Currently, antiviral drugs against COVID-19 and various therapeutic drugs according to the symptoms of COVID-19 are being developed all over the world. Early COVID-19 symptoms are only mild cold symptoms. However, many cases have been reported in which the symptoms worsen rapidly and progress to severe pneumonia that requires respiratory management by mechanical ventilation. In a large multicenter randomized, open-label study in hospitalized patients has been performed in the UK, this study showed reduced case fatality in patients receiving dexamethasone compared to patients receiving standard treatment. The Japanese Society of Infectious Diseases recommends treatment with dexamethasone for COVID-19 patients with chronic eosinophilic pneumonia or idiopathic organizing pneumonia (1).

However, in the course of medical care for the COVID-19, cases of bacterial pneumonia is observed not a few. In general, prescribing corticosteroids (steroids) for bacterial pneumonia exacerbates bacterial pneumonia. Therefore, the diagnosis of pneumonitis in COVID-19 patients should be made with caution.

The differential diagnosis of COVID-19 pneumonitis from common bacterial pneumonia is not easy. In COVID-19 clinical practice guidelines in Japan the Infectious Diseases Society, the following items are specified.

1 COVID-19 pneumonitis has cough but no sputum.
2 In bacterial pneumonia, there is an increase in white blood cells, especially neutrophils.
However, in pneumonitis of COVID-19, the increase in white blood cells is not observed.
3 Elevated hepatobiliary enzymes are common in COVID-19 pneumonitis.

The differential diagnosis between bacterial pneumonia and COVID-19 pneumonitis must be made with caution, as treatment strategies for each pneumonitis are different and affect the prognosis of patients.

From the current research results, SARS-CoV2 has been confirmed to be transmitted from person to person. The main transmission routes for COVID-19 are droplet infection and contact transmission. As a preventive measure against droplet infection and contact infection of SARS-CoV-2, it is effective to thoroughly wash hands and disinfect hands and wear a mask (2). In addition, the Japanese Ministry of Health, Labor and Welfare recommends covering the nose and mouth with a mask or scarf when going out, even if there are no symptoms. Further research on prevention of COVID-19 is underway.

Disclosure of potential conflicts of interest
The authors declare no potential conflicts of interest.

Research funding
This clinical research was performed with the support of the following research funding: Japan Society for the Promotion of Science for TH (grant No. 19K09840), START-program Japan Science and Technology Agency (JST) for TH (grant No. STSC20001), and National Hospital Organization Multicenter clinical study for TH (grant No. 2019- Cancer in general-02).

Acknowledgments
We thank all the medical staffs and co-medical staffs for providing and helping medical research at National Hospital Organization Kyoto Medical Center. We appreciate Crimson Interactive Japan Co., Ltd. for revising and polishing our manuscript by Native English.

References
1. https://www.mhlw.go.jp/content/000650160.pdf
2. https://www.mhlw.go.jp/content/10900000/000607629.pdf                  JAMA, New Engl J Med. Published on December 18, 2020.                              by Kyoto@Takuma H