2.5. Infectivity assay
The viral titer was determined by 50% cell-culture-infective-dose (CCID50) of Reed and Muench approach. Briefly, the serial tenfold dilutions of a pooled virus fraction obtained from different purification step in Eagle’s Minimum Essential Medium (pH=7.3) was inoculated for 48 h on IBRS2 monolayer cell culture prepared in 96-well micro plate and was incubated for 72 h after the inoculation. Virus infectivity was calculated as describe before (Frenz & Horváth, 1985).