WHO: Influenza Update N0 518
12 March 2025
In the Northern hemisphere, activity decreased or remained the same in most countries. Increased activity was reported in a few countries in Central America and the Caribbean, Tropical South America, Northern, Western and Middle Africa, Central, South and South-East Asia, and Eastern Europe. Activity remained elevated in North America and Central America and the Caribbean with co-circulation of A(H3N2) and A(H1N1)pdm09 viruses, Tropical South America (predominantly A(H3N2)), Northern (predominantly A(H3N2)) and Western Africa (predominantly A(H1N1)pdm09), Europe (A and B viruses co-dominant), Central Asia (predominantly B viruses), Western and Southern Asia (predominantly A(H3N2)) and South-East and Eastern Asia (predominantly A(H1N1)pdm09). Read More
Uncovering the Burden of Influenza-Associated Illness across Levels of Severity in the Kingdom of Saudi Arabia Across Three Seasons (J Epidemiol Glob Health. 2025 Mar 20;15(1):47.)
This study aims to quantify influenza-associated severe acute respiratory infection (SARI) hospitalization rates and estimating the broader influenza burden across severity levels. Data from four sentinel hospitals in three regions of Saudi Arabia were analyzed across three influenza seasons: 2017-2018, 2018-2019, and 2022-2023. The John Hopkins University/WHO Seasonal Influenza Burden Disease Estimator (Flutool) was employed to extrapolate national estimates of influenza-associated hospitalizations, deaths, and mild/moderate cases. The average annual SARI hospitalization rate was 294 per 100,000 population (95% CI: 288-300). Influenza-associated SARI hospitalization rates averaged 48 per 100,000 population (95% CI: 45-50), with the highest burden observed in individuals aged 65 years and older (269 per 100,000, 95% CI: 240-301) and children aged 0-4 years (118 per 100,000, 95% CI: 107-131). Seasonal variation was noted, with the highest rates in the 2017-2018 season. National estimates suggested a substantial burden, with influenza-associated hospitalizations totalling 17,678 in 2017-2018, 7,683 in 2018-2019, and 13,982 in 2022-2023. The flutool analysis estimated annual influenza-associated deaths ranging from 30 to 4,441 and mild/moderate cases reaching up to 6.3 million in the most severe season. This study demonstrates a significant burden of influenza-associated SARI hospitalizations in Saudi Arabia, with the highest rates observed in the elderly and young children. Read More
Global meta-analysis of short-term associations between ambient temperature and pathogen-specific respiratory infections, 2004 to 2023. Euro Surveill. 2025 Mar;30(11):2400375.
The researchers conducted pathogen-specific analyses, sensitivity analyses, subgroup analyses and meta-regression. A total of 137 studies were eligible for meta-analysis. The pooled and single-study estimates revealed that the incidence of respiratory syncytial virus (RR = 0.14), influenza virus (IV) (RR = 0.40), human metapneumovirus (RR = 0.48), human coronavirus (HCoV) (RR = 0.21) and SARS-CoV-2 (RR = 0.52) decreased per 1° C temperature rise, while that of human parainfluenza virus (HPIV) (RR = 2.35), human bocavirus (HBoV) (RR = 1.86) and MERS-CoV (RR = 1.05) increased. The risk of infection was lower for IVA, IVB, HCoV-229E and HCoV-OC43, while HPIV-3, and HBoV-1 were at increased risk. The risk of Streptococcus pyogenes pharyngitis (RR = 0.46) decreased per 1° C temperature rise, while Pseudomonas aeruginosa (RR = 1.04) and Legionella pneumophila infections (RR = 2.69) increased. As the climatic conditions will become warmer, public health policy makers should act to develop pathogen adaptation strategies. Read More
Infectiousness of raw (unpasteurised) milk from influenza H5N1-infected cows beyond the USA (Lancet Microbe Online first 101107March 18, 2025)
Pasteurisation effectively inactivates the H5N1 influenza virus, ensuring the safety of pasteurised cow milk for consumers. In the USA, 99% of commercial milk complies with safety standards, and no live H5N1 virus has been found in retail milk, though 20% of samples contain genetic traces. Studies confirm raw milk from affected states contains infectious H5N1, with cattle likely contracting the virus through milk and milking practices. Farm workers may contract it via exposure to raw milk or contaminated equipment. The virus shows a strong affinity for mammary glands, leading to high viral loads in milk.
