Superinfection is frequently detected among individuals infected by human immunodeficiency virus type Ⅰ (HIV-1).Superinfection occurs at similar frequencies at acute and chronic infection stages but less frequently than primary infection.This observation indicates that the immune responses elicited by natural HIV-1 infection may play a role in curb of superinfection;however,these responses are not sufficiently strong to completely prevent superinfection.Thus,a successful HIV-1 vaccine likely needs to induce more potent and broader immune responses than those elicited by primary infection.On the other hand,potent and broad neutralization responses are more often detected after superinfection than during monoinfection.This suggests that broadly neutralizing antibodies are more likely induced by sequential immunization of multiple different immunogens than with only one form of envelope glycoprotein immunogens.Understanding why the protection from superinfection by immunity induced by primary infection is insufficient and if superinfection can lead to cross-reactive immune responses will be highly informative for HIV-1 vaccine design.