2019 novel coronavirus disease (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is becoming a serious global public health crisis 1 . Thus far, more than 5,000,000 people have been diagnosed with COVID-19 worldwide with over 330,000 deaths 2 . Coincidently, influenza viral infection were also prevalent during this period. In previous studies, co-infection of influenza virus has been reported in a small number of Middle East respiratory syndrome coronavirus (MERS-CoV) patients, while the risk of increased or decreased severity of diseases in these co-infectious patients was still controversial 3 . Moreover, it has been found that influenza A virus positive will increase diagnostic difficulties in SARS-CoV-2 infected patients 4 . And influenza viral positive patients were normally ruled out for testing for SARS-CoV-2 in China, who may become SARS-CoV-2 shedder if the co-infection did occur. Therefore, it is crucial to investigate the clinical features and impact of co-infection on COVID-19 patients.
A single-centered retrospective study was performed in Tongji hospital (Wuhan, China) between January 12, 2020 and February 21, 2020 during the COVID-19 outbreak. SARS-CoV-2 infection was confirmed by viral nucleotide positive in qRT-PCR and influenza virus infection was diagnosed by influenza virus IgM antibody positive in serum. According to test results, patients This article is protected by copyright. All rights reserved.
were grouped into three group, including SARS-CoV-2 single positive, co-infection with influenza A virus or co-infection with influenza B virus.
Continuous variables were compared using Analysis of Variance (ANOVA) or Kruskal-Wallis test, as appropriate. Categorical variables were assessed using Pearson's χ² test or Fisher's exact test, as appropriate. All significance tests were 2-tailed, and P ≤ .05 were considered statistically significant different. The results were analyzed using SPSS for Windows, version 26.0 (IBM Corp, Armonk, NY)
A total of 307 patients were diagnosed as SARS-CoV-2 positive during the outbreak ( Figure 1A) . Continuous variables were described as median and interquartile range (IQR) or mean and standard deviation (SD) and differences were assessed using Analysis of Variance (ANOVA) or Kruskal-Wallis test. Categorical variables were expressed as number (%) and differences between groups were assessed using Pearson's χ² test or Fisher's exact test. a, ANOVA; b, Kruskal-Wallis test; c, Pearson's χ² test; d, Fisher's exact test. P < 0.05 was bold. n=131, 153 or 23 individually unless indicated. (D) Disease outcomes for patients among three groups. Favourable means disease alleviated or recovered; poor means disease aggravated with certain deaths. The differences between groups were assessed using Pearson's χ² test or Fisher's exact test.