A study led by the Oswaldo Cruz Institute (IOC/FiocA study by Ruz analyzed the circulation of influenza viruses in Brazil throughout the 2025 season and confirmed that the vaccines used during that period were effective against the main strains circulating in the country.
The influenza virus, which causes the flu, is characterized by its diverse types and subtypes and a high rate of genetic variation, requiring periodic vaccine updates and continuous surveillance.
Based on data from the national Influenza Virus Laboratory Surveillance network, collected between August 2024 and August 2025, the analysis indicated the predominance of influenza A(H1N1)pdm09 in Brazilian territory, in addition to the simultaneous circulation of other virus lineages, such as influenza B (Victoria) and the A(H3N2) subtype.
Isolated cases of strains containing mutations that may affect the response to Oseltamivir, the main antiviral used in the Brazilian Unified Health System (SUS), were also identified. According to the authors, the findings were isolated and do not indicate widespread transmission, but they reinforce the importance of constant genomic and antigenic monitoring, which allows tracking how the virus changes over time and evaluating potential impacts on vaccines and treatments.
According to virologist Paola Resende, from the Laboratory of Respiratory Viruses, Exanthematic Viruses, Enteroviruses and Viral Emergencies at IOCThe analysis reinforced the importance and effectiveness of vaccination, even in the face of the genetic variations observed between 2024 and 2025.
"From a laboratory standpoint, both genetic analyses and antigen tests showed that the vaccines were able to inhibit the viruses circulating in Brazil, confirming the vaccine's effectiveness during that period," he explained.
The researcher warned, however, about the low vaccination rates, especially among priority groups defined by the Ministry of Health, such as the elderly, children, pregnant women, and people with comorbidities. According to her, expanding vaccination coverage is fundamental to strengthening collective protection and reducing the risk of severe cases and hospitalizations.
Following the changes caused by the Covid-19 pandemic in the dynamics of respiratory virus circulation, influenza is gradually returning to its seasonal pattern in Brazil, which reinforces the importance of influenza monitoring.
In this context, the results were presented in the article 'Molecular and antigenic overview of influenza viruses circulating in Brazil during the 2025 season., released as a preprint in January 2026.
The analyses were based on more than 106 samples collected in different regions of Brazil between August 2024 and August 2025, originating from both cases of influenza-like illness and severe acute respiratory syndrome (SARS).
The work combined genetic sequencing of the viruses — to identify subtypes, lineages, and mutations — with antigen tests — which assess whether the circulating viruses continue to be recognized by the antibodies induced by the vaccine strains.
Regarding the samples analyzed, approximately 12,8% tested positive for influenza. Of the total confirmed cases, influenza A(H1N1)pdm09 was responsible for the largest share of infections, accounting for approximately 40% of the records, followed by influenza B of the Victoria lineage, with about 30%. The A(H3N2) subtype showed more limited circulation, around 7% of cases.
In records of severe acute respiratory syndrome associated with influenza, A(H1N1)pdm09 also stood out as the main agent, being present in most hospitalized cases and in almost half of the deaths confirmed due to influenza in the analyzed period.
Genomic analyses indicated the simultaneous circulation of different subgroups of influenza A and B viruses in distinct regions of the country. According to virologist Paola Resende, this behavior reflects the natural dynamics of the virus, which can vary over time and manifest itself differently between regions, influenced by factors such as climate and the movement of people.
"The circulation of the influenza virus does not follow a fixed pattern. Once it enters the country, its spread depends on factors such as the flow of people, climate, and regional characteristics, which can generate peaks at different times throughout the year," he summarized.
Despite this genetic diversity, the subtypes analyzed remained compatible with the strains used in the vaccines. Thus, vaccination continues to be an effective strategy for protection against influenza.
The researchers also investigated genetic markers associated with resistance to antivirals, such as oseltamivir, the main drug used to treat influenza in the Brazilian public health system (SUS). According to the authors, the findings were rare, did not spread, and do not indicate a loss of effectiveness of the antiviral, but they reinforce the importance of ongoing surveillance.
The study is the result of work by the national Influenza Virus Laboratory Surveillance network, which operates in an integrated manner with the global influenza monitoring system coordinated by the WHO. In total, the initiative involved the participation of 46 institutions and 116 co-authors.
