—Several points raised by Dr Krause merit clarification. First, Merck was granted a US license for Vaqta (hepatitis A vaccine, inactivated), its hepatitis A vaccine, on March 29,1996; Vaqta went on the market in May 1996. The compositions of Vaqta and Havrix differ (Table). Whether these compositional differences will translate into better safety and tolerability is a scientific question that can only be answered by comparative trials. These trials for rare postvaccination adverse events will, by definition, require adverseexperience report surveillance of large populations of vaccinees that should be accruable over 1 or 2 years.