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Background. Norovirus infection is the leading cause of acute nonbacterial gastroenteritis. Histoblood group antigens (HBGAs) are host susceptibility determinants for Norwalk virus (NV) infection. We hypothesized that antibodies that block NV-HBGA binding are associated with protection from clinical illness following NV exposure. Methods. We developed an HBGA blocking assay to examine the ability of human serum to block the interaction of NV viruslike particles with H type 1 and H type 3 glycans. Serum samples from persons who were experimentally challenged with NV were evaluated. Results. There was a high correlation between the H type 1 and H type 3 synthetic glycan assays (r=0.977; P<.001); the H type 1 assay had higher quantitative sensitivity (P<.001). Among 18 infected secretor-positive individuals, blocking titers peaked by day 28 after challenge and were higher for individuals who did not develop gastroenteritis than for those who developed gastroenteritis on days 0, 14, 28, and 180 (P<.05 for each). In addition, 6 of 6 subjects without gastroenteritis had measurable prechallenge blocking titers (>25), compared with 2 of 12 subjects with gastroenteritis (P=.002). Conclusions. Blocking antibodies correlate with protection against clinical NV gastroenteritis. This knowledge will help guide the evaluation of new vaccine strategies and the elucidation of the nature of immunity to the virus. Trial registration. ClinicalTrials.gov identifier: NCT00138476.

BackgroundHepatitis C virus (HCV) quasispecies diversity is more likely to affect early viral decline during treatment of hepatitis C than is having human immunodeficiency virus (HIV) infection. We evaluated the influence of HCV therapy on changes in the nonstructural 5A (NS5A) protein MethodsFifteen patients with HCV genotype 1 infection with or without HIV infection were recruited for the present study, and the decrease in the HCV RNA level was measured at early time points. The evolution of HCV NS5A quasispecies within the first week was analyzed by comparing the clones observed at later times in the study with the baseline consensus sequence of individual patients. The response to therapy was defined as an early response (ER; ie, an HCV RNA level <615 IU/mL at week 4) or a slow response (SR; ie, a detectable HCV RNA level at week 4) ResultsHIV infection did not affect early viral kinetics. At baseline, lower diversity was seen in NS5A and in the amino and carboxyl termini of patients with an ER, compared with those with an SR. Rapid evolution of the NS5A genetic region occurred in patients with an ER (P=.01) but not in those with an SR (P=.73). The evolution was the result of an increase in the number of amino acid substitutions in the carboxyl region (P=.02) in patients with an ER ConclusionsSelective pressure appears to result in more-marked changes in individuals with an ER than in those with an SR. The carboxyl terminus was subject to the most change and may be an important determinant of phenotypic resistance to interferon-based therapy