The flu shot protects against only 40 percent of this year's viruses, the Centers for Disease Control said recently. On Thursday, the FDA decided to replace all three of the vaccine's influenza strains for next winter. Wait, why can't the scientists put every strain there is into the vaccine?
It wouldn't be worth the effort, even if it were feasible. There are thousands of influenza subtypes infecting people around the world, but very few are likely to make someone in the United States sick. Vaccinating people against a disease they're never going to get is a risky proposition: We don't know how the body would respond to a barrage of flu vaccinations. The patient might also develop a strong immune response to an insignificant strain, while skimping on antibodies for a nastier virus.
That's not to say that we're better off putting just three strains of influenza in a vaccine. Five or six might offer better protection than what we have today without running afoul of the problems described above. But there are other reasons not to cram more into the flu shot: For each added strain, you need about 15 micrograms of protein, so a diverse vaccine yields a larger dose. This means it might hurt more when you get a flu shot, or you might need to get multiple shots. People could also suffer greater side effects and allergic reactions. Some scientists are investigating universal flu vaccines, which work in a different way; the shots under development target proteins that don't change much between strains, meaning one vaccine could protect against many strains.
In the meantime, there's the matter of manufacturing the extra strains. Pharmaceutical companies already have their hands full developing vaccines for just three forms in the United States. (Just four years ago, a national shortage forced government officials to discourage Americans from getting shots unless they were sick, infants, or elderly.) To make a single vaccine dose for a given strain, the virus must be injected into a chicken embryo; to manufacture enough doses for the whole country can take half a year. But the drug companies don't have enough eggs to make vaccines for 20 distinct viruses, let alone thousands. (There's reason for hope, however. By using bacteria, yeast, or mammalian cells to incubate viruses, scientists may be able to speed up production significantly.)
Developers would also need to undergo a lengthy process to show their new product is safe. Even if it did get approval in time for flu season, manufacturing costs would be much higher. That could make the flu shots more expensive for patients and result in fewer doses being administered.