Recombinant HA can be produced using different expression systems. When expressed in E. coli the resulting HA protein gave rise to the induction of hemagglutination inhibition (HI) titers upon immunization of mice [15] . However, as proper folding and trimerization of the HA protein requires multiple posttranslational modifications including glycosylation and disulfide bond formation, expression of the HA protein in higher eukaryotic systems is likely to result in superior antigenicity. Thus, mammalian cellderived HA trimers were found to induce much higher levels of neutralizing antibodies than similarly produced monomeric HA protein [16] . The baculovirus expression system has been used to produce strain specific HA antigens in insect cells, which were shown to protect against a HPAI H5N1 challenge [17] . Such fullsize HA proteins may, however, be limited in their efficacy because the membrane proteins may not retain their native membrane-bound structure upon purification [18] . Hence, the production of recombinant, soluble, stable HA trimers that are secreted from the cells seems like an attractive alternative approach. Such HA trimers, expressed either in insect or mammalian cells, were indeed shown to elicit neutralizing antibodies [16] and to partially protect mice against HPAI H5N1 challenge infection [19] .