients. Results: A total of 18,665 ILI patients were enrolled and swabbed. Among them, 6362(34.1 %) tested positive for influenza, major epidemic strain was A(H3N2) and B(Victoria). The overall IVR was 8.7 %, and the differences of IVR by gender, age, region, chronic conditions and month of onset were statistically significant(P<0.05). The adjusted VE against all influenza was moderate at 44.8 %, with the highest for B (Victoria) at 52.2 %, the highest for 19-59 age at 72.4 %, and the highest when vaccinated only in current season at 48.3 %. Conclusion: Our study suggested the influenza vaccine has moderate effectiveness, with the best VE against B (Victoria), followed by A(H3N2) and A(H1N1)pdm09 in Beijing, 2023/24 season. Meanwhile, the influenza VE was relatively high in school-age children and the elderly. Consistent long-term studies are required in the future to evaluate the protect effect of influenza vaccine.

[Objective]To understand the epidemic characteristics of influenza in Beijing from 2023 to 2024,and to provide a scientific basis for the prevention and control of influenza.[Methods]Data on influenza-like illness(ILI)from secondary level and above hospitals,etiology surveillance data,and influenza clusters outbreaks data from 2023-2024 were used to analyze the epidemic trend and pathogenic characteristics of influenza.Furthermore,an influenza comprehensive index was used to categorize the epidemic intensity at the severity level.[Results]A total of 2 065 857 ILI cases were reported in 2023-2024 epidemic season,and the percentage of ILI was 3.67%.The age group of 5-14 years accounted for the highest proportion of ILI(30.48%).A total of 41 766 throat swabs from ILI were detected,with a positive rate of 17.28%.A(H3N2)(51.86%)and B Victoria(41.93%)were the most prevalent subtypes of influenza virus.Clustered influenza outbreaks occurred mainly in primary schools(57.78%)and middle schools(35.55%),mainly caused by the influenza A(H3N2)subtype(85.93%).According to the influenza comprehensive index(I),the period of influenza activity and above(I＞0.5)lasted for a total of 37 weeks,accounting for 71.15%of the entire influenza season.[Conclusion]Compared with previous years,the epidemic level of influenza in Beijing was increased in 2023-2024,and the peak time became earlier.The comprehensive index method can objectively evaluate the level of influenza epidemic and provide suggestions for the future prevention and control of influenza in Beijing.

Background:Characterizing the paradigm and impact of long COVID is crucial for addressing this worldwide health challenge. This study aimed to investigate the prevalence of long COVID one year after primary Omicron infection and characterize differences in long-term health consequence between participants with persistent long COVID and those who fully recovered. Methods:This a community-based cross-sectional study conducted from December 2023 to March 2024 at the China-Japan Friendship Hospital and 16 administrative districts in Beijing. 12,789 participants infected with Omicron between December 2022 and January 2023 were recruited through stratified multistage random sampling and included in the final analysis. Of them, 376 participants with persistent long COVID and 229 without long COVID were matched for further physical examinations. The primary outcome was the prevalence of long COVID one year after infection. Secondary outcomes included muscle strength, exercise capacity, health-related quality of life (HRQoL), mental health, work status, laboratory tests, and examinations. Findings:Among 12,789 participants (media [IQR] age, 48.4 [37.3 to 61.4] years; 7817 females [61.1%]), 995 of them (7.8%) experienced long COVID within one year, with 651 (5.1%) having persistent symptoms. Fatigue (598/995 [60.1%]) and post-exertional malaise (367/995 [36.9%]) were the most common symptoms. Brain fog had the lowest resolution proportion as 4.2% within one year. The odds of long COVID increased with reinfections (odds ratios for one reinfection 2.592 [95% CI: 2.188 to 3.061]; two or more: 6.171 [3.227 to 11.557]; all p < 0.001). Participants with persistent long COVID had markedly lower muscle strength (upper-limb: 26.9 ± 12.4 vs. 29.1 ± 14.5 Kg; lower-limb: 40.0 [27.0 to 62.0] vs. 43.0 [28.0 to 59.0] s), worse exercise capacity and poorer HRQoL, and meaningful difference in laboratory tests results compared to those without long COVID. They also exhibited significantly higher proportions of abnormal lung function (FEV1 %pred<80%: 13.0% vs. 2.0%; DLco %pred<80%: 32.7% vs. 19.9%) and lung imaging abnormalities (23.5% vs. 13.6%). Interpretation:The considerable health burden of long COVID and the progression of neurological symptoms following Omicron infection warrant close monitoring. Utilizing professional questionnaires and developing reliable diagnostic tools are necessary for improving diagnosis and treatment of long COVID. Funding:This work was supported by Beijing Research Center for Respiratory Infectious Diseases (BJRID2024-012), Chinese Academy of Medical Sciences Innovation Fund for Medical Sciences (2022-I2M-CoV19-005/CIFMS 2021-I2M-1-048), the National Natural Science Foundation of China (82241056/82200114/82200009), the New Cornerstone Science Foundation.

