The US keeps millions of chickens in secret farms to make flu vaccines. But their eggs won't work for coronavirus

(CNN)Across the United States, prized chickens are laying life-saving eggs at secret farms.

Few people know where the chickens are kept -- their locations are undisclosed as a matter of national security.
Each day, hundreds of thousands of their eggs are trucked to facilities, where they are protected by guards and multimillion-dollar, government-funded security systems.
    But these eggs aren't for breakfast; they're the source of your common flu shot.
    For the past 80 years, much of the world has relied on chicken eggs for the production of influenza vaccines.
    About 174.5 million doses of the flu vaccine were distributed across the US this flu season through the end of February, of which an estimated 82% were egg-based, according to the Centers for Disease Control and Prevention (CDC).
    With each egg producing one vaccine, that means the US might have used 140 million eggs this flu season alone.
    To prepare for annual flu seasons, as well as possible pandemics, the US government has invested tens or hundreds of millions of dollars over the past 15 years to ensure there are enough eggs for vaccines.
    But now the world faces a new crisis: the novel coronavirus, which has infected more than 423,000 people globally and killed more than 19,000 since the virus emerged last December, according to Johns Hopkins University.
    There is no vaccine yet for the virus; and because it's different than the influenza virus, traditional methods like using eggs won't work. As scientists race to find a cure, the huge US stockpile of eggs won't be of any help.

    The rise of the egg-based vaccine

    Scientists started exploring the use of eggs in vaccine production in the 1930s.
    Researchers in England conducted the first trials on their armed forces in 1937, and the year after the US found it could protect its military with the flu shot.
    A working egg-based vaccine was ready for the US public by the 1940s.
    Trays of chicken eggs are loaded onto racks during a demonstration at a Sanofi Pasteur facility in Swiftwater, Pennsylvania in July 2007.
    Here's how it works today: The US CDC and other labs partner with the World Health Organization (WHO) to choose certain virus strains to send to private vaccine manufacturers. The flu can mutate and strains can change each year, meaning new vaccines are needed for every flu season.
    The selected virus is injected into a fertilized hen's eggs, where it incubates and replicates for a few days -- just as it would do inside a human host.