All vaccines have side effects, and our COVID vaccines developed in record time are no exception. Many of the common side effects you experience after vaccination, such as fever or nausea, are actually because our immune system is doing its thing. They’re the best indication that your immune system is getting pounded and trained, so it’s ready to face the coronavirus ‘in the field’ if the opportunity presents itself.

But since you might have had no side effects following your COVID vaccine, does this mean you’re less protected against the coronavirus? Not at all.

According to Pfizer’s clinical trial, 50% of the participants did not experience any significant side effects, yet more than 90% developed immunity against the virus. Likewise, the Moderna vaccine causes side effects in only one in ten people, yet the vaccine had a 95% efficacy. Possible reactions to vaccines include headache, fever, injection site pain, muscle and joint pain, and fatigue.

These stats clearly show that a lack of side effects doesn’t mean you are less immune than those who experience side effects. The reason why some people develop side effects while others are spared has to do with the way our immune system develops immunity against viruses.

The vast majority of authorized COVID vaccines prime the immune system by inserting a piece of viral protein found on the outer envelope of the coronavirus, known as the spike protein. This protein alone cannot infect or make you sick. However, a branch of the immune system known as innate immunity will be immediately activated by the protein. The mechanisms of the innate immune system include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. This rapid response initiates inflammation, which is often felt as fever and pain.

So, it is due to this innate immune response that some people developed common side effects a day or two after receiving their jab. These resolve in three days tops.

You shouldn’t be worried if you don’t have any of these side effects because the aim of any vaccine is to provide long-lasting immunity and this is achieved solely by activating another branch of immunity: adaptive immunity.

Adaptive immunity does not trigger inflammation, although it may contribute to it significantly if it is already occurring for other reasons. In other words, you can be immune and perfectly protected against the coronavirus without any outward sign that your immune system is primed.

Adaptive immunity does not trigger inflammation, although it may contribute it significantly if it already present for other reasons. In other words, you can be immune and perfectly protected against the coronavirus without any outward sign that your immune system is primed.

Why some people are more likely to have vaccine side effects

Some demographics are more prone to vaccine side effects than others. Those aged 65 or older, for instance, are known to have fewer vaccine-related side effects. This may be due to the gradual age-related decline in immune activity.

A person’s biological sex can also play a role. According to a CDC study published earlier this year, 79% of the reported side effects following COVID vaccination were among women. Scientists think this could be due to the dampening effect of testosterone on inflammation, thereby reducing common symptoms like fever and pain.

Likewise, people suffering from certain conditions, such as rheumatoid arthritis or multiple sclerosis, who have to take immunosuppressive medication, may experience fewer side effects due to the dampened immunity.

But regardless of sex or the use of immunosuppressants, the COVID vaccines — and all vaccines in general — should provie immunity against viral infection despite the apparent lack of side effects.