WHO: Influenza Update N° 525
May 7, 2025 | Emergency Situational Updates
In the Northern hemisphere, influenza positivity continued to decline or remained stable in most countries. Influenza positivity remained elevated in Central America and the Caribbean and Tropical South America (predominantly A(H1N1)pdm09), in Western Africa (predominantly A(H1N1)pdm09), Eastern Africa (predominantly A viruses), in Europe and Western Asia (predominantly A(H3N2) and B), in Southern Asia (predominantly A(H3N2)), Eastern Asia (predominantly A(H1N1)pdm09 and B) and South-East Asia (predominantly A viruses). In the Southern hemisphere, positivity remained stable with small increases in Temperate South America. Read More
T Cell Immune Response to Influenza Vaccination When Administered Prior to and Following Autologous Chimeric Antigen Receptor-Modified T Cell Therapy.
Infectious Disease 2025;31,May 5: 327-338
Chimeric antigen receptor-modified T (CAR-T) cell therapies are gaining wider use in relapsed and refractory malignancies. However, data on vaccination in this population is lacking. The authors evaluated T cell responses in an established cohort of CAR-T recipients and healthy controls before and after 2019 to 2020 influenza vaccination. Peripheral blood mononuclear cells were isolated from healthy controls and patients who received the 2019 to 2020 influenza vaccine pre- or post-CD19, CD20, or B cell maturation antigen CAR-T. T cells were expanded in vitro for 10 days with peptide libraries for hemagglutinin (HA) and nucleoprotein from the 2019 to 2020 vaccine influenza A strains and analyzed by flow cytometry following interferon-γ/tumor necrosis factor-α (IFNγ/TNFα) intracellular staining. Antibody response was evaluated by a hemagglutination inhibition assay. Twenty-nine participants, including eight immunocompetent adults, seven pre-CAR-T, and 14 post-CAR-T patients, were evaluated. IFNγ+/TNFα+ T cell responses after influenza vaccination in healthy controls varied with an increased response to HA-Kansas after vaccination in 7/8 individuals. In the pre-CAR-T cohort, there was a rise in CD4+ T cell response to HA-Brisbane in 6/7 patients after vaccination that remained detectable in 3/4 evaluable patients 90 days post-CAR-T. Five of six patients who lacked an antibody response nonetheless demonstrated a T cell response to HA-Brisbane. There was no association between time since CAR-T administration, baseline immunoglobulin G, or absolute lymphocyte count and change in CD4+ T cell IFNγ+/TNFα+ response pre- to postvaccine for the post-CART cohort. These data demonstrate that cell-mediated immunity to the influenza vaccine can be elicited in patients vaccinated pre-CAR-T and sustained post-CAR-T, filling an important gap from lack of humoral responses. Read More
Intranasally administered whole virion inactivated vaccine against clade 2.3.4.4b H5N1 influenza virus with optimized antigen and increased cross-protection.
Virology Journal 2025;22:May5, 131.
The global spread, frequent antigenic changes, and pandemic potential of clade 2.3.4.4b highly pathogenic avian influenza H5N1 underscore the urgent need for robust cross-protective vaccines. Here, we developed a clade 2.3.4.4b H5N1 whole inactivated virus (WIV) vaccine strain with improved structural stability, productivity, and safety. By analyzing the evolutionary trends of clade 2.3.4.4b H5N1 viruses, we identified a key mutation (R90K) that increases heat stability while preserving antigenicity. Additionally, the PB2 gene of PR8 was replaced with a prototypical avian PB2 gene to increase replication efficiency in embryonated chicken eggs and reduce replication efficiency in mammalian cells, thereby improving productivity and biosafety. We found that our optimized clade 2.3.4.4b H5N1 vaccine strain (22W_KY), inactivated with binary ethylenimine (BEI), had superior antigen internalization into respiratory epithelial cells compared to those inactivated with formaldehyde or beta-propiolactone. Following intranasal administration to mice, the BEI-inactivated 22W_KY also elicited significantly stronger systemic IgG, mucosal IgA, and T-cell responses, especially in the lungs. Protective efficacy studies revealed that the BEI-inactivated 22W_KY vaccine provided complete protection against heterologous viral challenges and significant protection against heterosubtypic viral challenges, with no weight loss and complete suppression of the viral load in the respiratory tract in 2 of 3 mice. These results indicate that the BEI-inactivated 22W_KY vaccine could serve as a promising candidate for a safe, stable, cost-efficient, and broadly protective intranasal influenza vaccine against zoonotic and pandemic threats. Read More
Influenza Vaccination and Cardiovascular Outcomes in Patients with Coronary Artery Diseases: A Placebo-Controlled Randomized Study, IVCAD.
