Seasonal influenza is an important cause of severe respiratory infection worldwide, leading to an estimated 3–5 million cases of severe illness and 290, 000–650, 000 respiratory deaths annually 1. Since late 2025, public attention has focused on the high activity of influenza A in countries of the Northern Hemisphere, largely associated with the drifted A (H3N2) clade 2a. 3a. 1 subclade K (J. 2. 4. 1) 2. However, in the same year, Brazil experienced an exceptionally intense and highly atypical influenza season, which was driven mainly by A (H1N1) pdm09 D. 3. 1 and A (H3N2) J. 2. 3 3. Despite its magnitude, this epidemic has received comparatively little public and international attention. Here, we describe this unprecedented epidemic and discuss the potential contributions of the circulating virus subtypes, as well as the roles of delayed vaccination and low vaccination coverage in shaping this atypical season. In Brazil, severe Influenza cases are monitored through the severe acute respiratory infections (SARI) national surveillance system, which has provided mandatory and universal reporting since 2009. To quantify the severity of the 2025 season, we estimated the historical trend values 4 for Influenza A SARI cases using a probabilistic Bayesian framework proposed by Picinini Freitas et al. 5 This method fits a Bayesian model to historical weekly case counts to estimate the expected number of cases in a season. We fit this model using data from SARI confirmed as Influenza A from 2016 to 2024, excluding COVID-19 pandemic years (2020–2022). Based on percentiles of the predictive distribution, four categories of probabilistic epidemic bands reflecting the predicted cases level for the 2025 season are defined: below average, typical (≤ 50%) ; moderately high, fairly typical (50%–75%) ; fairly high, atypical (75%–90%) ; and exceptionally high, very atypical (> 90%) (Figure 1A). We also estimated severity thresholds for Influenza A season using the moving epidemic method 6 (Supporting Information). The 2025 Influenza A season was classified as exceptionally high and very atypical, with an unprecedented 28, 270 severe cases reported, almost 1. 6 times the 90th percentile of the predicted cases (17, 710). This was also the most severe season ever recorded for Influenza A since the establishment of the SARI universal surveillance during the pandemic of 2009 (14, 251 registered cases) (Figures 1A and S1). In addition to the atypical number of cases, we also observed an atypical seasonality pattern in 2025, with two distinct waves (Figure 1A). Influenza A has a marked seasonal pattern in Brazil, with one epidemic wave beginning around March (late summer) and decreasing after its peak in May (autumn), with a low activity during spring and summer months. Surprisingly, a second, and unexpected, wave of Influenza A started around August and extended through all of the spring months and into the beginning of summer. The first and more intense wave was dominated by A (H1N1) pdm09, whereas A (H3N2) predominated in the smaller second wave (Figure 1B). Resende et al. 3. identified A (H1N1) pdm09 D. 3. 1 and A (H3N2) J. 2. 3 as the main subclades circulating in Brazil until early August, both having good antigenic similarity to the 2025 Southern Hemisphere vaccine components. Up to November 11, 2025, the A (H3N2) subclade K was not detected in Brazil 7. In addition, an interim vaccine effectiveness evaluation across eight countries in the Southern Hemisphere, including Brazil, showed that the 2025 seasonal vaccine provided moderate protection against influenza-associated hospitalizations 8. A key point is that this unprecedentedly large Influenza A epidemic happened in Brazil at a time of low adherence to vaccination. In 2025, the Ministry of Health purchased 73. 6 million doses of Influenza vaccine to immunize 50. 4 million people from priority groups, such as older adults and individuals with comorbidities 9. The vaccination campaign for this group in most parts of the country began on April 7 (Week 15), but only 14. 6% of the purchased doses (10. 7 million) were administered before the number of cases reached an exceptionally high level (Week 17) (Figure 1C). Subsequently, some municipalities expanded vaccination eligibility to the entire population aged 6 months older. Overall, only 57. 4 million people were immunized, achieving a total coverage of 26. 9% of the entire population (213. 4 million). Among priority groups specifically, only 26. 4 million individuals were immunized, corresponding to a coverage of 52. 4%, far below the Brazilian Ministry of Health target of 90% 10. The low vaccination coverage and the delayed timing of vaccine uptake in 2025 indicate that individuals in high-risk groups were not well protected against the dominant clades of influenza A this season, which could have largely contributed to the unprecedented number of severe cases observed. In addition, characteristics of the main circulating viruses, such as severity and transmissibility, might also have contributed to this intense epidemic, which needs further investigation. We hope this report draws attention to the severe influenza A season that occurred in the Southern Hemisphere and highlights the likely impact of delayed vaccination uptake and low vaccination coverage on this unprecedented epidemic. Tatiana P. Portella: conceptualization, investigation, writing – original draft, methodology, visualization, formal analysis, data curation, writing – review and editing, software. Laís Picinini Freitas: investigation, writing – original draft, methodology, visualization, data curation, writing – review and editing, conceptualization, formal analysis, software. Paola Cristina Resende: investigation, writing – review and editing, conceptualization. Marcelo Ferreira da Costa Gomes: investigation, writing – review and editing, data curation, methodology, conceptualization, formal analysis, software. Leonardo S. Bastos: conceptualization, investigation, methodology, visualization, supervision, writing – review and editing, software. T. P. P. is funded by the Foundation for Scientific and Technological Development in Health—FIOTEC (VPEIC-004-FIO-23) and National Council for Scientific and Technological Development—CNPq (89102/2025-0), L. P. F. is supported by the Visiting Researcher Program of the Escola Nacional de Saúde Pública Sergio Arouca (Sergio Arouca National School of Public Health), Fundação Oswaldo Cruz (ENSP/Fiocruz), and L. S. B. is funded by FAPERJ (E-26/204. 098/2024) and CNPq (302603/2025-5) grants. The authors declare no conflicts of interest. The data that support the findings of this study are available in Boletim InfoGripe (https: //github. com/infogripe/BoletimInfoGripe). These data were derived from the following resources available in the public domain: Sivep-Gripe (> 2022; https: //dados. gov. br/dados/conjuntos-dados/srag-2021-e-2022), Sivep-Gripe (2013–2018; https: //dados. gov. br/dados/conjuntos-dados/srag-2013-2018), and Sivep-Gripe (2009–2012; https: //dados. gov. br/dados/conjuntos-dados/srag-2009-2012). Figure S1: Estimated probabilistic epidemic bands and the fixed thresholds from the moving epidemic method for the 2025 season (horizontal lines), compared with the observed number of severe acute respiratory infection cases caused by Influenza A from 2009 to 2025, excluding COVID-19 pandemic years (2020–2022). 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Portella et al. (Wed,) studied this question.