Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Neighbor-joining tree of M gene segment sequences of historic subtype H1N1 strains aligned with those of several recent vaccine strains (denoted by Δ) since the reemergence of H1N1 with A/USSR/90/77. A/Brevig Mission/1/1918 and A/New Caledonia/20/99 are underlined and in grey to emphasize the distance between these two strains.
