Children younger than 5 years had more than eightfold higher odds of influenza co-infection with other respiratory viruses compared with older adults, according to a recent study.
A retrospective study, published in Pathology, examined influenza epidemiology and co-infections with other respiratory viruses in two metropolitan health districts in New South Wales, Australia, between 2018 and 2023. Data was collected from 386,283 respiratory specimens, of which 3.3 percent tested positive for influenza. Influenza activity varied across the study period, with markedly reduced detections during 2020 and 2021, when fewer than 60 cases were identified and positivity rates were below 0.2 percent. In contrast, 2019 recorded 3,312 cases with a positivity rate of 14.4 percent.
Following the relaxation of pandemic-related measures, influenza activity increased, with positivity rates of 2.1 percent in 2022 and 3.9 percent in 2023. Influenza A virus predominated throughout, with peak activity occurring earlier in 2022 and 2023 than in most pre-pandemic years. Influenza B virus activity was higher in 2019 and 2023 compared with other years.
Of 12,778 influenza-positive specimens, 2,010 were tested for a full panel of respiratory viruses. Among these, 17.2 percent demonstrated co-infections, most frequently with rhinovirus. Analysis indicated higher odds of influenza co-infections in children younger than 5 years and in those aged 5 to 17 years compared with adults aged 65 years and older. Increased odds of co-infections were also observed in 2022 .
Severe acute respiratory syndrome coronavirus 2 was detected in 1.3 percent of influenza-positive specimens, all involving influenza A. Respiratory syncytial virus appeared more frequently as a co-infecting virus in 2022 than in earlier years.
“In response to the COVID-19 pandemic, overall influenza testing volumes at our study sites expanded significantly (7.3-fold in 2022 and 3.8-fold in 2023 compared to 2019) due to the implementation of multiplex PCR assays for simultaneous SARS-CoV-2/IFV/RSV detection,” noted the study authors.
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