>Respiratory infections are associated with high morbidity and mortality and are a major global health problem [1] . Acute respiratory infections are caused by multiple respiratory pathogens, including viruses and bacteria. Viral-bacterial co-infections, which have become increasingly common and a global concern, can lead to substantial complications, causing higher morbidity and adverse prognosis [2] . Previous studies have reported low positive detection rates of targeted pathogens related to acute respiratory infections, owing to the limited number of detected pathogens and variations in the sensitivity of diagnostic methods [3-4] .

This study aimed to estimate the end-of-season influenza vaccine effectiveness (VE) for the 2024/25 season in Beijing, China. Methods: We used a test-negative design (TND) to assess influenza VE among outpatients with influenza-like illness (ILI) enrolled through the influenza virological surveillance in sentinel hospitals in Beijing from week 44, 2024 to week 14, 2025. Cases were ILI patients who tested positive for influenza; controls were those who tested negative. Results: Among 18,405 ILI patients tested, 3690 (20.0%) were positive for influenza, with A(H1N1)pdm09 as the predominant strain (98.9%). The overall influenza vaccination coverage was 12.4%. Adjusted VE was 48.3% (95%CI: 40.4%–55.3%) against any influenza and 48.2% (95%CI: 40.3%–55.1%) against A(H1N1)pdm09, with the highest VE observed in adults aged 18–59 years (79.0%). The adjusted VE was similar for those vaccinated in 2023/24 only (53.1%) or both 2023/24 and 2024/25 seasons (50.8%), but lower for those vaccinated only in the 2024/25 season (48.5%). The adjusted VE was higher during the epidemic period (52.5%) than in the pre-epidemic (48.1%) and post-epidemic (35.3%) periods. Conclusions: Our findings indicate moderate VE against laboratory-confirmed influenza, especially A(H1N1)pdm09, during the end of the 2024/25 season in Beijing, China. Influenza vaccination provided protective effects across different epidemic periods. These timely estimates support ongoing public health communication and immunization strategies.

What is already known about this topic?:The H3N8 avian influenza virus (AIV) demonstrates considerable capacity for interspecies transmission and has been documented in multiple mammalian hosts, including equine and canine species. During 2022-2023, three laboratory-confirmed human infections with H3N8 were reported in China, heightening public health concerns about the zoonotic spillover potential of H3 subtype AIVs. What is added by this report?:This study reports the isolation of a genetically reassorted, low-pathogenicity H3N8 avian influenza virus (AIV) from an islet in Niukouyu Wetland Park, Beijing Municipality - the first detection of this viral strain in a wild environment within the city. Throat swabs collected from park staff tested negative for influenza viruses. Phylogenetic analysis demonstrated that the viral hemagglutinin gene originated from the Eurasian lineage, while the neuraminidase gene was derived from the North American lineage. Although no direct evidence of human infection has been documented, multiple mutations identified in the virus's internal genes are associated with enhanced replication capacity, increased virulence, and improved adaptation to mammalian hosts. These molecular features indicate a potential risk for cross-species transmission to humans. What are the implications for public health practice?:Given the potential threat that H3N8 AIVs pose to mammalian species, including humans, this study emphasizes the critical need to strengthen influenza surveillance networks and broaden monitoring efforts specifically targeting H3 subtype AIVs.

