CD55 and infection: Thus, vaccination and prior infection cannot account for the lack of infection in V0S0 children or susceptibility of V1S1 children, and further host factors (reviewed in ref. 27) such as genotype for key immune pathways (e.g., IFTIM3, CD55, IL28B) or antigenic gaps in prior immunity (no previous or recent H1N1 exposure) should be investigated28.