2.3 Infectivity assay (TCID50)
Samples that potentially contained infectious virus were titrated by tissue culture infectious dose 50% (TCID50) assay, that is, eightfold replicates of serial half-log sample dilutions were incubated with cells for 5-7 days and CPE was assessed microscopically. Virus concentrations were calculated according to the Poisson distribution and expressed as viral load (log10[TCID50]).