Influenza Update N0 517 (WHO March 12, 2025)

 In the Northern hemisphere, influenza activity in general remained the same or decreased except in a few countries in Central America and the Caribbean, Europe and Western Asia. Activity remained elevated in North America (predominantly A(H1N1)pdm09), Central America and the Caribbean and Tropical South America (predominantly A(H1N1)pdm09 and A(H3N2)), Western and Northern Africa (predominantly A(H1N1)pdm09), Europe (all seasonal subtypes), Asia (Central: predominantly B viruses, Eastern: predominantly A(H1N1)pdm09, Western: predominantly A(H3N2), Southern: all seasonal subtypes and South-East Asia: co-circulation of A(H1N1)pdm09 and B viruses).

In the Southern Hemisphere, activity was low except in single countries in Eastern Africa and South-East Asia and Oceania, where elevated activity was observed. Read More

The Global Influenza Hospital Surveillance Network: A Multicountry Public Health Collaboration. (Influenza Other Respir Viruses. 2025 Mar;19(3):e70091.)

There is the need for robust and coordinated surveillance to guide global health responses. Established in 2012, the Global Influenza Hospital Surveillance Network (GIHSN) addresses this need by collecting clinical and virological data on persons with acute respiratory illnesses across a network of hospitals worldwide. GIHSN utilizes a standardized patient enrolment and data collection protocol across its study sites. It leverages pre-existing national infrastructures and expert collaborations to facilitate comprehensive data collection. This includes demographic, clinical, epidemiological, and virologic data, and whole genome sequencing (WGS) for a subset of viruses. Sequencing data are shared in the Global Initiative on Sharing All Influenza Data (GISAID). GIHSN uses financing and governance approaches centered around public-private partnerships. Over time, GIHSN has included more than 100 hospitals across 27 countries and enrolled more than 168,000 hospitalized patients, identifying 27,562 cases of influenza and 44,629 of other respiratory viruses. GIHSN has expanded beyond influenza to include other respiratory viruses, particularly since the COVID-19 pandemic. In November 2023, GIHSN strengthened its global impact through a memorandum of understanding with the World Health Organization, aimed at enhancing collaborative efforts and data sharing for improved health responses. GIHSN exemplifies the value of integrating scientific research with public health initiatives through global collaboration and public-private partnerships governance. Future efforts should enhance the scalability of such models and ensure their sustainability through continued public and private support. Read More

Public perceptions and influencing factors of seasonal influenza vaccine uptake in Makkah region, Saudi Arabia: a cross-sectional study (Fron Public Health 2025; Feb 18:13:1534176.)

This  research investigates the prevalence and influencing factors of influenza vaccine uptake among the public in Makkah region, Saudi Arabia. This cross-sectional study was conducted in Saudi Arabia between February and June 2024. The main researcher developed the questionnaire, which was reviewed by five academics and then piloted with 20 individuals for validation. An online questionnaire was used, targeting residents aged 18 and over with internet access. A snowball sampling method was applied. Data were collected via Google Forms and analyzed using SPSS version 26, employing descriptive statistics such as frequencies, percentages, and means. The total number of respondents is 450 participants. Vaccination uptake showed 65.1% had received the vaccine, with 31.1% confident it is safe. Despite this, 58.8% did not get vaccinated this season. Confidence in the vaccine’s effectiveness was 59.5%, though 41.2% reported breakthrough infections. Accessibility was generally rated easy (57.7%). Social pressure (17.4%), trust in health institutions (36.9%), and COVID-19 concerns (36.7%) significantly influenced vaccination decisions. The likelihood of vaccination next season displayed polarized views. Read More

Recommendations announced for influenza vaccine composition for the 2025–2026 northern hemisphere influenza season (WHO Feb 28, 2025)

The World Health Organization (WHO) today announced the recommendations for the viral composition of influenza vaccines for the 2025-2026 influenza season in the northern hemisphere. The WHO recommends that trivalent vaccines for use in the 2025–2026 northern hemisphere influenza season contain the following: 

Egg-based vaccines

an A/Victoria/4897/2022 (H1N1)pdm09-like virus

an A/Croatia/10136RV/2023 (H3N2)-like virus

a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

       Cell culture-, recombinant protein- or nucleic acid-based vaccines

an A/Wisconsin/67/2022 (H1N1)pdm09-like virus

an A/District of Columbia/27/2023 (H3N2)-like virus

a B/Austria/1359417/2021 (B/Victoria lineage)-like virus. 

The recommendation for the B/Yamagata lineage component of quadrivalent influenza vaccines remains unchanged from previous recommendations:

a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus. Read More

Severity Scale of Influenza and Acute Respiratory Illness Hospitalizations to Support Viral Genomic Surveillance: A Global Influenza Hospital Surveillance Network Pilot Study (Influenza Other Resp Viruses (2025 Mar;19(3):e70085.)

