(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.
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.
Scientists then harvest fluid containing the virus from the egg. They inactivate the virus so it can no longer cause disease, and purify it, leaving scientists with the virus antigen.
The antigen is the crucial element -- it's a substance released by the virus that triggers your immune system to respond. That's how the vaccine prepares your immune system for a real infection.
The entire process, from the arrival of the egg to the publicly available vaccine, takes at least six months, according to the CDC.
The government gets its vaccines from a handful of manufacturing companies, which source their eggs from numerous farms in secret locations. The farms' locations are "proprietary information," said a spokesperson from Sanofi Pasteur, the biggest flu vaccine manufacturer in the US, citing "security sensitivities."
The eggs don't come cheap: the Department of Health and Human Services (HHS) spent $42 million on a three-year contract with one company to ensure a year-round supply of high-quality eggs, according to a 2017 report from the US Government Accountability Office (GOA).
That's just one company from one time period; the HHS began contracting a handful of manufacturers and egg farms in 2005, meaning there are likely many more contracts worth tens of millions of dollars.
And these eggs are precious: if the supply chain is compromised, it could mean a nationwide shortage of crucial flu vaccines.
So, obviously, the eggs have bodyguards.