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Table 3 Development of similarity criteria

From: Linking healthcare associated norovirus outbreaks: a molecular epidemiologic method for investigating transmission

Point mutations (n nucleotides) Similarity probability formula The probability (expressed as a percent) that two viruses will differ by n or more nucleotides given that they are from the same outbreak. Average number of viral sequences that would have to be sequenced from the same outbreak to have 1 sequence with n nucleotide changes (1/(a)n)
1 nucleotide an = a1 17.2% 6.2
2 nucleotides an = a2 2.96% 38.5
3 nucleotides an = a3 0.509% Approx. 250
4 nucleotides an = a4 0.088% Approx. 1000
Any changes   < 20% > 5