inclusion criteria: * adults (≥18 years of age) with active severe acute respiratory syndrome coronavirus 2 (sars-cov-2) infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. * a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care \[covid-19 related or otherwise\]), as defined by an 8 point ordinal scale. * male participants: * a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. * female participants: * a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: * not a woman of childbearing potential (wocbp). or * a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. * ability to provide informed consent signed by the study participant or legally authorized representative. * ability and willingness to participate in telephone/telemedicine follow-up visits if needed. * zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: * adults (≥18 years of age) with active severe acute respiratory syndrome coronavirus 2 (sars-cov-2) infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. * a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care \[covid-19 related or otherwise\]), as defined by an 8 point ordinal scale. * male participants: * a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. * female participants: * a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: * not a woman of childbearing potential (wocbp). or * a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. * ability to provide informed consent signed by the study participant or legally authorized representative. * ability and willingness to participate in telephone/telemedicine follow-up visits if needed. * zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: adults (≥18 years of age) with active severe acute respiratory syndrome coronavirus 2 (sars-cov-2) infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. male participants: a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. female participants: a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: not a woman of childbearing potential (wocbp). or a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. ability to provide informed consent signed by the study participant or legally authorized representative. ability and willingness to participate in telephone/telemedicine follow-up visits if needed. zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: adults (≥18 years of age) with active severe acute respiratory syndrome coronavirus 2 (sars-cov-2) infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. male participants: a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. female participants: a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: not a woman of childbearing potential (wocbp). or a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. ability to provide informed consent signed by the study participant or legally authorized representative. ability and willingness to participate in telephone/telemedicine follow-up visits if needed. zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: - adults (≥18 years of age) with active sars-cov-2 infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. - a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. - male participants: - a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. - female participants: - a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: - not a woman of childbearing potential (wocbp). or - a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. - ability to provide informed consent signed by the study participant or legally authorized representative. - ability and willingness to participate in telephone/telemedicine follow-up visits if needed. - zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: - adults (≥18 years of age) with active sars-cov-2 infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. - a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. - male participants: - a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. - female participants: - a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: - not a woman of childbearing potential (wocbp). or - a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. - ability to provide informed consent signed by the study participant or legally authorized representative. - ability and willingness to participate in telephone/telemedicine follow-up visits if needed. - zilucoplan only: antibiotic prophylaxis: all participants must be willing to take antibiotic prophylaxis concomitantly, starting with the first dose of zilucoplan or placebo.

inclusion criteria: - adults (≥18 years of age) with active sars-cov-2 infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. - a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. - male participants: - a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. - female participants: - a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: - not a woman of childbearing potential (wocbp). or - a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. - ability to provide informed consent signed by the study participant or legally authorized representative. - ability and willingness to participate in telephone/telemedicine follow-up visits if needed.

inclusion criteria: - adults (≥18 years of age) with active sars-cov-2 infection confirmed by laboratory tests and/or point of care tests (eg, commercial or public health assay, which is approved for emergency use). if no diagnostic test results are available that have been obtained during the previous 72 hours, then a test should be performed as part of the screening assessment. - a score of grade 2 (hospitalized, on invasive mechanical ventilation or ecmo), grade 3 (hospitalized, on noninvasive ventilation or high-flow oxygen devices), grade 4 (hospitalized, requiring supplemental oxygen), or grade 5 (hospitalized, not requiring supplemental oxygen, requiring ongoing medical care [covid-19 related or otherwise]), as defined by an 8 point ordinal scale. - male participants: - a male participant must agree to use contraception during the treatment period and for at least 6 weeks after the last dose of study treatment and refrain from donating sperm during this period. - female participants: - a female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies: - not a woman of childbearing potential (wocbp). or - a wocbp who agrees to follow the contraceptive guidance during the treatment period and for at least 6 weeks after the last dose of study treatment. - ability to provide informed consent signed by the study participant or legally authorized representative. - ability and willingness to participate in telephone/telemedicine follow-up visits if needed.