In method 1, the nucleotide difference (3.2) among five Singaporean sequences was used to adjust the sequence errors and mutations that occurred during cell culture. In method 2, the number of variants between two sequences was reduced by 2 and the divergence time was increased by 14 days. In method 3, the nucleotide variants that were observed only once among the isolates were excluded. t = range of divergence time (days). μ = mutation rate (per site per year).