Effectiveness and Safety of Long-acting Antibody (LAAB) to Prevent COVID-19 among High-risk Population in Thailand: a 6-month Retro-prospective Cohort Study

Abstract

During COVID-19 pandemic, evidence showed lower immunity after infection and vaccination among immunocompromised individuals. In July 2022, a prophylaxis campaign using long-acting antibodies (LAAB), tixagevimab-cilgavimab was launched in Thailand to decrease hospitalizations among high-risk groups. To evaluate the real-world effectiveness and safety of LAAB for high-risk populations, a 6-month retro-prospective cohort study was conducted starting in March 2023. We included 1,249 participants aged ≥18 years with high-risk conditions who tested negative for COVID-19 using antigen test kits during the campaign in Thailand’s central, northern, and northeastern regions. Participants provided blood samples for anti-S and anti-N IgG testing and were monitored weekly by phone for six months for acute respiratory symptoms and were screened if COVID-19 was suspected. Positive cases were further tested with RT-PCR and sequencing. We matched 600 individuals who received the study drug tixagevimab-cilgavimab (exposed) by age and comorbidities to 600 individuals who did not receive the drug (non-exposed). Predominant strain was the omicron sublineage XBB. One participant who did not receive the drug was hospitalized without respiratory failure. Anti-N IgG was positive and high levels of anti-S IgG were observed. The effectiveness of tixagevimab-cilgavimab in preventing COVID-19 infections or hospitalizations among high-risk groups was not seen. Existing immunity from previous infections and vaccinations likely influenced these results. No serious adverse events related to the drug were reported. Despite these findings, there is a potential prophylactic role of LAAB for immunocompromised groups in the early phase of a pandemic while effective vaccines and treatments are unavailable.