BACKGROUND: Casirivimab and imdevimab (REGEN-COV™) markedly reduces risk of 
hospitalization or death in high-risk individuals with Covid-19. Here we explore 
the possibility that subcutaneous REGEN-COV prevents SARS-CoV-2 infection and 
subsequent Covid-19 in individuals at high risk of contracting SARS-CoV-2 by 
close exposure in a household with a documented SARS-CoV-2-infected individual.
METHODS: Individuals ≥12 years were enrolled within 96 hours of a household 
contact being diagnosed with SARS-CoV-2 and randomized 1:1 to receive 1200 mg 
REGEN-COV or placebo via subcutaneous injection. The primary efficacy endpoint 
was the proportion of participants without evidence of infection (SARS-CoV-2 
RT-qPCR-negative) or prior immunity (seronegative) who subsequently developed 
symptomatic SARS-CoV-2 infection during a 28-day efficacy assessment period.
RESULTS: Subcutaneous REGEN-COV significantly prevented symptomatic SARS-CoV-2 
infection compared with placebo (81.4% risk reduction; 11/753 [1.5%] vs. 59/752 
[7.8%], respectively; P<0.0001), with 92.6% risk reduction after the first week 
(2/753 [0.3%] vs. 27/752 [3.6%], respectively). REGEN-COV also prevented overall 
infections, either symptomatic or asymptomatic (66.4% risk reduction). Among 
infected participants, the median time to resolution of symptoms was 2 weeks 
shorter with REGEN-COV vs. placebo (1.2 vs. 3.2 weeks, respectively), and the 
duration of time with high viral load (>104 copies/mL) was lower (0.4 vs. 1.3 
weeks, respectively). REGEN-COV was generally well tolerated.
CONCLUSIONS: Administration of subcutaneous REGEN-COV prevented symptomatic 
Covid-19 and asymptomatic SARS-CoV-2 infection in uninfected household contacts 
of infected individuals. Among individuals who became infected, REGEN-COV 
reduced the duration of symptomatic disease, decreased maximal viral load, and 
reduced the duration of detectable virus.(ClinicalTrials.gov number, 
NCT04452318.).