A(H1N1)pdm09 vaccination of healthcare workers: improved immune responses in low responders following revaccination.

TO THE EDITOR—I read with great interest the article by Pathirana et al [1], which demonstrated that a 2-dose vaccination regimen with ASO3 adjuvant is needed to improve immune responses against 2009 pandemic influenza A virus subtype H1N1 (A[H1N1]pdm09) infection in low responders. The authors, in their discussion, suggested that the rapid induction of antibodies in the responders may have been due to a recall response to prior infection/vaccination. Indeed, that seems to be the case, although they did not elaborate on this in the results section. According to Figure 2A in their article [1], at the time of vaccination at least 10 of 25 subjects in the control (ie, responder) group had detectable and significant hemagglutination inhibition (HI) titers against A(H1N1)pdm09, which ranged from 8–64, while only 2 of 15 low responders had HI titers (8 in one low responder and 32 in the other). It would be interesting if the data from these 10 subjects were analyzed and shown separately, to allow assessment of