Reply to Sambhara.

TO THE EDITOR—We are pleased to respond to the interesting questions raised in Sambhara’s letter regarding our article [1]. How has prior exposure to 2009 pandemic influenza A virus subtype H1N1 (A[H1N1]pdm09) or previous vaccination influenced the A(H1N1)pdm09specific vaccine responses? In Norway, mass vaccination was initiated before the peak of the pandemic, and healthcare workers (HCWs) were prioritized for early vaccination [2]. Hospital policy was that all HCWs who experienced respiratory symptoms were required to have a A(H1N1)pdm09-negative result of polymerase chain reaction analysis before returning to work. None of our HCWs reported experiencing an influenza-like illness prior to or after vaccination. All subjects (10 controls and 2 low responders) who had prevaccination hemagglutination