Six strains of vaccinia were isolated from 3,000 doses of Dryvax and found to exhibit significant variation in virulence.

The strain with the most similar virulence to the overall Dryvax mixture was selected and grown in MRC-5 cells to make the ACAM1000 vaccine.

After a successful Phase I trial of ACAM1000, the virus was passaged three times in Vero cells to develop ACAM2000, which entered mass production at Baxter.

The United States ordered over 200 million doses of ACAM2000 in 1999–2001 for its stockpile, and production is ongoing to replace expired vaccine.

[11] Other vaccines containing live viruses include measles, mumps, rubella, polio and chickenpox.

The needle is dipped into the vaccine solution and used to prick the skin several times in the upper arm.

It will cause a localized infection, with a red itchy sore produced at the vaccination site within three to four days.