Viral Time Bomb: Defensive Measures

Because flu virus mutates from year to year, flu vaccine must be reformulated annually to effectively counter the bug. The process, which was developed more than five decades ago, takes nearly a year.

January-February

The World Health Organization's multinational Global Influenza Surveillance Network begins epidemiological research across the globe to determine the most likely strains of influenza virus to be in circulation during the following flu season.

Based on this data, the Food and Drug Administration recommends a vaccine formula containing fragments from the three most virulent strains of virus believed likely in the United States.

Vaccine manufacturers estimate market demand for the vaccine, and negotiate contracts with farmers to provide fertile eggs for vaccine production.

Spring

Flu virus from the three strains is injected into 9- to 11-day-old eggs, infecting the embryos. (A single egg is used for each strain.) Two days later, the viral fragments are harvested, purified and combined. Hundreds of millions of eggs are used in the process.

Summer

The vaccine is tested on sheep, which produce antibodies that protect against the virus and are used in the final formulation of the vaccine.

Flocks of chickens are hatched to reach maturity and begin laying eggs by the following December and January, to begin annual production the following year. Eggs from new-laying hens are more reliable in producing vaccine; therefore, aged flocks are killed at the end of vaccine production in the summer.

Fall

By September, the vaccine is usually ready for shipment

In October, mass inoculations begin

Sources: World Health Organization; Centers for Disease Control and Prevention