MLD₅₀ of wt and recombinant H1N1pdm viruses in Balb/c mice.

*Ca/04, A/California/04/09 (H1N1); ma-Ca/04, mouse-adapted Ca/04; maHA1:7Ca/04, recombinant carrying the HA gene from ma-Ca/04 and remaining 7 genes from Ca/04; maHA-PA2:6Ca/04, recombinant carrying the HA and PA genes from ma-Ca/04 and remaining 6 genes from Ca/04; maHA-NP2:6Ca/04, recombinant carrying the HA and NP genes from ma-Ca/04 and remaining 6 genes from Ca/04; maHA-PA-NP3:5Ca/04, recombinant carrying the HA, PA, and NP genes from ma-Ca/04 and remaining 5 genes from Ca/04; NL/602, A/Netherlands/602/09 (H1N1); NY/18HA1:7NL/602, recombinant carrying the HA gene from A/New York/18/09 (H1N1) and remaining 7 genes from NL/602; maHA1:7NL/602, recombinant carrying the HA gene from ma-Ca/04 and remaining 7 genes from NL/602.