From your body’s perspective, a flu shot looks a lot like an oncoming case of the flu. Made from weakened flu viruses, flu vaccines trigger our bodies to pump antibodies and immune cells into the bloodstream. This enables our bodies to mount a fast defense against real flu viruses. Producing the vaccines, however, is hardly so efficient. Current methods require five to six months of labor, a fair share of educated guesswork—and a good supply of eggs.
Pick a virus. Because vaccine production takes so long, public-health officials must predict problem flu strains nearly a year in advance. The U.S. Centers for Disease Control and Prevention then develops stocks of each virus and distributes them to vaccine makers. That’s why there are two flu shots this year: production of the vaccine against H1N1 virus did not begin until the virus emerged as a pandemic in late spring.
Egg, meet virus. Tiny needles inject live viruses into fertilized chicken eggs. It takes about one egg to make one dose of vaccine, which means somewhere around 100 million eggs are needed to create seasonal flu vaccines. And that’s one of the major limitations of the current process. If a strain of avian flu started infecting humans, for example, there might not be enough healthy chickens and healthy eggs to produce vaccines. Scientists are exploring new approaches that would replace this step with something faster and more reliable, such as engineered cells that could churn out virus-like proteins.
Keep the eggs warm. The eggs are incubated in ideal virus-growing conditions for a number of days, allowing each virus injected into an egg to reproduce hundreds of times over.
Crack ’em open. Big machines slice off the tops of the eggs and invert them to collect the fluid inside, which at this point is teeming with live viruses.
Kill the virus. Chemicals are used to deactivate the viruses and chop them into pieces. In the case of the seasonal flu vaccine, particles from three flu strains get mixed together at this point to create a vaccine that can guard against multiple varieties of flu.