COVID-19 Monoclonal Antibodies: How They Worked, Why They Fell Out of Use, and What Replaced Them

Monoclonal antibodies (mAbs) are laboratory-engineered proteins designed to mimic the immune system's ability to neutralize SARS-CoV-2. During the early pandemic, they were a critical treatment option for high-risk outpatients, but their utility has dramatically shifted as the virus evolved.

Mechanism of Action

These antibodies bind specifically to the spike protein on the surface of the coronavirus, physically blocking the virus from attaching to and entering human cells. They were administered either through intravenous (IV) infusion or subcutaneous injection, typically in outpatient infusion centers.

Historical Use and Current Status

• Early pandemic (2020–2022): Combinations such as bamlanivimab/etesevimab, casirivimab/imdevimab (REGEN-COV), sotrovimab, and bebtelovimab demonstrated significant reductions in hospitalization and death among high-risk, non-hospitalized patients.
• Omicron era: As Omicron subvariants (BA.2, BA.4/BA.5, BQ.1, XBB, etc.) accumulated mutations in the spike protein, nearly all authorized mAbs lost their neutralizing activity. By early 2023, the FDA had revoked authorization for all treatment-use mAbs in the U.S.
• Pemivibart (Pemgarda): In March 2024, the FDA authorized pemivibart for pre-exposure prophylaxis only in moderately to severely immunocompromised individuals aged 12 and older who are unlikely to mount an adequate immune response to vaccination. It is not authorized for treatment of active COVID-19.

Key Limitations That Led to Their Decline

• Rapid variant susceptibility changes: The virus evolves faster than new mAbs can be developed, tested, and authorized, requiring constant genomic surveillance.
• Not a substitute for vaccination: Vaccines remain the primary prevention strategy; mAbs were always a complement, not a replacement.
• Logistical hurdles: IV infusion requires healthcare facility visits, creating access barriers and supply-chain complexities.
• Cost and equity concerns: High per-dose costs and uneven distribution exacerbated healthcare disparities.

Current Standard of Care for High-Risk Individuals

• Vaccination: Updated 2024–2025 formula mRNA vaccines (targeting JN.1 lineage) are the cornerstone of prevention.
• Oral antivirals: Paxlovid (nirmatrelvir/ritonavir) and Lagevrio (molnupiravir) for high-risk outpatients started within 5 days of symptom onset.
• IV antiviral: Remdesivir (3-day outpatient course) for high-risk patients who cannot take oral antivirals.
• Pre-exposure prophylaxis: Pemivibart for eligible immunocompromised patients.

Bottom line: If you are at high risk for severe COVID-19, discuss the current therapeutic options with your healthcare provider. Guidelines continue to evolve alongside the virus.