According to this document, the dosage would be a one-time IV within 48 hours of exposure: 5 mg/kg IV over 1 h. This vastly differentiates smallpox treatment with cidofovir from the treatment of CMV with the same, since treatment of CMV is ongoing for the life of the patient. According to this study, they believe this one-time, one-hour IV dose will significantly attenuate or even avoid infection with smallpox if administered within 48 hrs of infection - certainly worth the difficulty of administration, and certainly worth the risk of renal damage if administered with proper accompanying medications (probenecid)...
That's a problem. Smallpox, from what I read, doesn't show obvious symptoms for a few days post infection. So how do you know the person doesn't have the flu, or some other disease? How do you know for sure who has been exposed to Smallpox?