Table 1.

Plasma Viremia in Patients with Multiple Sclerosis Who Were Treated with Natalizumab.^*

Variable	Initial Yr of Treatment (N = 26)	Treatment >24 Mo (N = 23)	Healthy Volunteers (N = 18)
	number of patients
Level of JC virus DNA
Mean, 43 copies/ml (range, 13–510)	10	0	0
Mean, 40 copies/ml (range, 21–126)	0	7	0
Seropositivity
Seropositive (antibody titer, ≥2560)	6†	5‡	12
Seronegative (antibody titer, <2560)	4§	2¶	6

^*Viremia was significantly more common in treated patients than in healthy controls (P = 0.003 by Fisher’s exact test).

^†Three patients had viremia at baseline, and one patient each had viremia at 3, 6, and 9 months.

^‡Two patients had viremia at 25 months, and one patient each had viremia at 30, 33, and 36 months.

^§One patient had viremia at baseline, and one patient each had viremia at 3, 6, and 9 months.

^¶One patient had viremia at 29 months, and one patient had viremia at 33 months.