Herd immunity to GII.4 noroviruses is supported by outbreak patient sera.

Noroviruses (NoVs) of genogroup II, cluster 4 (GII.4), are the most common cause of outbreaks of acute gastroenteritis worldwide. During the past 13 years, GII.4 NoVs caused four seasons of widespread activity globally, each associated with the emergence of a new strain. In this report, we characterized the most recent epidemic strain, GII.4-2006 Minerva, by comparing virus-like particle (VLP) antigenic relationships and histo-blood group antigen (HBGA) binding profiles with strains isolated earlier. We also investigated the seroprevalence and specificity of GII.4 antibody in the years prior to, during, and following the GII.4 pandemic of 1995 and 1996 using a large collection of acute- and convalescent-phase serum pairs (n = 298) collected from 34 outbreaks. In a surrogate neutralization assay, we measured the blockade of HBGA binding using a panel of GII.4 VLPs representing strains isolated in 1987, 1997, 2002, and 2006 and a GII.3 VLP representing a strain isolated in the mid-1990s. Serum titers required for 50% HBGA blockade were compared between populations. In general, blockade of GII.4 VLP-HBGA binding was greater with convalescent-phase outbreak sera collected near the time of origin of the VLP strain. Heterotypic genotypes did not contribute to herd immunity against GII.4 NoVs based on their inability to block GII.4 VLP binding to HBGA. However, previous exposure to GII.4 NoV followed by infection by GII.3 NoV appeared to evoke an immune response to GII.4 NoV. These results support the hypothesis that herd immunity is a driving force for GII.4 evolution in the U.S. population. The data also suggest that complex patterns of cross-protection may exist across NoV genotypes in humans.