In our center up to 25% of the hospitalized patients with COVID-19 progress and need an intensive care unit. It is urgent to find measures that can avoid this progression to severe stages of the disease. We hypothesize that the use of anti-inflammatory drugs used at the time they start hyperinflammation episodes could improve symptoms and prognosis of patients and prevent their progression sufficiently to avoid their need for be admitted to an Intensive Care Unit.

Inclusion Criteria:

1. Age ≥ 18 years old.

2. Hospitalized patient (or documentation of a hospitalization plan if the patient is in an emergency department) with illness of more than 5 days of duration with evidence of pneumonia by chest radiography / tomography computed chest and meets at least one of the following requirements:

   1. Non-critical patient with pneumonia in radiological progression and / or
   2. Patient with progressive respiratory failure

3. Laboratory confirmed SARS-CoV-2 infection (by PCR) or other commercialized analysis or public health in any sample collected 4 days before the randomization or COVID-19 criteria following the defined diagnostic criteria at that time in the center.

4. Be willing and able to comply with the study related procedures / evaluations.

5. Women of childbearing potential * should have a negative serum pregnancy test before enrollment in the study and must commit to using methods highly effective contraceptives (intrauterine device, bilateral tubal occlusion, vasectomized couple and sexual abstinence).

6. Written informed consent. In case of inability of the patient to sign the informed consent, a verbal informed consent from the legal representative or family witness (or failing this, an impartial witness outside the investigator team) will be obtained by phone.

When circumstances so allow, participants should sign the consent form. The confirmation of the verbal informed consent will be documented in a document as evidence that verbal consent has been obtained.

Exclusion Criteria:

1. Patient who, in the investigator's opinion, is unlikely to survive> 48 hours after the inclusion in the study.

2. Presence of any of the following abnormal analytical values at the time of the inclusion in the study:

   • absolute neutrophil count less than 2000 / mm3;
   • AST or ALT> 5 times the upper limit of normality;
   • platelets <50,000 per mm3.

3. Patients with respiratory support greater than or equal to 60%

4. In active treatment with immunosuppressants or previous prolonged treatment (more 3 months) of oral corticosteroids for a disease not related to COVID-19 at a dose greater than 10 mg of prednisone or equivalent per day.

5. Known active tuberculosis or known history of tuberculosis uncompleted treatment.

6. Patients with active systemic bacterial and / or fungal infections.

7. Patients who have received previous treatment with IL6 inhibitor (tocilizumab, sarilumab).

8. Participants who, at the investigator's discretion, are not eligible to participate, regardless of the reason, including medical or clinical conditions, or participants potentially at risk of not following study procedures.

9. Patients who do not have entry criteria in the Intensive Care Unit.

10. Pregnancy or lactation.

11. Known hypersensitivity to siltuximab or to any of its excipients (histidine, histidine hydrochloride, polysorbate 80 and sucrose).