Objective To investigate the epidemic characteristics of influenza in children and the features of severe influenza.
Methods From January 2016 to September 2022, 1 600 samples from hospitalized cases of severe acute respiratory tract infection and 7 660 samples from outpatients with influenza-like illness were collected. Influenza virus was detected by real-time RT-PCR. Other respiratory viruses in the samples of severe hospitalized cases and some samples of outpatients were detected. Clinical features of influenza virus infection and co-infection were analyzed.
Results The positive rate of influenza virus in the 1 600 hospitalized cases of severe acute respiratory infection was 6.63% (106 cases). H1N1, H3N2, BV and BY were deteted in 49.06% (52 cases), 17.92% (19 cases), 29.25% (31 cases) and 3.77% (4 cases) of the 106 cases, respectively. The positive rate of influenza virus in the 7 660 out-patient cases was 15.01% (1 150 cases), and H1N1, H3N2, BV and BY were detected in 22.17% (255 cases), 30.96% (356 cases), 41.39% (476 cases) and 5.48% (63 cases) of the infected cases, respectively. Influenza A (H1N1) virus was more likely to cause severe influenza in children (χ2=37.978, P<0.001), while seasonal H3N2 and BV strains were less likely to cause severe influenza in children (χ2=7.871, P=0.005; χ2=5.948, P=0.015). There was no statistically significant difference in the positive rates of BY lineage in the two groups. Severe influenza mainly occured in the peak season of influenza epidemic. There was no significant difference in the clinical manifestations between the children infected with the four different influenza viruses. In the 106 severe cases of influenza, the co-infection rate of influenza virus with other respiratory viruses was 17.92% (19 cases), while the co-infection rate reached 34.81% (47 cases) in 135 outpatient cases of influenza. The difference in the co-infection rates was statistically significant between outpatient and hospitalized cases (χ2=10.734, P=0.001).
Conclusions Influenza A (H1N1) virus was more likely to cause severe influenza in infants and young children in comparison with seasonal H3N2 and BV. There was no significant difference in the clinical features of influenza caused by H1N1, H3N2, BV and BY. Co-infection of influenza virus with other respiratory viruses is not a major risk factor for severe influenza in infants.