The reads remaining after filtering were then compared to sequences in the NCBI non-redundant nucleotide and protein databases using BLASTn and BLASTx respectively. Virus-derived sequences were detected in all 7 positive control samples and in 45/123 (37%) of previously negative serum samples (Table 1 ). In 78/123 (63%) samples, we were unable to identify virus sequence by our detection criteria (Methods).