Antibodies elicited by vaccination. Mice (n = 8) were vaccinated via intramuscular injection at weeks 0 and 3 with VLPs (clade 1 or clade 2) or recombinant HA, or PBS only (Mock). At week 5, serum from each group was pooled and tested for anti-HA antibodies by ELISA. The endpoint dilution titer was determined against Indo/05 rHA or VN/04 rHA. Pre-immune sera from mice had no detectable specific anti-HA antibodies.