Purpose

Since the pandemic of coronavirus disease-19 (COVID-19), the incidence of influenza has decreased significantly, but there are still few reports in the short period before and after the pandemic period. This study aimed to explore influenza activity and dynamic changes before and during the pandemic.

Methods

A total of 1,324,357 influenza-like illness (ILI) cases were reported under ILI surveillance network from Jan 1, 2018 to Sep 5, 2021 in Nanjing, of which 16,158 cases were detected in laboratory. Differences of ILI and influenza was conducted with the chi-square test.

Results

The number of ILI cases accounted for 8.97% of outpatient and emergency department visits. The influenza-positive ratio (IPR) was 7.84% in ILI cases. During the COVID-19 pandemic, ILI% and IPR dropped by 6.03% and 11.83% on average, respectively. Besides, IPR rose slightly in Week 30-35 of 2021. Not only differences in gender, age and employment status, but also the circulating strains had changed from type A to B through the COVID-19 pandemic.

Conclusion

The level of influenza activity was severely affected by COVID-19, but it seemed that it is inevitable to be vigilant against the co-circulation in the future.

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