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Influenza B lineage circulation and hospitalization rates in a subtropical city, Hong Kong, 2000-2010.

BACKGROUND
A need for quadrivalent vaccines to cover both lineages of influenza B has been raised. Information on the circulation status of influenza B lineages and the associated hospitalization rates is important to assist evidence-based decision making. This retrospective study revealed the situation in a subtropical city over a 10-year period.


METHODS
Sequences of 268 influenza B isolates were analyzed to identify the circulating pool of virus lineages for each year. Hospital records and population census data were used to estimate annual age-specific hospitalization rates.


RESULTS
Cocirculation with 2 influenza B lineages was found in 9 of the 10 years. Only in 6 of the 10 years had the vaccine strain successfully matched with the lineage that was found in >50% of the circulating pool. Six years were predominated by one lineage (occupying >80% of the circulating pool), and these years had higher (average, 1.4-fold) hospitalization rates. Matching between vaccine and circulating lineage was achieved only in 2 of the 6 "predominated years." The Yamagata lineage accounted for most (5/6) of the predominated years. Overall, 24% of influenza admissions were due to influenza B, and influenza B contributed to a higher proportion (41.9%) among children and young teenagers (5-14 years old).


CONCLUSIONS
Cocirculation with 2 influenza B lineages is common in the subtropical region. To predict the next predominant lineage proves to be difficult. Influenza B accounts for a substantial fraction of influenza-associated hospitalizations, especially among children and young teenagers. Quadrivalent vaccines may improve the effectiveness of influenza vaccination programs.