This is a confirmatory, randomized, placebo-controlled, multi-center, double-blinded phase II/III study. The study population consists of male or female intensive care unit (ICU) patients with a need for mechanical ventilation for more than 48 hours, aged between 18 and 80 years.

This is a confirmatory, randomized, placebo-controlled, multi-center, double-blinded phase II/III study. The study population consists of male or female intensive care unit (ICU) patients with a need for mechanical ventilation for more than 48 hours, aged between 18 and 80 years.

Eight-hundred patients will be enrolled at approximately 50 study centers. Informed consent (i.e., from the patient or from the patient's legally authorized representative) or waiver will be obtained according to regional requirements prior to any study related procedures. Patients will be randomized to receive either IC43 100 mcg or placebo and will receive the first vaccination on Day 0. The second vaccination will be applied on Day 7. In case ICU discharge occurs before Day 7, immunization will be done at the hospital ward.

Inclusion Criteria:

• male or female patients admitted to an intensive care unit (ICU) with need for mechanical ventilation for at least 48 hours, aged between 18 and 80 years at Visit 0
• written informed consent or waiver according to the national regulations
• no childbearing potential or negative pregnancy test

Exclusion Criteria:

• Sequential Organ Failure Assessment (SOFA) < 4 on Day 0
• Patients <6 months post organ transplantation
• readmission to ICU during the current total hospital stay on Day 0
• patients admitted to ICU within 2 days after surgery
• patients admitted to ICU due to trauma
• elective surgery until Day 28 after first vaccination