The Ouch-Factor Behind a Flu Shot: Explained

Shraddha Chakradhar

It’s flu season and that means a lot of people have been getting their flu shots. This means that a fair number of you have also been feeling the after-shot soreness. Keep reading to find out why even a tiny prick can result in extended pain.

Published December 30, 2013 at 7:07 pm

Flu season is here! Here in the United States at least, flu season has been in full swing since October, with pharmacies and doctors’ offices posting signs encouraging people to get their flu shot. If you’ve ever had a flu shot (or any shot, for that matter), you know to expect some pain to go with that healthy dose of immunity. But what you may not know is why. And what’s even more surprising: the pain is good. 

Despite the latest needles being significantly small, the size of a needle does make a difference when it comes to the amount of pain you may experience when getting a shot. Needle sizes, specifically how thick they are, are measured in gauges, with a lower gauge representing a thicker diameter and a higher gauge representing a thinner diameter. With flu shots, the most commonly used needles are between 20-gauge and 24-gauge, which represent diameters between 0.023 inches and 0.012 inches. The length of a needle, however, is much higher, ranging from between 1 inch to 1.5 inches. 

One major difference between a shot and a pinprick is that a pin would not normally penetrate your skin as deeply as a shot does. Flu shots are typically administered intramuscularly, meaning that it penetrates past your skin layers, a subcutaneous layer of fat, and then reaches the muscle layer. Beginning in 2011, however, a subcutaneous method of administration has also been made available in which the needle is not inserted as deeply. There is no major difference in the two types, however, as far as immunity and side…

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Shraddha Chakradhar

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Shraddha uses Beacon to tell stories about everyday science, medicine and health. She believes a lot of science reporting stops at the surface, leaving out the most interesting details and the most important data. Her stories will look deeper into the science of everyday things to give you a keener understanding of their scientific underpinnings.

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Needle gauges, lengths and penetration depth aside, the most important reason why a flu shot hurts is that your body is responding to the many agents within a flu shot. And many doctors believe that pain after getting a shot is actually a good sign. It’s a sign that everything is functioning like it should, with the body launching the expected attack against the inactivated viruses, or antigens, that are found within seasonal flu shots (the nasal spray version of the vaccination has live attenuated viruses, which are weakened but live forms of the microorganisms). When the body recognizes these antigens, the immune system launches a coordinated effort against them, by-products of which are the symptoms of an infection—soreness, inflammation, and in some cases, fever. These symptoms serve as signs not only to us at the organismal level that something is going on within the body, but also sound the alarm of an invasion to cells in the immune system.