What’s in a flu vaccine ?

ALL FLU VACCINES start with flu viruses: genetic material packaged in an envelope of proteins and fats, studded with yet more proteins—antigens—that push the body’s immune system into action.

Manufacturers grow the flu viruses in fertilized chicken eggs—hundreds of millions every year. Scientists inject the viruses into the allantoic fluid between the embryo and the shell, where the viruses replicate.

Without formaldehyde, this vaccine would just be infectious flu in a bottle.

As the flu virus replicates, it steals some fatty membrane
from the egg to hold its pro­teins and genetic material together. This detergent pulls the fat out like a grease stain, leaving free virus bits behind—they’re less likely to cause side effects than the whole virus.

All gelatin is made from animal collagen; this version, which keeps vaccine components stable during storage and shipping, comes from pig parts.

Some people (wrongly) think this preservative makes vaccines dangerous. Actually, it makes multidose vials safer, keeping them free of bacteria and fungi even after repeated ­needle insertions.