The findings parallel earlier results that found adding a strain of influenza B could improve the effectiveness of a flu vaccine nasal spray and a traditional intramuscular vaccine that is injected as a shot in the arm muscle."We found adding a fourth strain to the vaccine increases the chance the vaccine will match the circulating flu B strains," said lead research author Geoffrey Gorse, professor of internal medicine at the Saint Louis University in the US.

Flu vaccines can be trivalent - containing two strains of influenza A and one of influenza B - or quadrivalent - including two strains of A and two of B. Both are available to fight influenza.Scientists create a flu vaccine annually based upon the strains of influenza they predict will circulate for the next season.Despite rigorous modelling practices, the virus in the vaccine occasionally does not match the circulating strain of influenza.

There are two lineages of B flu strains, and 50 percent of the time in the past decade, the trivalent vaccine B strain did not match the circulating B strain.The quadrivalent vaccine has both B strains in it.During the study, 3,355 volunteers who were between 18 and 64 years of age were vaccinated at 38 sites in the United States.They were randomised to receive one of three vaccines: the quadrivalent flu vaccine that contained two A flu strains and both lineages of the B strains - the licensed trivalent intradermal vaccine for the 2012-2013 flu season or an alternate trivalent intradermal vaccine that contained two A strains and the B strain that was not in the licensed seasonal flu vaccine.

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