The effect of subtype-specific influenza illness in young children on the probability of influenza illness in the following year was analyzed for placebo recipients in four 2-year vaccine efficacy studies. There was evidence of homosubtypic protection but no evidence of heterosubtypic protection; however, a moderate-to-low level of heterosubtypic protection could not be ruled out. This analysis supports previous observations that children do not develop lasting, clinically significant heterosubtypic protection after influenza illness.
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From *Medical and Scientific Affairs and †Biostatistics, MedImmune, LLC, Gaithersburg, MD.
Accepted for publication April 13, 2011.
Supported by MedImmune.
Both authors are employees of MedImmune, LLC, Gaithersburg, MD.
The authors have no other funding or conflicts of interest to disclose.
Address for correspondence: Christopher S. Ambrose, MD, Medical and Scientific Affairs, MedImmune, LLC, One MedImmune Way, Gaithersburg, MD 20878. E-mail: firstname.lastname@example.org.
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