Epidemiology of influenza under the coronavirus disease 2019 pandemic in Nanjing, China

Abstract

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 the ILI surveillance network from January 1, 2018, to September 5, 2021, in Nanjing, of which 16 158 cases were detected in a laboratory. Differences in ILI and influenza were conducted with the χ² 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 this, 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 seems that it is inevitable to be vigilant against the co-circulation in the future.

Keywords: COVID-19; epidemiology; etiology; influenza; influenza-like illness.

Figure 1

Age groups of influenza‐like illness in five sentinel hospitals in Nanjing, January 1, 2018 to September 5, 2021 (192 weeks). Abbreviations: NDTH, Nanjing Drum Tower Hospital; NFH, Nanjing First Hospital; NJH, Nanjing Jiangning Hospital; NMUCH, Children's Hospital of Nanjing Medical University; NMUSH, the Second Affiliated Hospital of Nanjing Medical University

Figure 2

Time series chart of influenza‐like illness (ILI) and ILI% and influenza‐positive ratio in Nanjing, January 1, 2018 to September 5, 2021 (192 weeks). There are 52 weeks in 2018, 2019, 2021, and 53 weeks in 2020. The horizontal ordinate label interval is 13 weeks

Figure 3

The influenza‐positive ratio for gender (A), age (B), employment (C), and sentinel hospital (D) in 2018–2021. Abbreviations: NDTH, Nanjing Drum Tower Hospital; NFH, Nanjing First Hospital; NJH, Nanjing Jiangning Hospital; NMUCH, Children's Hospital of Nanjing Medical University; NMUSH, the Second Affiliated Hospital of Nanjing Medical University. ns refers to p‐value > 0.05; * refers to 0.05 ≥ p‐value > 0.01; ** refers to 0.01 ≥ p‐value > 0.001; *** refers to p‐value ≤ 0.001

Figure 4

The number of specimens detected for influenza and influenza‐positive ratio in Nanjing, January 1, 2018 to September 5, 2021 (192 weeks). There are 52 weeks in 2018, 2019, 2021, and 53 weeks in 2020. The horizontal ordinate label interval is 13 weeks

Figure 5

Influenza types for gender (A), age (B), employment (C), and sentinel hospital (D) in 2018–2020. Abbreviations: NDTH, Nanjing Drum Tower Hospital; NFH, Nanjing First Hospital; NJH, Nanjing Jiangning Hospital; NMUCH, Children's Hospital of Nanjing Medical University; NMUSH, the Second Affiliated Hospital of Nanjing Medical University. ns refers to p‐value > 0.05; * refers to 0.05 ≥ p‐value > 0.01; ** refers to 0.01 ≥ p‐value > 0.001; *** refers to p‐value ≤ 0.001. In 2021, 19 influenza cases were laboratory‐confirmed, which led to no statistically significant difference