Table 1.

Baseline antibody status prior to treatment with peginterferon beta1a.

	Delayed treatment*		Peginterferon beta1a 125 μg
	Every 2 weeks (n = 228)	Every 4 weeks (n = 227)	Every 2 weeks (n = 512)	Every 4 weeks (n = 501)
Anti-IFN beta1a BAb positive, n (%)	1 (<1)	7 (3)	16 (3)	8 (2)
Antipeginterferon beta1a NAb positive, n (%)**	0	3 (1)	8 (2)	2 (<1)
Anti-PEG Ab positive, n (%)	18 (8)	12 (5)	25 (5)	27 (5)

^*Delayed treatment groups: patients who received placebo in year 1 and switched to peginterferon beta1a in year 2. Pretreatment baseline measure taken at the beginning of year 2 in these groups.

^**NAb assay was performed only on anti-IFN BAb-positive samples, per tiered assay design; percentages are given as a proportion of the total safety population.

Ab, antibody; BAb, binding antibody; IFN, interferon; NAb, neutralizing antibody; PEG, polyethylene glycol.