In addition, vaccine - makers that use eggs can not begin developing new vaccines that target new virus strains until the U.S. Centers for Disease Control and Prevention (CDC) creates a live - virus reference strain for these companies to work with, a process that could take several weeks. (scientificamerican.com)
By comparing the amount of mutation, says Smith, «you can say with a high degree of confidence how many bacterial generations separate an unknown strain from closely related reference strains». (newscientist.com)
At that time, Keim noted a very close genetic similarity between the Porton Down strains and what had become the standard laboratory reference strain used in experiments and vaccine development, known as the Ames Ancestor strain. (sciencedaily.com)