The half-maximal inhibitory concentration (IC50) of chloroquine inhibition of SARS-CoV replication in Vero E6 cells, 8·8 μmol/L, is substantially lower than the plasma concentrations that are reached in humans when the drug is prescribed to treat malaria at a dose of 25 mg/kg over 3 days.1 For long-term prophylaxis, even lower doses could be used.