Virus isolation efficiency
For primary virus isolation, an aliquot of the 20 clinical samples was inoculated into the three MDCK cell lines and embryonated hens’ eggs. In MDCK-2 and MDCK-3 cells all viruses grew after one blind passage following primary inoculation (Table 1). All five influenza A(H1N1) and B Victoria-lineage viruses but only 60% of the B Yamagata-lineage viruses grew at the second passage in MDCK-1 cells, whereas 60% of influenza A(H3N2) viruses grew on the third passage. For comparison, isolation efficiency in eggs was 60% for influenza A(H1N1) and influenza B Victoria-lineage, 40% for influenza A (H3N2), and 20% for influenza B Yamagata-lineage at passage levels E3, E4, E3, and E3, respectively. The characteristics of viruses isolated in embryonated hens’ eggs will be presented elsewhere [38]. Overall, both anchorage dependent and suspension MDCK cells were more sensitive than eggs by at least an order of magnitude for primary isolation of influenza A and B viruses.