Conversely, the lack of side effects can be worrying: my immune system doesn’t seem to be doing what it should be doing … does that mean it can’t protect me?
Rest assured, this does not mean anything like that. Pfizer’s clinical trials of the vaccine show that half of the participants experienced no significant side effects during the trial, and 90% of them developed immunity to the virus. And the recommendations after the injection of the Moderna vaccine show that one in ten people can experience common side effects, while the vaccine protects 95% of the people who take it.
These differences in responses can be explained by looking at how the immune system develops protective immunity against viruses when a vaccine is launched. Most COVID-19 vaccines use a viral protein found on the outer envelope of the coronavirus, called a spike protein, to mimic a natural viral infection and trigger an immune response.
A branch of the immune response known as innate immunity reacts almost immediately to viral spike protein. It attacks it, causing inflammation, which is characterized by fever and pain. Thus, it is the innate immune response that causes the common side effects that people experience a day or two after vaccination.
Long-term specific immunity, which is the ultimate goal of any vaccination, is achieved only by activating the second branch of the immune response: adaptive immunity. Adaptive immunity is triggered by components of innate immunity and results in the production of T cells and antibodies that protect against infection upon subsequent exposure to the virus.
Unlike innate immunity, adaptive immunity cannot induce inflammation, although recent research suggests it can contribute significantly to this. In some people, the inflammatory response of the innate and adaptive immune systems is exaggerated and manifests itself as a side effect. In other cases, although it functions normally, it does not reach levels that would cause noticeable side effects. In any case, immunity to the virus has been established.
What causes a different immune response?
Scientists have noticed that people over 65 have fewer side effects from the vaccine. This can be explained by the gradual decline in immune activity associated with age. Although this phenomenon may be due to a drop in antibody levels, those affected remain immune to the virus.
Men and women react differently. In an American study, 79% of reported side effects were from women. This gender imbalance may have something to do with testosterone. It tends to reduce inflammation and hence the side effects associated with it. Men have more testosterone than women, so they may have fewer side effects.
People with chronic inflammatory conditions such as rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis who take immunosuppressants to control their symptoms may experience fewer side effects due to a weakened inflammatory response. This does not mean that it does not exist. A 2020 study compared antibody levels between people taking immunosuppressive drugs and those who did not. The former produced fewer antibodies, but none of them contained antiviral antibodies.
The side effects of vaccines should not be taken as an indicator of the effectiveness of the vaccine. Despite the variety of immune responses after injection, most people achieve immunity to coronavirus after vaccination, regardless of the presence, absence or severity of side effects.