Impairment of multicycle influenza virus growth in Vero (WHO) cells by loss of trypsin activity.

We demonstrated that influenza virus replication in Vero (WHO) cells, a subline of African green monkey kidney cells, is impaired by rapid inactivation of trypsin in the culture fluids. Trypsin inactivation was caused by a factor secreted by Vero cells into the media. Repeated addition of trypsin to the culture medium of influenza virus-infected Vero cells restores the multicycle growth pattern of influenza A virus strains, allowing high yields to be obtained at a low multiplicity of infection. These findings may permit efficient use of Vero (WHO) cells in the production of influenza vaccines.