The purpose of this study was to analyze the relationship between the microbial community, host immunity and the prognosis of patients with severe COVID-19.

Methods: Bronchoalveolar lavage specimens of patients with severe COVID-19 were collected and studied to analyze the relationship between the composition of patients' microecology and the occurrence and development of disease.

Inclusion Criteria:

• 1) Respiratory distress, respiratory rate ≥ 30 times/minute

  2) At rest, oxygen saturation ≤ 93%

  3）PaO2/FiO2 ≤300mmHg

  4) Shock

  5) Respiratory failure requires mechanical ventilation

  6) Complicated with other organ failure requires ICU monitoring and treatment

  7) Or in case of any of the following conditions, even if none of the above conditions is found, it is also managed as a critical case: ① pulmonary imaging shows that the lesion has significantly progressed more than 50% within 24-48 hours; ② Age>60 years old; ③ Complicated with serious chronic diseases (including diabetes, hypertension, coronary heart disease, malignant tumor, structural lung disease, pulmonary heart disease and immunosuppressed population)

Exclusion Criteria:

• 1) Pregnant women

  2) Age<18

  3) Mental disorders

  4) Patients with long-term mechanical ventilation for more than 60 days

  5) Disagreement group researcher

Patients with severe COVID-19 infection after endotracheal intubation