In Brazil, this network effort directly involves three National Influenza Centers (NICs) — the Evandro Chagas Institute (IEC), the Adolfo Lutz Institute (IAL), and the Oswaldo Cruz Institute itself — which receive samples sent by the Central Public Health Laboratories (LACENs) and perform complementary laboratory analyses.
The work also relies on the integrated actions of municipal, state, and national epidemiological surveillance agencies, strengthening the country's capacity to detect, monitor, and respond to the circulation of influenza viruses.
In addition to monitoring the circulation of viruses in the country and the world, the network subsidizes international meetings responsible for defining and updating the strains that make up the flu vaccine.
Much of the data generated by the study was submitted to the WHO in the form of a technical report in September 2025, contributing to the definition of the composition of the influenza vaccine for the Southern Hemisphere for the 2026 season.
"There is a global network that works tirelessly to monitor influenza viruses, and this work supports the international meetings that decide on updating the strains that make up the flu vaccine," he highlighted.
In addition to guiding the composition of vaccines and antiviral drugs, influenza surveillance plays a strategic role in the early detection of emerging risks, including the emergence of viruses with pandemic potential.
During the analyzed season, a rare case of influenza A(H3N2)v was recorded in Paraná, associated with exposure to swine. Although the investigation did not identify sustained transmission between humans, the episode highlighted the importance of maintaining systems capable of rapidly detecting viruses that cross the species barrier.
“Influenza has epidemic and pandemic potential. Therefore, it is essential to maintain sensitive and constant monitoring, capable of quickly identifying both mutations associated with antiviral resistance and zoonotic events, before they spread,” stated Paola Resende.
According to the authors, the circulation of influenza viruses in humans and animals creates opportunities for genetic rearrangements that can give rise to new variants. In this context, genomic and epidemiological monitoring not only supports immediate public health actions, but also strengthens the country's capacity to respond to future epidemic and pandemic scenarios, in coordination with the Ministry of Health and the World Health Organization.
A study led by the Oswaldo Cruz Institute (IOC/FiocA study by Ruz analyzed the circulation of influenza viruses in Brazil throughout the 2025 season and confirmed that the vaccines used during that period were effective against the main strains circulating in the country.
The influenza virus, which causes the flu, is characterized by its diverse types and subtypes and a high rate of genetic variation, requiring periodic vaccine updates and continuous surveillance.
Based on data from the national Influenza Virus Laboratory Surveillance network, collected between August 2024 and August 2025, the analysis indicated the predominance of influenza A(H1N1)pdm09 in Brazilian territory, in addition to the simultaneous circulation of other virus lineages, such as influenza B (Victoria) and the A(H3N2) subtype.
Isolated cases of strains containing mutations that may affect the response to Oseltamivir, the main antiviral used in the Brazilian Unified Health System (SUS), were also identified. According to the authors, the findings were isolated and do not indicate widespread transmission, but they reinforce the importance of constant genomic and antigenic monitoring, which allows tracking how the virus changes over time and evaluating potential impacts on vaccines and treatments.
According to virologist Paola Resende, from the Laboratory of Respiratory Viruses, Exanthematic Viruses, Enteroviruses and Viral Emergencies at IOCThe analysis reinforced the importance and effectiveness of vaccination, even in the face of the genetic variations observed between 2024 and 2025.
"From a laboratory standpoint, both genetic analyses and antigen tests showed that the vaccines were able to inhibit the viruses circulating in Brazil, confirming the vaccine's effectiveness during that period," he explained.
The researcher warned, however, about the low vaccination rates, especially among priority groups defined by the Ministry of Health, such as the elderly, children, pregnant women, and people with comorbidities. According to her, expanding vaccination coverage is fundamental to strengthening collective protection and reducing the risk of severe cases and hospitalizations.
Following the changes caused by the Covid-19 pandemic in the dynamics of respiratory virus circulation, influenza is gradually returning to its seasonal pattern in Brazil, which reinforces the importance of influenza monitoring.
In this context, the results were presented in the article 'Molecular and antigenic overview of influenza viruses circulating in Brazil during the 2025 season., released as a preprint in January 2026.
