On Wednesday, the US Food and Drug Administration announced it had approved FluMist Quadrivalent, a vaccine to prevent seasonal influenza in people aged 2 to 49 years. This is the first quadrivalent flu vaccine, that is one that contains four strains of flu virus, the agency has approved.

FluMist Quadrivalent, made by MedImmune LLC of Gaithersburg in Maryland, contains weakened forms of two influenza A strains and two influenza B strains.

The vaccine is administered as a nasal spray, like the trivalent form, FluMist.

The FDA approved FluMist Quadrivalent after reviewing safety and effectiveness studies conducted previously for the trivalent vaccine, together with three new clinical studies that examined the quadrivalent formulation in about 4,000 children and adults in the US.

The results of the studies showed that the immune responses from FluMist Quadrivalent were similar to those of FluMist.

The most commonly reported side effects in children and adults were runny or stuffy nose, with headache and sore throat reported in adults only.

There are two types of flu virus that cause illness and death in humans: influenza A and B.

Every year, scientists get together to decide which strains of flu virus to put into next season’s vaccines.

They decide which particular A and B strains to include in the vaccine based on which strains have been prevalent recently, for instance in other parts of the world.

A vaccine approved by the FDA usually contains three strains: two of the A virus and one of the B.

But in a typical flu season, there could be a second B strain going around, or what sometimes happens, is that the B strain in the vaccine ends up not being the one that eventually circulates and causes illness.

By including a second B strain, a vaccine increases the chances of adequate protection against the strains that actually end up circulating once the season arrives.

Dr Karen Midthun, director of the Center for Biologics Evaluation and Research at the FDA, told the press:

“Illness caused by Influenza B virus affects children, particularly young and school-aged, more than any other population.”

“A vaccine containing the four virus strains most likely to spread and cause illness during the influenza season offers an additional option to aid in influenza prevention efforts,” she added.

Flu seasons are unpredictable, with some seeing many more deaths than others. For instance, between 1976 and 2007,some seasons saw fewer than 5,000 deaths linked to flu in the US, but there were also others where more than 40,000 people died from flu-related illness.

Written by Catharine Paddock PhD