Both were to become infected, and one of the groups of mice would get the preventive vaccine. The specific type of vaccinia virus used in the experiment doesn't grow well in mice, said Berzofsky, so Arichi apparently ''spiked'' some fluid from the mice with vaccinia virus to make it seem the vaccinia virus had taken hold and was growing. Arichi worked at NIH from December 1995 through May 1999, said a spokeswoman for the National Cancer Institute. A spokesman for the Proceedings of the National Academy of Sciences said this was the first retraction of a paper this year. Last year there were four retractions, and there were two in 1999, said the spokesman.
Retractions can stem from honest mistakes in experiments, as well as data fabrication.