The analyses were based on more than 106 samples collected in different regions of Brazil between August 2024 and August 2025, originating from both cases of influenza-like illness and severe acute respiratory syndrome (SARS).
The work combined genetic sequencing of the viruses — to identify subtypes, lineages, and mutations — with antigen tests — which assess whether the circulating viruses continue to be recognized by the antibodies induced by the vaccine strains.
Regarding the samples analyzed, approximately 12,8% tested positive for influenza. Of the total confirmed cases, influenza A(H1N1)pdm09 was responsible for the largest share of infections, accounting for approximately 40% of the records, followed by influenza B of the Victoria lineage, with about 30%. The A(H3N2) subtype showed more limited circulation, around 7% of cases.
In records of severe acute respiratory syndrome associated with influenza, A(H1N1)pdm09 also stood out as the main agent, being present in most hospitalized cases and in almost half of the deaths confirmed due to influenza in the analyzed period.
Genomic analyses indicated the simultaneous circulation of different subgroups of influenza A and B viruses in distinct regions of the country. According to virologist Paola Resende, this behavior reflects the natural dynamics of the virus, which can vary over time and manifest itself differently between regions, influenced by factors such as climate and the movement of people.
"The circulation of the influenza virus does not follow a fixed pattern. Once it enters the country, its spread depends on factors such as the flow of people, climate, and regional characteristics, which can generate peaks at different times throughout the year," he summarized.
Despite this genetic diversity, the subtypes analyzed remained compatible with the strains used in the vaccines. Thus, vaccination continues to be an effective strategy for protection against influenza.
The researchers also investigated genetic markers associated with resistance to antivirals, such as oseltamivir, the main drug used to treat influenza in the Brazilian public health system (SUS). According to the authors, the findings were rare, did not spread, and do not indicate a loss of effectiveness of the antiviral, but they reinforce the importance of ongoing surveillance.
The study is the result of work by the national Influenza Virus Laboratory Surveillance network, which operates in an integrated manner with the global influenza monitoring system coordinated by the WHO. In total, the initiative involved the participation of 46 institutions and 116 co-authors.
In Brazil, this network effort directly involves three National Influenza Centers (NICs) — the Evandro Chagas Institute (IEC), the Adolfo Lutz Institute (IAL), and the Oswaldo Cruz Institute itself — which receive samples sent by the Central Public Health Laboratories (LACENs) and perform complementary laboratory analyses.
The work also relies on the integrated actions of municipal, state, and national epidemiological surveillance agencies, strengthening the country's capacity to detect, monitor, and respond to the circulation of influenza viruses.
In addition to monitoring the circulation of viruses in the country and the world, the network subsidizes international meetings responsible for defining and updating the strains that make up the flu vaccine.
Much of the data generated by the study was submitted to the WHO in the form of a technical report in September 2025, contributing to the definition of the composition of the influenza vaccine for the Southern Hemisphere for the 2026 season.
"There is a global network that works tirelessly to monitor influenza viruses, and this work supports the international meetings that decide on updating the strains that make up the flu vaccine," he highlighted.
In addition to guiding the composition of vaccines and antiviral drugs, influenza surveillance plays a strategic role in the early detection of emerging risks, including the emergence of viruses with pandemic potential.
During the analyzed season, a rare case of influenza A(H3N2)v was recorded in Paraná, associated with exposure to swine. Although the investigation did not identify sustained transmission between humans, the episode highlighted the importance of maintaining systems capable of rapidly detecting viruses that cross the species barrier.
“Influenza has epidemic and pandemic potential. Therefore, it is essential to maintain sensitive and constant monitoring, capable of quickly identifying both mutations associated with antiviral resistance and zoonotic events, before they spread,” stated Paola Resende.
According to the authors, the circulation of influenza viruses in humans and animals creates opportunities for genetic rearrangements that can give rise to new variants. In this context, genomic and epidemiological monitoring not only supports immediate public health actions, but also strengthens the country's capacity to respond to future epidemic and pandemic scenarios, in coordination with the Ministry of Health and the World Health Organization.
The non-profit reproduction of the text is allowed as long as the source is cited (Comunicação / Instituto Oswaldo Cruz)
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