The ingredients in the flu vaccine vary by brand, but most contain at least three influenza viruses — usually two type A and one type B, according to the National Vaccine Information Center. Injectable vaccines that come in multi-dose containers also contain thimerosal.
The vaccine FluMist Quadrivalent, manufactured by MedImmune, contains one A/H1N1 and one A/H3N2 virus strain. In addition, the vaccine contains one strain from the B/Yamagata/16/88 influenza lineage and one from the B/Victoria/2/87 lineage, reports the Food and Drug Administration. Other ingredients include porcine gelatin, monosodium glutamate, dibasic potassium phosphate and monobasic potassium phosphate. Some doses of the vaccine contain gentamicin sulfate, but only in residual amounts less than 0.015 micrograms per milliliter.
The Fluzone High-Dose vaccine contains three virus strains: one type A H1N1, one type A H3N2 and one type B strain. Other ingredients include an isotonic sodium chloride solution buffered by sodium phosphate, 100 micrograms or less of formaldehyde, and octylphenol ethoxylate. Fluzone High-Dose does not contain gelatin or preservatives, notes the FDA.
Some flu vaccines contain thimerosal, an additive that prevents bacteria and fungi from growing in the solution while it is in storage, according to the Centers for Disease Control and Prevention. Thimerosal contains a safe type of mercury that flushes out of the body.