According to a study conducted in the United States, between 1995 and 2000 the number of people affected by the drug-resistant virus increased considerably. Until 1998, 3.4% had resistant viruses and 12.4% in 2000. During the research, the capacity of 15 drugs was measured and it was found that those containing resistant viruses took longer than normal to reduce the level of virus in the blood. Therefore, researchers recommend testing resistance when choosing medicines. The results have been published in the New England Journal of Medicine.