This study aimed to establish a Severity Scale for influenza and other acute respiratory infections (ARI), requiring hospitalization, for surveillance and research purposes (the SevScale). The authors used a subset of data from the Global Influenza Hospital Surveillance Network database, including data from different geographical areas and income levels. To quantify the underlying concept of severity, an item response model was developed using 16 indicators of severity related to the hospital stay. Each patient in the dataset was assigned a Severity Score and a Severity Category (low, medium, or high severity). Finally, we compared the model scores across different subgroups. Data from 9 countries were included, covering between 4 and 11 seasons from 2012 to 2022, with a total of 96,190 ARI hospitalizations. The initial model successfully highlighted severity disparities across patient subgroups. The SevScale proved to be a promising method to define severity for influenza vaccine strain selection, surveillance, and research. Read More

Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023-2024 (Emerg Infect Dis 2025 Mar 10;31(4)

During 2023-2024, highly pathogenic avian influenza A(H5N1) viruses from clade 2.3.2.1c caused human infections in Cambodia and from clade 2.3.4.4b caused human infections in the Americas. We assessed the susceptibility of those viruses to approved and investigational antiviral drugs. Except for 2 viruses isolated from Cambodia, all viruses were susceptible to M2 ion channel-blockers in cell culture-based assays. In the neuraminidase inhibition assay, all viruses displayed susceptibility to neuraminidase inhibitor antiviral drugs oseltamivir, zanamivir, peramivir, laninamivir, and AV5080. Oseltamivir was ≈4-fold less potent at inhibiting the neuraminidase activity of clade 2.3.4.4b than clade 2.3.2.1c viruses. All viruses were susceptible to polymerase inhibitors baloxavir and tivoxavir and to polymerase basic 2 inhibitor pimodivir with 50% effective concentrations in low nanomolar ranges. Because drug-resistant viruses can emerge spontaneously or by reassortment, close monitoring of antiviral susceptibility of H5N1 viruses collected from animals and humans by using sequence-based analysis supplemented with phenotypic testing is essential. Read More

Reduced risks of influenza-associated hospitalization and complications following vaccination among over 2 million older individuals: a nationwide study using target trial emulation framework. (BMC Medicine 2025 Mar 13;23(1):157.)

An observational study with a target trial emulation framework aimed to evaluate the clinical effectiveness of an influenza vaccine on influenza infection, complications, and associated healthcare utilization and costs, was conducted in Taiwan. 1,214,392 propensity-score-matched pairs of vaccinated and unvaccinated older populations from the influenza season of 2018/2019 were identified from Taiwan’s National Health Insurance Research Database. VE (estimated as 1 minus hazard ratio [HR]*100%) and the HRs were used for influenza events and associated complications, respectively. Primary analyses show 14% (10-18%) of VE against influenza-associated hospitalization, irrespective of age, frailty status, and underlying influenza risk. Compared with non-vaccination, having an influenza vaccination significantly reduced risks of influenza-associated death by 30%, various respiratory by 12-26%, cardiovascular complications by 39-47%, and acute kidney injury by 23%. Approximately savings of USD 3,000,000 in total from averting influenza-associated hospitalization following vaccination were found. Read More

COVID-19 and influenza hospitalizations and the role of COVID-19 vaccination in the post-pandemic period: A cross-sectional study from Saudi Arabia (Vaccine 2025 Feb 26:52:126937).

This study examines the clinical characteristics of influenza and COVID-19 patients in Saudi Arabia and evaluates the impact of COVID-19 vaccination on hospital and ICU admission risks.  a cross-sectional study of 235 patients (113 with influenza and 122 with COVID-19) admitted between October 2023 and March 2024 to Saudi public hospitals. Data on demographic factors, comorbidities, vaccination status, and ICU admission were collected from medical records. Logistic regression models were used to investigate associations between COVID-19 vaccination and clinical outcomes, adjusting for potential confounders. COVID-19 patients had higher rates of comorbidities but lower rates of ICU admissions than influenza patients (18.0 % versus 5.3 %) and (7.4 % versus 13.3 %), respectively. COVID-19 vaccination was associated with a higher likelihood of receiving the seasonal influenza shot (OR [95 % CI] = 4.43 [1.29, 15.26]) and a lower likelihood of COVID-19 hospital admission (OR [95 % CI] = 0.39 [0.19, 0.81]) and ICU admission (OR [95 % CI] = 0.31 [0.12, 0.80]). COVID-19 vaccination was significantly associated with a reduced risk of COVID-19 hospital admission and ICU admission in the post-pandemic period. These findings emphasize the importance of vaccination in mitigating severe outcomes from COVID-19 and influenza infections. Read More

Registry-Based Surveillance of Severe Acute Respiratory Infections in Norway During 2021-2024 (Influenza Other Respir Viruses 2025 Feb;19(2):e70080.)

The investigators aimed to describe the surveillance system and evaluate selected attributes to inform the establishment of a permanent SARI surveillance system in Norway. Data from polymerase chain reaction (PCR) analyses were available for 10 respiratory pathogens including SARS-CoV-2, influenza virus and respiratory syncytial virus (RSV). We identified 214,730 SARI cases, of whom 82%, 73% and 53% were tested for SARS-CoV-2, influenza virus and RSV. Case peaks were predominantly driven by one or a combination of these pathogens. Median time between admission and a registered SARI diagnostic code was 5 (lower-upper quartile 3-10) days. Nowcasting and alternative case definitions for SARI with COVID-19, influenza and RSV improved the timeliness. The ICD-10 codes for LRIs and COVID-19 captured only ~55% of the cases in the age group of 0-29 years compared to the routine case definition, where URIs were included. The authors recommend establishing a permanent SARI surveillance system. Read More

Impact of COVID-19 pandemic on influenza vaccination rates among healthcare workers and the general population in Saudi Arabia: A meta-analysis (Human Vaccine Immunotherapeutics 2025; 21(1):2477954.)

The authors conducted a meta-analysis of cross-sectional studies to statistically examine IVRs before and after the COVID-19 pandemic among the general population and HCWs in Saudi Arabia. The meta-regression analysis showed a significant correlation among the general population was observed between the IVR and the timing of the study, with a mean effect size estimate of 14.3 (95% CI = 5.7-22.9; p < .001). Among HCWs, no significant relationship was observed between the IVR and the timing of the study, with a mean effect size estimate of 6.7 (95% CI = -19.3-32.7; p = .5). COVID-19 might have contributed to a rise in IVR among HCWs, whereas the general population has seen a decline in IVR. Read More