Vaccines 2025, 13(5), 472
Influenza infection is associated with cardiovascular morbidity and mortality; however, the effect of influenza vaccination on cardiovascular outcomes is not fully understood. This clinical trial aimed to investigate the correlation between cardiovascular outcomes and influenza vaccine (FluVac) in coronary artery disease (CAD) subjects. The study was a randomized single-blinded placebo-controlled trial. Enrolled CAD subjects received 0.5 mL of 2007–2008 trivalent FluVac (15 µg hemagglutinin of each of Solomon Islands/3/2006 (H1N1), Wisconsin/67/2005 (H3N2), and Malaysia/2506/2004 (B)). The subjects were followed up at 1 month (hemagglutinin (HA) antibody titers) and at 12 months post-vaccination for evaluation of outcomes (influenza-like episodes, acute coronary syndrome (ACS), myocardial infarction (MI), coronary revascularization, and death). In total, 278 eligible CAD subjects were randomized to receive either FluVac (n = 137) or a placebo (n = 141), of which consequently 131 and 135 subjects completed the study. Cardiovascular deaths (3/131 [2.29%] vs. 3/135 [2.22%]) and all-cause deaths (4/131 [3.05%] vs. 4/135 [2.96%]) were similar in both groups. Adverse cardiovascular events, including ACS, MI, and coronary revascularization, were less frequent in the vaccine group but did not reach statistical significance. The magnitude of the antibody change and serologic response (≥4-fold HI titer rise) of all three antibodies were significantly higher in the vaccine group compared to the placebo but did not correlate with cardiovascular outcomes in the FluVac group. The authors conclude that influenza vaccine may improve cardiovascular outcomes, though this improvement is not correlated with post-vaccination antibody titers. Despite the controversy, influenza vaccination is recommended in the CAD population. Read More
Knowledge, attitudes, practices and vaccine acceptance towards seasonal influenza vaccination among international travelers: a cross-sectional survey in Thailand.
Travel Medicine and Infectious Disease 2025;66 (July-August):102863
Influenza is a common but preventable disease. International travelers encounter significant risks in contracting influenza. The cross-sectional, questionnaire-based study was conducted on international travelers while visiting the Thai Travel Clinic at the Hospital for Tropical Diseases in Bangkok, Thailand. From May to November 2024, 250 Thai and 229 non-Thai international travelers were enrolled. Most participants reported sufficient knowledge about influenza, with 86.4 % achieving a score of ≥60 % while expressing mild concern of infection [mean perceived risk score of 3.1 ± 2.4 (range from 0 to 10)]. When regarding preventive measures, 72.2 % reported regular hand hygiene, while only 19.4 % regularly wore masks in public. Influenza vaccine acceptance was 38.2 %. The most influential reason for vaccination was healthcare personnel’s advice. In the multivariable analysis, the independent factors which affected vaccine acceptance were travelers’ nationality, age, purpose of travel, destination country, and perceived risk score. The authors conclude that international travelers had sufficient influenza knowledge but low awareness relating to influenza prevention methods and influenza vaccinations. Low vaccine acceptance rates were observed among international travelers. Travel consultations should focus on influenza awareness while traveling and associated preventative measures. Read More
Influenza vaccine effectiveness among primary and secondary school students in Shenzhen during the 2023/24 influenza season.
Emerging Microbes & Infections 2025, 14: 2490531
During the winter influenza peaks, a total of 2689 ILI patients were collected from the two hospitals, with 58.61% of the participants vaccinated. Of the ILI patients, 761 (28.3%) were laboratory-confirmed influenza infection, with 76.7% being A(H3N2) and 22.8% B/Victoria. After adjusting for covariates, the adjusted VE (aVE) was 57.06% (95% CI, 48.59–64.13%). In the subtype/lineage and age subgroup analysis, influenza VE against B/Victoria was higher than A(H3N2), and decreased with age. More, influenza VE (aVE = 19.63% [95% CI, −29.13–49.98%]) against all subtypes/lineages were not statistically significant for the spring-summer peak. Overall, influenza vaccines provided significant protection against influenza- associated outpatient visits among students in Shenzhen during 2023/24 winter influenza season. The findings underscore the importance of maintaining high vaccination coverage. Read More
Interference between SARS-CoV-2 and influenza B virus during coinfection is mediated by induction of specific interferon responses in the lung epithelium.
