We thank Souto and colleagues for bringing their study to our attention. We stand by the first statement that was made with regard to warfarin sodium alone and not to all oral anticoagulants in general. Our study was performed to demonstrate the safety of intramuscular influenza vaccination in patients receiving long-term anticoagulant therapy with warfarin, the most widely used oral anticoagulant agent in North America. However, the study by Souto et al1 confirms the lack of any interaction with acenocoumarol, a short-acting coumarin compound that is very similar to warfarin. With two of the three 4-hydroxy coumarin derivatives (warfarin sodium, acenocoumarol, and dicumarol) showing no interaction with influenza vaccine, perhaps it is safe to say that there is no significant interaction between influenza vaccine and the coumarin group of oral anticoagulants.
We take exception to the statement made by Souto and colleagues about administering influenza vaccine subcutaneously. The effectiveness of