The coming era of quadrivalent human influenza vaccines: who will benefit?

Influenza vaccines form the mainstay of public health and personal protection against infection with seasonal influenza viruses. These vaccines are designed to protect people against infection with the currently circulating influenza viruses. Since the late 1970s, this has required the use of a trivalent vaccine consisting of two influenza A viruses and one influenza B virus. However, since the early 2000s, a second lineage of B viruses has regularly circulated in many countries that is quite distinct, with only low levels of cross protection between the two lineages. Due to the difficulties in determining which B lineage will circulate, and matching this with the vaccine to be administered some 6-9 months later, there has been an increasing interest in the development of quadrivalent influenza vaccines, containing two influenza B viruses representing both lineages. Development has been rapid and we are now on the cusp of a new generation of influenza vaccines becoming available. This paper discusses the issues and rationale behind this welcome development and who is likely to benefit most.