To the Editors:
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Coronavirus disease 2019 (CoVID-19) is a new infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated from Wuhan in China and has now spread globally. However, despite the concern focused on SARS-CoV-2, influenza virus continues to circulate and cause disease. Here we report a mixed infection. Physicians should be alert that a positive test for influenza does not rule out the possibility of COVID-19 disease.

The SARS-COV-2 outbreak in late December of 2019 in Wuhan, China, has caused many infections and deaths globally. SRAS-COV-2 is a new respiratory tract transmitted disease mainly through respiratory droplet and close contact, aerosol but fecal-oral route is also suspected. As of March 19, 2020, a total of 23,473 cases, and 9840 deaths were reported.^[@R1]^

In China, several respiratory viruses are also now active including influenza, parainfluenza virus, respiratory syncytial virus, adenovirus, and now SARS-COV-2. Unfortunately, according to the World Health Organization influenza website,^[@R2]^ the respiratory illness indicators and influenza activity remained elevated overall in the northern hemisphere which are in a "flu" season. The weekly report of the influenza surveillance reported that the United States now has its highest pneumonia and influenza mortality since 2004, except for the 2009 pandemic.^[@R3]^ During the SARS pandemic in 2003, Yang et al^[@R4]^ found that the patients with fever, cough or sore throat had a 5% of influenza virus positive rate, and with SARS infection reportedly increasing at the meantime. This raises the concerns that there might be mixed infections of seasonal influenza and the novel coronavirus. Thus, we do think there might be a change of mixed infection of the influenza virus and SARS-COV-2. The Chinese experts from China-Japan Friendship Hospital have reported a case coinfection with influenza A virus and SARS-CoV-2.^[@R5]^ So, in an era when testing for COVID-19 is in short supply in many areas of the world, when patients have a positive test for influenza, one cannot assume that they do not also have SARS-CoV-2 infection. Measures should be taken to enhance the respiratory infectious diseases surveillance systems and screen the people with fever, cough or sore throat for both viruses with oral, nasopharyngeal and anal swabs.

Di Wu, MMed

Jianyun Lu, MMed

Xiaowei Ma, MMed

Qun Liu, MMed

Dedong Wang, MMed

Yuzhou Gu, MMed

Yongguang Li, BSc

Weiyun He, BSc

COVID-19 Emergency Response Unit, Guangzhou Center for Disease Control and Prevention, Guangzhou, People's Republic of China

This work was supported by the National Natural Science Foundation of China Grants (81803325), Medical Science and Technology Project of Guangzhou (20191A011064), Guangdong Medical Science and Technology Research Project (A2019379).

The authors have no conflicts of interest to disclose.

Di Wu and Jianyun Lu contributed equally to this letter.
