In this first-in-humans dose escalation study, SA55 will be evaluated for safety, tolerability, and pharmacokinetics. The study is intended to enable future studies of SA55-injection's efficacy in preventing and treating COVID-19.

This is a Phase I, first time in human, randomised, double-blind, placebo-controlled, and dose escalation study.

A Screening Period of up to 14 days (Day -14 through Day 0); A Treatment Period during which participants will be resident at the hospital from Day 0, 1 day before IMP administration (on Day 1) . A Follow up Period lasting 183 days after the IMP dose.

The study will be conducted at a single study centre in Beijing, China.

Inclusion Criteria:

• Male or Female 18-65 on the day of enrollment;
• Overall good health condition, with no significant abnormalities during screening and/or pre medication physical examinations;
• Male subjects weighing ≥ 50.0kg; Female subjects with a weight of ≥ 45.0kg and a BMI between 18.0-28.0kg/m2 (including boundary values);
• The subjects and their sexual partners did not have any family planning during the study period, voluntarily took effective contraceptive measures, and did not have plans to donate sperm or eggs; Or for surgical sterilization (female: bilateral fallopian tube ligation, bilateral ovariectomy or hysterectomy; male: vas deferens ligation or vas deferens blockage surgery);
• Volunteer to participate in the experiment, cooperate with the visit, and sign an informed consent form before the start of the study.

Exclusion Criteria:

• Individuals who are known to be allergic to the investigational drug, any component in the preparation, or other similar drugs;
• The test results of hepatitis B B surface antigen (HBsAg), hepatitis C antibody (HCV Ab), HIV antibody (HIV Ab) or Treponema pallidum antibody were positive;
• Suffering from severe neurological disorders (epilepsy, seizures or convulsions) or mental illness, with a family history of mental illness, and neurological and psychiatric problems associated with various organic disorders;
• Abnormal coagulation function (such as coagulation factor deficiency, coagulation disorders, platelet abnormalities) or obvious bruising or coagulation disorders diagnosed by a doctor;
• Other uncontrolled chronic or severe disease history, including but not limited to cardiovascular disease, hematological system disease, liver and kidney disease, digestive system disease, respiratory system disease, malignant tumor, history of major organ transplantation, or any other disease or physiological condition that the researcher believes can interfere with the test results;
• The target injection site (deltoid muscle of the upper arm) may interfere with drug administration or local reaction observation due to skin damage, inflammation, ulcers, rashes, and scars;
• (Female subjects) who are pregnant (including those who have tested positive for pregnancy) or are breastfeeding;
• Have received any SARS-CoV-2 neutralizing antibody injection that is already on the market or in research before screening;
• Participated in clinical trials of other drugs or medical devices within 3 months prior to screening;
• Individuals with a body temperature greater than 37.0 ℃ on the day of medication (Day 1);
• 4 weeks prior to screening for clinically significant acute or chronic disease exacerbations, or a history of surgery;
• It is known that there is a history of SARS CoV-2 infection within 3 months (including positive detection of COVID-19 nucleic acid/antigen during enrollment and screening) or COVID-19 vaccine has been vaccinated;
• Blood donation>400 mL or significant blood loss>400 mL within 3 months prior to medication; Donate plasma or platelets within one month before medication;
• Within the first 6 months of screening, use of immunoglobulins or blood products, immune modulators such as corticosteroids and thymosin α Or blood stimulating drugs;
• Received attenuated live vaccine within 14 days prior to screening, or received subunit or inactivated vaccine within 7 days;
• Those who have used any prescription drugs, over-the-counter drugs, Chinese herbal medicines, and functional vitamins within the first 14 days of screening;
• Smokers who smoke ≥ 5 cigarettes per day within the first 3 months of screening, or who cannot guarantee giving up smoking during the trial period;
• The weekly alcohol consumption in the three months before screening was more than 14 drinking units (1 drinking unit=14g 100% alcohol=360mL beer, or 150mL wine; or 45mL distilled wine/Baijiu), or alcohol abstinence was not allowed during the test, or alcohol breath test was positive before enrollment;
• Have a history of drug abuse or dependence or recreational drug use within 2 years prior to screening, or have a positive urine drug abuse screening before enrollment;
• According to the judgment of the researchers, the subjects have any other factors that are not suitable for participating in clinical trials.

The study will be blinded for all placebo controlled dose groups, ie, the principal investigator (PI), all clinical staff involved in the clinical study, the participants, and the study monitor will remain blinded, unless safety concerns or a regulatory requirement necessitate unblinding.