Whether you’re planning a trip abroad, have poked yourself with a dirty thorn, or want to prepare for flu season, sitting down for a vaccine shot is not usually a big deal. Barring any fear or apprehension about needles, it’s one quick alcohol swab, a jab into your deltoid, and a bandage.

Some people, however, report that some vaccine shots feel more like a harpoon, with pain upon injection and residual soreness. Does the level of discomfort have anything to do with the vaccine being administered, how the vaccine is made, the patient, or the health care provider?

According to Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, injection pain is a normal consequence of a shot designed to provoke an immune system response. Dr. Messonnier told The Wall Street Journal that inflammation around the injection site is an indication the vaccine is working.

When a vaccine is injected, antigens are introduced into the body. These proteins allow white blood cells to battle against viruses. When they’re jabbed into your arm, your body mounts a defense at the injection site, leading to inflammation. (Yes, this is why your arm gets sore after a flu shot.)

Some vaccines tend to hurt a little more than others, like ones targeting hepatitis A and B and DTaP (for diphtheria, tetanus, and pertussis). It’s not totally clear why, but it’s possible that additives designed to strengthen the immune system, like aluminum salts or monophosphoryl lipid A, are the culprit. “These are safe ingredients added to the vaccine specifically to create a stronger immune response,” Messonnier told the paper, adding that some people might be more sensitive to them than others.

These additives aren’t the only reason vaccines can sting. The pH level of the solution (which can be acidic), the volume, and the temperature can also affect whether there’s discomfort.

If you or your child are needle-averse, you can try distraction techniques like music, request a numbing cream, or take an over-the-counter pain reliever to combat any post-injection soreness.

Incidentally, the shot probably won’t hurt any more or less if it’s delivered into your buttocks, as was the practice at one time. While some vaccines work well in fatty tissue, like MMR (measles, mumps, and rubella, which gets directed to the fat near your triceps), many do not. And because flu vaccines are often administered in public places like pharmacies, it’s probably best that they stick to your upper extremities.

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