Early detection of A/California/7/2004-like A(H3N2) strains in Norway during the 2004-2005 season, and emergence of a novel H3N2 genetic sub-variant toward the end

Abstract

Influenza activity in Norway during the 2004-2005 season was unusually slow in its development despite the early occurrence of an influenza A(H3N2) drift variant. Analysis of initial import and domestic cases in August-November gave early warning of a novel influenza A(H3N2) antigenic variant that had drifted further than the Southern Hemisphere A/Wellington/1/2004-like strains, and this novel variant, subsequently represented by the A/California/7/2004 vaccine strain, came to predominate in the Northern Hemisphere influenza season in most countries. After the peak of activity in mid-March, yet another H3N2 genetic variant (A/Norway/745/2005 lineage) came to predominate. While antigenically similar to A/California/2004, this new variant seems to have been able to persist in the face of increased herd immunity against the dominating variant through March-May when the major variant had vanished almost completely.