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 effects. 

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.  

A specialized type of white blood cells, T-cells, are regularly on patrol-duty, monitoring the blood stream for foreign particles and alerting the rest of the system if antigens are recognized. When antigens present themselves in the blood, antibodies are made by another type of specialized white blood cells known as B-cells. These antibodies are then dispatched to attack and destroy the antigens presented by the vaccine. The antibodies stick to the surface of the antigens, in a mechanism akin to solving a jigsaw puzzle. B-cells create an antibody that fits in specifically with the molecules in an antigen. By being locked into the antigen, the antibody is able to manipulate the antigen in such a way as to stop it from moving further within the body. In the process, the antibodies also alert other cells in the immune system to the presence of antigens. 

In most cases, your body is successfully able to overcome the foreign agents, both at the time of vaccination and in case of a future infection. Only some of the B-cells that were involved in antibody production actually respond to the antigen. While many antibody-presenting B-cells are dispatched, some never get involved in the attack and instead become memory cells, ready to launch an attack the next time they spot the same enemy. Memory T-cells also function similarly, remembering the antigen from a previous encounter and launching an attack as soon as the antigen presents itself again in the future. 

But that’s the thing: the flu shot changes from year to year, which explains why, even if you’ve been diligent in getting a shot every year, you still experience pain. Every year’s flu shot is a mix of what scientists have deemed that year’s mix of flu viruses to beware. They spend months preparing the solution, adding between three and four types of viruses to the vaccine. The vaccine is tested in cell cultures, animal models and, finally, humans before it is made commercially available. In many cases, these concoctions also have other chemicals, known as adjuvants, mixed in. 

“Adjuvants are added to vaccines to make them more potent,” said Dr. Richard Malley, a pediatric infectious disease specialist at Children’s Hospital Boston. “Often, they are aluminum phosphate or aluminum hydroxide additives that drive a better immune response in people.” But a major side effect of adjuvants: pain. These adjuvants trigger inflammation and pain responses within the body but have the added benefit of boosting immunity. 

Not surprisingly, the benefits of getting a vaccination far outweigh the costs. But if it helps, despite decades of administering and developing vaccines, pain is still one of the first things vaccine researchers take into consideration, according to Malley. So the next time you get a shot, think of the many fighting components in your body working to protect you, but most importantly, embrace the pain.