Safety, Tolerability and Immunogenicity of a Maternal Respiratory Syncytial Virus (RSV) Vaccine (MKK900) in Healthy Adult Women

Inclusion Criteria: 1. Adult females 18-49 years old on the day of vaccination. 2. Generally healthy, as established by medical history and clinical examination before vaccination and absence of unresolved acute disease or acute exacerbation of chronic disease. 3. Intent to reside in the area of the study site for the throughout the study visits and also available for phone follow ups after vaccination. 4. Signing an Informed Consent Form indicating that the purpose, procedures and potential risks and benefits of the study have been explained including an opportunity to ask questions. 5. Females must not be of childbearing potential OR those who are of childbearing potential must be non-pregnant and non-lactating and willing to use acceptable, highly effective methods of contraception from 28 days prior to vaccination on Day -1 through to 90 days after vaccination. Females must also agree not to donate ova from the first dose of the study vaccine until at least 90 days after vaccination. Exclusion Criteria: 1. Recent RSV exposure/vaccination: History of RSV infection within 6 months or prior RSV vaccination. 2. Pregnancy/lactation: Pregnant, breastfeeding, or planning pregnancy within 90 days post-vaccination. 3. Recent investigational products: Received any investigational product within 30 days or plans study participation during this trial. 4. Recent vaccines: Live vaccine within 30 days or any other vaccine within 14 days before study vaccination, or planned vaccination within 3 months after. 5. Blood products: Received immunoglobulins or blood products within 6 months. 6. Recent blood donation: Donation/loss of \>450 mL of blood or components within 14 days of Day 1. 7. Active illness: Acute illness or acute flare of chronic disease within 3 days before vaccination. 8. Vaccine allergies: Allergy to vaccine components (especially aminoglycosides) or severe reactions to past vaccines. 9. Neurological conditions: History of seizures, epilepsy, encephalopathy, or significant neurological disorders. 10. Mental health concerns: Any mental illness that may affect study compliance or AE reporting. 11. Malignancy: Active cancer or cancer within past 5 years (except adequately treated skin or cervical lesions). 12. Splenic issues: Asplenia, functional asplenia, or splenectomy. 13. Immunodeficiency/autoimmune disease: Congenital/acquired immunodeficiency or autoimmune diseases per Investigator judgement. 14. Immunosuppressive therapy: Systemic immunosuppressants (e.g., prolonged steroids) within 3 months; inhaled/topical steroids allowed. 15. Severe chronic diseases: Severe cardiac, pulmonary, hepatic, renal disease, or diabetes. 16. Coagulation issues: History of thrombocytopenia or bleeding disorders contraindicating IM injection. 17. Fever/infection: Fever \>38°C or active systemic infection within 7 days of vaccination. 18. Hypertension: Abnormal or uncontrolled high blood pressure at screening (≥140/90 mmHg). 19. Known or suspected infection with HBV, HCV, or HIV. 20. Alcohol or drug abuse, including regular alcohol intake of \>14 drinks/week or \>4 drinks/day. 21. Positive drug screen (amphetamines, barbiturates, benzodiazepines, cocaine, THC, methadone, methamphetamine, opiates, PCP, tricyclic antidepressants) or positive alcohol breath test at Screening or Day -1. 22. Inability to assess injection site due to tattoos or skin conditions on both deltoids. 23. Study site employees involved in the protocol or with access to study data. 24. Any other condition that may impact participant safety or interfere with study assessments, as judged by the Investigator.