Globally, the spread of H5N1 among dairy cattle, such as in Egypt, poses significant risks. In Egypt, where H5N1 is endemic, limited surveillance and unsafe practices, like consuming raw milk and inadequate protective equipment for workers, heighten concerns. The virus’s ability to infect mammals and its potential for mutation underscore the need for active surveillance and monitoring of milk production declines. Addressing these challenges is critical to preventing wider outbreaks and ensuring dairy safety. Read More
Exploration of the Impact of Air Pollutants on the Influenza Epidemic After the Emergence of COVID-19: Based on Jiangsu Province, China (2020-2024) (Frontiers in Public Health Volume 13 – 2025 | doi: 10.3389/fpubh.2025.1555430)
Non-pharmaceutical interventions (NPIs) during the COVID-19 pandemic altered influenza transmission patterns, yet the age-specific effects of air pollutants on influenza dynamics remain unclear. Utilizing influenza surveillance data of Jiangsu Province from 2020 to 2024, we integrated generalized additive quasi-Poisson regression model and distributed lag non-linear models (DLNM) to quantify lagged effects and exposure-response relationships between air pollutants (NO2 , SO2 , PM2.5) and influenza risk across young, middle-aged and elderly groups. The NO2 and SO2 both showed significant positive effects in all age groups.The effect of NO2 is most significant in the young group (RR=5.02, 95 % CI: 4.69-5.37), while SO2 exhibited the most pronounced effects in middle-aged and elderly groups (RR=4.22, RR=8.31, respectively). PM2.5 elevated risks in young (RR=1.99, 95% CI: 1.87-2.12) and elderly (RR = 1.45, 95% CI: 1.07-1.94) groups. Air pollutant impacts on influenza transmission are age-dependent: NO2 dominates in younger populations, whereas SO2 disproportionately affects older adults. These findings highlight age-related vulnerability to air pollution and the need for targeted public health strategies for different population subgroups. Read More
Effectiveness of Influenza Vaccination in Children Aged 6 to 59 Months during the Inaugural 2022-2023 Season in Andalusia, Spain (J Pediatrics 21 March 2025, 114558)
Population-based, retrospective cohort study during the 2022-2923 influenza season of 287,661 children aged 6 to 59 months at the beginning of the inaugural 2022-2023 vaccination campaign. Adjusted vaccine effectiveness (VE) was calculated by estimating multivariable logistic regression models for likelihood of influenza diagnosis in vaccinated and unvaccinated children.
In children aged 6 to 59 months, VE was 51% (95% CI: 48-53%) for preventing infection and 69% (95% CI: 56-79%) for preventing influenza-related hospitalization. In children aged 6 to 23 months, VE was 53% (95% CI: 48-57%) for preventing influenza and 76% (95% CI: 57-87%) for preventing hospitalization due to influenza. In both subpopulations, VE was significantly higher with the two-dose vaccination schedule. Influenza vaccination shows moderate effectiveness in preventing influenza and high effectiveness in preventing influenza-related hospitalization in children aged 6 to 59 months. This highlights the importance of strengthening influenza vaccination coverage in this age group. Read More
Two-step wastewater surveillance reveals co-circulation of respiratory pathogens during the 2023-2024 influenza season in a low-resource setting. (Frontiers Environmental Sci Engineering March 17, 2025)
Clinical surveillance for respiratory pathogens has traditionally been challenging in low-resource settings, such as Western China. A low-cost wastewater monitoring network offers an alternative solution. To explore this, we first compared the sensitivity of a MeltArray-based qPCR assay, which detects 25 respiratory pathogens, with singleplex qPCR using both mock and real wastewater samples. We then employed this MeltArray assay to detect these respiratory pathogens in wastewater from a low-income region in Xi’an city from September 2023 to January 2024. Following this, qPCR and MLST were employed to quantify the dynamics of positive respiratory pathogens and confirm their genotypes. Results showed unusual surges in sewage influenza A virus (IAV) and adenovirus levels starting in October 2023, persisting until late December. Additionally, influenza B virus (IBV) outbreaks were identified beginning in late December. These findings matched the positivity rates reported by a sentinel hospital. For coronaviruses, HCoV-229E/OC43 were consistently detected in wastewater, while SARS-CoV-2 was occasionally found. The qPCR assays revealed continuous increases in sewage Mycoplasma pneumoniae and Hemophilus influenzae concentrations since September, both peaking in October. Genotyping confirmed the circulation of specific bacterial genotypes in the region. Read More
