MDCK cell-derived virus seed, exhibiting 2.6 HAU at 24 hours post infection (hpi; data not shown), was used to infect five consecutive passages of Vero cells. Adaptation of the virus to Vero cells resulted in increased virus yields within shorter time frames in the new host system: in the first passage of Vero cells 2.1 HAU were obtained at 96 hpi, whereas in the fifth passage a titer of 2.7 HAU at 72 hpi was reached (data not shown). The HA N-glycosylation pattern of the MDCK cell-derived IVA-Uruguay seed exhibited 25 different characteristic peaks in the range of 160 bp to 400 bp. Of these a total ure 1A) . In comparison to MDCK cell-derived HA, the Vero cell-derived antigen showed a tendency towards smaller glycan structures. The relative abundance of each peak over all Vero passages only varied marginally with standard deviations (SD) ≤ 2.1 % (table 1). The increase in virus titers within shorter time frames suggests increased viral fitness, during adaptation from MDCK to Vero cells. An impact of HA N-glycosylation on properties of the virus, i.e. virus replication, has already been descried [8] [9] [10] [11] [12] . Interestingly, the glycan pattern stabilized soon after the first passage in Vero cells. This clearly indicates that further increase in HA titer did not depend on changes in the HA N-glycosylation pattern.