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Differential clinical manifestations and viral load in influenza A(H1N1) pdm09 infections: A comprehensive analysis

Influenza infections vary in severity, posing a heightened risk of severe outcomes, including fatality, for vulnerable groups such as children, pregnant women, the elderly, immunocompromised, and those with chronic conditions. Preventive measures are crucial to mitigate the impact on these high-risk populations. This study aimed to assess the impact of influenza viral load (VL) on the clinical spectrum of illnesses across different patient groups. Three groups were established: asymptomatic (AS), symptomatic outpatients (OP), and hospitalized patients (HP).

The findings revealed 7.16% positive cases for H1N1pdm09, with children aged 5 to 11 showing the highest detection rates. AS patients had the lowest VL, differing significantly from symptomatic patients. Higher VL was observed within the first two days of illness onset. Among HP receiving antiviral treatment, 50% achieved complete viral clearance after 12 days. The study emphasizes the potential role of AS patients in viral spread and suggests considering prolonged antiviral treatment for patients with underlying diseases and severe symptoms.



Influenza viral load (VL) can be a decisive factor in determining the antiviral efficacy in viral clearance.


This study aimed to evaluate the rate of infection and the role of influenza VL on the clinical spectrum of illnesses among different patient groups attended at a tertiary hospital in Brazil.


Samples were collected from patients presenting acute respiratory infections from 2009 to 2013. Overall, 2262 samples were analyzed and distributed into three groups: (i) asymptomatic (AS); (ii) symptomatic outpatients (OP); and (iii) hospitalized patients (HP). VL (expressed in Log10 RNA copies/mL) was calculated through a quantitative real-time one-step reverse transcription-polymerase chain reaction (RT-PCR) assay aimed at the M gene, with human RNAseP target as internal control and normalizing gene of threshold cycle values.


A total of 162 (7.16%) H1N1pdm09 positive samples were analyzed. Patients aged from 0.08 to 77 years old [median ± standard deviation (SD): 12.5 ± 20.54]. Children with 5 to 11 years old presented the highest detection (p < 0.0001). AS patients had the lowest VL, with a significant difference when compared with symptomatic patients (p = 0.0003). A higher VL was observed within two days of disease onset. Ten patients (HP group) received antiviral treatment and were followed up and presented a mean initial VL of 6.64 ± 1.82. A complete viral clearance for 50% of these patients was reached after 12 days of treatment.


It is important to evaluate AS patients as potential spreaders, as viral shedding was still present, even at lower VL. Our results suggest that patients with underlying diseases and severe clinical symptoms may be considered for prolonged viral treatment.

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