Virology 2025;608:110556
Coinfections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus have represented a major health concern since the beginning of the COVID-19 pandemic. The continued spread and constant emergence of new SARS-CoV-2 variants mean that cocirculation and coinfection with seasonal respiratory viruses will continue. Despite the considerable contribution of influenza B virus (IBV) infections to global disease burdens, its interactions with SARS-CoV-2 remain largely unstudied. In this study, the authors sequentially coinfected lung epithelial cells with representative SARS-CoV-2 variants and IBV strains. They found that prior infection with IBV impaired SARS-CoV-2 D614G, Delta and Omicron BA.1 replication, but did not affect replication of the more recent Omicron EG.5.1 variant. The auhtors additionally show that pre-infection with SARS-CoV-2 reduces live attenuated influenza vaccine (LAIV) replication, suggesting vaccine effectiveness in children carrying SARS-CoV-2 pre-infections can be negatively impacted in coinfection. Both SARS-CoV-2 and IBV induced strong type III interferon (IFN) responses, whereas SARS-CoV-2 drove type I IFN production not seen in IBV infection, suggesting viral interference through specific IFN responses. Treatment with innate immune response inhibitors BX795 and Ruxolitinib abrogated viral interference between IBV and SARS-CoV-2 in coinfection, demonstrating that IFN-stimulated gene (ISG) responses play a vital role in viral interference. More specifically, we show that the magnitude and timing of ISG expression, triggered by the primary infecting virus in sequential coinfection, facilitates viral interference between IBV and SARS-CoV-2. Read More
Beyond respiratory distress: The impact of H1N1 influenza on circulatory failure
Acute Med Surg 2025 May 2;12(1):e70062.
H1N1 influenza infection can trigger acute cardiovascular events, including acute coronary syndrome (ACS) and AHF, particularly in individuals with pre‐existing conditions. Inflammation and hypoxia from H1N1 infection can exacerbate endothelial dysfunction and promote thrombogenesis. Additionally, physiological stress induced by H1N1 infection can exacerbate underlying conditions, as demonstrated in our thyroid storm case. 4 This severe hypermetabolic state strains the cardiovascular system and may cause AHF. During the H1N1 pandemic, recognizing the risk of dual‐organ failure, such as respiratory failure, including ARDS, and circulatory failure from fulminant myocarditis or secondary ACS, is critical. Given the severity and complexity of these complications, nationwide surveys are urgently needed to clarify and address the full impact of H1N1 on the respiratory and cardiovascular systems. Read More
Assessment of adulthood immunization knowledge, attitudes, and behavior.
Rev. Assoc. Med. Bras. 2025;71:3
Adulthood vaccination has not reached adequate levels, both in Turkey and around the world. The aim of this study was to identify the knowledge, attitude, and behavior of vaccination in those aged 18 years. Questionnaires were applied to 686 participants attending Family Health Centers. Notably, 72.4% of people had at least one vaccination in adulthood. The most frequent vaccinations were tetanus (55.1%), influenza (26.8%), and hepatitis B (8.2%). PATH analysis found that the effect of variables with direct effects on vaccination (apart from the situation of thinking that vaccinations are necessary in adulthood) disappeared in the model in which adult vaccine recommendations were used as mediators. The adult vaccination situation is inadequate. It is necessary to inform society about adult vaccinations and recommend vaccination. Tools such as information given during health services and implementations such as social education and brochures, posters, media, and public information spots may be used with this aim.
Underreported influenza mortality in Central and Eastern Europe hinders the extension of seasonal influenza vaccination programs in older adults.
Vaccine 2025;56,May 22,127184
Several Central and Eastern European (CEE) countries have low seasonal influenza vaccination coverage of older adults coupled with severe underreporting of influenza-related deaths. Our objective was to project influenza mortality estimates for older adults in six CEE countries, building on high-quality mortality data from an EU-15 country with similar climate, population density, and seasonal influenza vaccination coverage. In addition, we aimed to compare the implications of the reported and projected influenza burden estimates on the economic value of extended influenza vaccination for older adults in an exemplary CEE country, Serbia. Multivariate regression modelling was used to adjust for differences in population health status between countries. Economic implication of underreporting influenza burden was investigated by using the VITALO decision analytic model. Locally reported and projected mortality rates were similar in Czech Republic and Slovenia, whereas projected mortality rates far exceeded locally reported influenza mortality rates in Poland, Hungary, Serbia, and Romania. Based on locally reported mortality rates in Serbia, increasing seasonal influenza vaccination coverage in the 65+ population to average coverage in the EU-27 would prevent 2.86 deaths and would generate 16.52 QALYs at the incremental cost of 2,847,994 EUR annually, which translates to 172,378 EUR/QALY incremental cost-effectiveness ratio (ICER). However, adopting Austrian influenza mortality rates adjusted to higher frailty prevalence in Serbia, increased vaccination coverage would prevent 28.96 deaths and generate 132.77 QALYs at the incremental cost of 2,803,675 EUR annually with an 21,116 EUR/QALY ICER, below Serbian willingness to pay threshold. Accordingly, extension of seasonal influenza vaccination in older adults would be a cost-effective public health intervention in Serbia. Underreporting of adult influenza mortality rates prevent policymakers from understanding the true economic value of influenza vaccination. Our approach is applicable in further countries with low reported influenza mortality rates. Read More