COVID-19 vaccines have been approved for distribution across the U.S. As of mid-January 2021, the Centers for Disease Control and Prevention reports that 31 million doses of vaccine have been distributed and about 12 million people in the U.S. have received a COVID-19 vaccination. That may sound like a lot, but to fully vaccinate the 332 million people living in the U.S. with two injections per person, we would need a whopping 664 million doses of vaccine.

Pharmaceutical labs are working hard to produce the COVID-19 vaccines to meet the demands both in the U.S. and worldwide. One of the most important parts of making vaccines is to make sure each dose is clean and safe to use—free of bacteria or contamination—because it’s being injected into your bloodstream. Toxins from certain bacteria that enter your bloodstream can cause you to become very sick or even die.

How do pharmaceutical companies make sure that each dose is safe? The answer lies within the horseshoe crab. Horseshoe crabs have bright blue blood that contains specialized immune cells that clot and turn their blood to gel when they come into contact with bacterial toxin. This is a natural defense that protects the animal from dangerous bacteria. Since the 1970s, scientists have used this natural characteristic to detect bacterial contamination in vaccines for mumps, measles and now COVID-19. If a sample from a batch of vaccine turns to gel when it’s exposed to the blood cells, this means that the batch is contaminated and must be thrown out. The horseshoe crab’s blood is so good at detecting contaminants that scientists haven’t found another substance that works as well.

Horseshoe crabs are collected off the coast of North America’s Atlantic coast and are sometimes held in ponds before researchers collect their blood. They are immediately released back into the ocean or kept in holding ponds for a few weeks after they are bled (called “biomedical bleeding”) by licensed harvesters. As you might imagine, being bled and held in an artificial pond can have a big effect on the physiology of the horseshoe crab. One research group found that horseshoe crabs will survive biomedical bleeding if less than 40% of their blood is collected at one time. However, biomedical bleeding will cause the immune cells in the blood to decrease and remain low for up to six days. Horseshoe crabs that are held in artificial ponds before bleeding have also been found to have fewer immune cells in the blood. Harvesters must take care during biomedical bleeding to ensure that less than 40% of total blood is removed and that the amount of time that horseshoe crabs spend in artificial ponds is limited.

When you get your COVID-19 vaccine—or the next time you visit the Atlantic coast—please remember to give a special thanks to those horseshoe crabs for keeping us safe and healthy!