Influenza B/Victoria viruses predominated during the 2021–2022 influenza season in Beijing, China, unlike most northern hemisphere countries, likely due to reduced international travel. We estimated influenza vaccine effectiveness (VE) against the B/Victoria lineage to provide a more comprehensive evaluation of 2021–2022 influenza VE. Between October 2021 and April 2022, patients aged ≥6 months with influenza-like illness (ILI) visiting outpatient departments in Beijing’s influenza virological surveillance system were enrolled. A test-negative design was used to assess VE against influenza B/Victoria, adjusting for sex, age groups, the presence of chronic diseases, onset-to-enrollment interval, and symptom onset timing. Of the 8,813 eligible patients, 1,787 (20.3%) tested positive for influenza B/Victoria only. 573/8813 (6.5%) were vaccinated against influenza. The adjusted effectiveness against B/Victoria for all ages was 16.6% (95% CI: −7.5% to 35.2%) overall. VE was low against influenza B/Victoria among medically attended ILI patients during the 2021–2022 season in Beijing, China.

Background: During the 2022–2023 influenza season, the influenza activities in most regions of China were postponed, including Beijing. The unusually delayed influenza epidemic posed a challenge to the effectiveness of the influenza vaccine. Methods: Using the test-negative design, we evaluated influenza vaccine effectiveness (VE) during the 2022–2023 influenza season against influenza A-associated outpatient and emergency-department-attended influenza-like illness (ILI) in Beijing, China, from 9 January to 30 April 2023. Results: The analysis included 8301 medically attended ILI patients, of which 1342 (46.2%) had influenza A(H1N1)pdm09, 1554 (53.4%) had influenza A(H3N2), and 11 (0.4%) had co-infection of the two viruses. VE against influenza A-associated ILI patients was 23.2% (95% CI: −6.5% to 44.6%) overall, and 23.1%, 9.9%, and 33.8% among children aged 6 months to 17 years, adults aged 18–59 years, and adults aged ≥60 years, respectively. VE against influenza A(H1N1)pdm09 and against influenza A(H3N2) were 36.2% (95% CI: −1.9% to 60.1%) and 9.5% (95% CI: −34.1% to 39.0%), respectively. VE of the group with vaccination intervals of 14–90 days (70.1%, 95% CI: −145.4 to 96.4) was higher than that of the groups with a vaccination interval of 90–149 days (18.7%, 95% CI: −42.4% to 53.6%) and ≥150 days (21.2%, 95% CI: −18.8% to 47.7%). Conclusions: A moderate VE against influenza A(H1N1)pdm09 and a low VE against influenza A(H3N2) were observed in Beijing during the 2022–2023 influenza season, a season characterized with a delayed and high-intensity influenza epidemic. VE appears to be better within three months after vaccination. Our findings indicate a potential need for the optimization of vaccination policies and underscore the importance of continuous monitoring of influenza to enhance vaccines and optimizing vaccination timing.

Background. The aim of this study was to investigate the changes of epidemic characteristics of influenza activity pre- and post-coronavirus disease 2019 (COVID-19) in Beijing, China. Methods. Epidemiologic data were collected from the influenza surveillance system in Beijing. We compared epidemic intensity, epidemic onset and duration, and influenza transmissibility during the 2022-2023 season with pre-COVID-19 seasons from 2014 to 2020. Results. The overall incidence rate of influenza in the 2022-2023 season was significantly higher than that of the pre-COVID-19 period, with the record-high level of epidemic intensity in Beijing. The onset and duration of the influenza epidemic period in 2022-2023 season was notably later and shorter than that of the 2014-2020 seasons. Maximum daily instantaneous reproduction number (R-t) of the 2022-2023 season (R-t = 2.31) was much higher than that of the pre-COVID-19 period (R-t = 1.49). The incidence of influenza A(H1N1) and A(H3N2) were the highest among children aged 0-4 years and 5-14 years, respectively, in the 2022-2023 season. Conclusions. A late, intense, and short-term peak influenza activity was observed in the 2022-2023 season in Beijing. Children <15 years old were impacted the most by the interruption of influenza circulation during the COVID-19 pandemic. Maintaining continuous surveillance and developing targeted public health strategies of influenza is necessary.

