Vero.E6 cells were seeded in a 96-well cell culture plate (Corning; 3340) 1 day prior to the assay, at a density of 20,000 cells per well. Mouse serum samples were heat inactivated at 56°C for 1 h prior to use. Serum dilutions were prepared in 1× minimal essential medium (MEM; Gibco) supplemented with 1% FBS. Virus was diluted to 10,000 50% tissue culture infectious doses (TCID50s)/mL, and 80 μL of virus and 80 μL of serum were incubated together for 1 h at RT. After the incubation, 120 μL of virus–serum mixture was used to infect cells for 1 h at 37°C. Next, the virus–serum mix was removed and 100 μL of each corresponding dilution was added to each well. A volume of 100 μL of 1× MEM were also added to the plates to get to a total volume of 200 μL in each well. Cells were incubated at 37°C for 3 days and later fixed with 10% paraformaldehyde (Polysciences) for 24 h. After 24 h, the paraformaldehyde solution was discarded and cells were permeabilized for intracellular staining. Staining and quantification were performed as previously described (40 , 44 (link), 54 (link), 55 (link)).