This episode is sponsored by Fabulous, an app that helps you form healthy habits that stick.

Click the link in the description to get a free one-week trial and 25% off a Fabulous subscription! [ intro ]. Imagine you just got your COVID-19 shot, and afterward, you’re just chilling in a cantina in Mos Eisley, and a Jedi comes along and cuts off your arm!

Your immediate question, of course, is whether you just lost immunity from the vaccine along with your arm. Surprisingly, the answer is probably not. And the reason has to do with why vaccines are typically injected into your arm.

The goal of a vaccine is to safely teach your immune system to recognize bits of a pathogen so it can recognize the full-blown pathogen later. These “bits” can be stuff like pieces of the particular germ, or the genetic instructions for those pieces, which your cells end up making. When you get a vaccine, the immune response starts at the injection site, which is why some of us get a sore arm afterward.

So you might think all the immunity action happens at that spot. But actually, vaccines start traveling around your body within minutes. See, vaccines are injected into your arm muscle because skeletal muscle is highly vascularized, meaning it has lots of blood vessels.

So, anything injected into your muscles moves into your blood quickly. From there, it circulates around your body, picking up speed as it rushes from the tiny capillaries to the larger veins and eventually through your heart. In addition, once it’s in a muscle, the vaccine drains into the lymphatic vessels.

It then flows into your lymph nodes, where immune cells learn to recognize the germ, and start making antibodies that defend your body from the pathogen. And these systems work within minutes. In 1997, researchers injected mice with a vaccine in their leg muscle, then surgically removed that muscle.

The mice developed a normal immune response even if their muscle was removed just ten minutes after vaccination, or even less. Apparently, it moved fast. But if muscle shots are so great, why not inject vaccines into even bigger muscles, like your glutes?

Actually, health officials once thought vaccinating people’s butts was a good idea. But they eventually realized backsides have downsides. Like, a 2006 study found that only 32 percent of buttocks injections hit muscle.

Most went into the fat, which doesn’t contain many blood vessels. So, best case scenario, the vaccine needs more time to exit your rear end. In fact, if the injection doesn't hit muscle, your vaccine could fail entirely or cause unpleasant side effects.

So technique matters, even for an arm injection, since people have varying amounts of fat in their arms. That means health practitioners might have to use different sized needles to make sure they hit their muscular target. Assuming everything goes well, if you got your vaccine and it’s been 10 minutes, you could hypothetically lose your arm in a freak encounter with a Jedi and still be immunized.

But - I wouldn’t test it. Maybe you’re looking to form new habits once we’re all back out in the world. If that’s the case, Fabulous can help you change up your routine.

Fabulous is a self care and habit forming app with over 20 million users. It was developed at Duke University, and uses behavioral research to help support your goals. For example, suppose you want to go for more regular walks.

You can tailor the app to your goals and use it to build your ideal personal routine. The first 100 people who click on the link in the description will get a free one-week trial and 25% off a Fabulous subscription! And checking them out helps us too -- so thanks. [ outro ].