Background. To estimate effect of COVID-19 control measures taken to mitigate community transmission in many regions, we analyzed data based on influenza surveillance system in Beijing from week 27th, 2014 to week 26th, 2020. Methods. We collected weekly number of influenza-like illness (ILI), weekly positive proportion of ILI and weekly ILI proportion in outpatients and the date of COVID-19 measures. We compared influenza activity indicators of influenza season 2019/2020 with preceding five seasons and built two ARIMAX models to estimate the effective of COVID-19 measures. Results. Compared with preceding five influenza seasons, ILIs, positive proportion of ILI, and duration of influenza epidemic period decreased from 13% to 54%, especially, the number of weeks from the peak to the end of influenza epidemic period, decreased from 12 to one. After natural decline considered, weekly ILIs decreased by 48.6% and weekly positive proportion dropped 15% in the second week after emergency response declared, and finally COVID-19 measures reduced 83%. Conclusions. We conclude public health emergency response can interrupt the transmission of influenza and other respiratory infectious diseases markedly. Keyword. COVID-19 control measures; influenza; ARIMAX

To assess the impact of absolute humidity on influenza transmission in Beijing from 2014 to 2019, we estimated the influenza transmissibility via the instantaneous reproduction number (Rt), and evaluated its nonlinear exposure-response association and delayed effects with absolute humidity by using the distributed lag nonlinear model (DLNM). Attributable fraction (AF) of Rt due to absolute humidity was calculated. The result showed a significant M-shaped relationship between Rt and absolute humidity. Compared with the effect of high absolute humidity, the low absolute humidity effect was more immediate with the most significant effect observed at lag 6 days. AFs were relatively high for the group aged 15-24 years, and was the lowest for the group aged 0-4 years with low absolute humidity. Therefore, we concluded that the component attributed to the low absolute humidity effect is greater. Young and middle-aged people are more sensitive to low absolute humidity than children and elderly.

Objective:To estimate the vaccine effectiveness (VE) of the seasonal influenza vaccination in prevention of hospitalized severe acute respiratory infection (SARI) in Beijing in the 2018/2019 epidemic season of influenza.Methods:Hospitalized SARI cases in 11 Beijing hospitals during 2018/2019 epidemic season of influenza were selected as study subjects. Vaccination status was determined from registry record system. Influenza virus infection was detected by RT-PCR. A binary logistic regression model was used to adjust influence from age, time of onset and underlying diseases to calculate the odds ratio (OR) between influenza virus nucleic acid positive and negative groups. VE was then calculated by 1-OR.Results:The overall estimated VE was -42.6% (95% CI: -152.7% to 19.5%) while VE of 5-59 group was 58.5% (-119.4% to 92.2%).VE of vaccination within 60 days was 15.8% (-142.8% to 70.8%). Conclusions:The protection effectiveness of influenza vaccine for hospitalized SARI patients in Beijing during 2018/2019 epidemic season of influenza were limited. Vaccination coverage in target groups should be enhanced.

Objective:To explore the association and the lag effect between meteorological factors and seasonal influenza activity in Beijing.Methods:Meteorological data and influenza surveillance data from 2014 to 2019 in Beijing were collected. Descriptive statistics were used to describe the data meteorological factors and influenza activity. A distributed lag non-linear model (DLNM) was used to analyze the relationship between influenza activity and meteorological factors.Results:From 2014 to 2019, there was a non-linear negative correlation between seasonal influenza activity and daily average temperature, relative humidity, and wind speed in Beijing. At -14.3 °C, the cumulative risk of daily average temperature was the highest ( RR=25.42, 95% CI: 11.84-54.57); At 8.0%, the cumulative risk of daily average relative humidity was the highest ( RR=36.61, 95% CI: 24.72-54.22); At 0.5 m/s, the cumulative risk of daily average wind speed was the highest ( RR=3.24, 95% CI: 1.75-6.00). Conclusions:The climate conditions of low temperature, low relative humidity, and low wind speed may be associated with the increased level of seasonal influenza activity in Beijing, which can increase the risk of influenza transmission.