Antiviral Activity of Berbamine Against Influenza A Virus Infection (Int. J. Mol. Sci. 2025, 26(6), 2819)
Berbamine (BBM) is a bibenzyl isoquinoline present in the traditional Chinese herbal medicine Berberis amurensisis Rupr. The present study demonstrates that BBM exerts strong antiviral efficacy against influenza A virus (IAV) infection. We examined the anti-IAV effect of BBM using green fluorescent protein (GFP)-expressing influenza A and H1N1 IAV. The fluorescence microscopy, fluorescence-activated cell sorting analysis, and plaque assay showed that BBM significantly hinders IAV infection. The immunofluorescence analysis confirmed the anti-influenza activity of BBM. From the time-of-addition and hemagglutination inhibition results, it is elucidated that the antiviral effect of BBM is closely related to its inhibitory effect against viral binding and entry at an early infection stage. Our findings imply that BBM has the potential to be developed as a potent antiviral drug against influenza viral infection. Read More
Mortality Risk Among Patients With Influenza Illness Admitted to the ICU: A Systematic Review and Meta-Analysis (Influenza and Other Respiratory Viruses 2025;19(3):e70073)
This systematic literature review (SLR) and meta-analysis investigated all-cause mortality risk among ICU-admitted patients with influenza in Europe.The authors observational studies conducted in Europe that reported mortality among patients ≥ 6 months of age with influenza admitted to the ICU. Studies published between January-2009 and December-2019 were included. Thirty-seven studies, reporting on 13,616 patients, were included. All-cause mortality ranged from 0% to 67%. The overall pooled mortality risk estimate was 0.24 (95% CI: 0.20, 0.27). Study heterogeneity was high (Cochran’s Q test p < 0.01, I2 = 93%). The sensitivity analysis using only studies identified as having low risk of bias produced a pooled mortality risk of 0.25 (95%CI: 0.21, 0.29). These results indicate that approximately a quarter of patients with influenza admitted to the ICU die, reinforcing the need for effective vaccination programs and treatment optimization. Read More
Human Cases of Highly Pathogenic Avian Influenza A(H5N1) — California, September–December 2024 (MMWR March 13, 2025 / 74(8);127–133)
Persons who work closely with dairy cows, poultry, or other animals with suspected or confirmed infection with highly pathogenic avian influenza (HPAI) A(H5N1) viruses are at increased risk for infection. In September 2024, the California Department of Public Health was notified of the first human case of HPAI A(H5N1) in California through monitoring of workers on farms with infected cows. During September 30–December 24, 2024, a total of 38 persons received positive test results for HPAI A(H5N1) viruses in California; 37 were dairy farm workers with occupational exposure to sick cows, and one was a child aged <18 years with an undetermined exposure, the first pediatric HPAI A(H5N1) case reported in the United States. All patients had mild illness. The identification of cases associated with occupational exposure to HPAI A(H5N1) viruses on dairy farms highlights the continued risk for persons who work with infected animals. The pediatric case was identified through routine surveillance. Given recent increases in the prevalence of HPAI A(H5N1) viruses among some animal populations, public health agencies should continue to investigate cases of HPAI A(H5N1) in humans as part of control measures, pandemic preparedness, to identify concerning genetic changes, and to prevent and detect potential human-to-human transmission of the virus. To date, no human-to-human transmission of HPAI A(H5N1) virus has been identified in the United States. Read More