According to a Business Insider article, clinical trials suggest that side effects are generally more pronounced among women and young adults, especially after the second dose.
According to the same article, the most common side effect of Pfizer (84%), Moderna (92%) and Johnson & Johnson (49%) vaccines is pain or swelling at the injection site. Other common effects include fatigue, headache and pain in the body or muscles.
Differences between doses
The effects between people who have not had Covid and those previously infected vary. For the former, the effects tend to be more numerous and severe after the second dose. Participants in the Pfizer study developed about twice as much chills and joint pain after the second dose as after the first. In the case of Moderna, the number rises to five times more. Fevers were also much more common among those who received the second dose than among those who received the first dose in both trials.
Those who had been infected with SARS-CoV-2 developed more side effects after the first dose. Effects such as fatigue, headaches and chills are more common among people with pre-existing immunity to coronavirus than in people who have never been infected. About 73 percent of those recovered developed side effects after the first dose of Pfizer or Moderna, compared with 66 percent who were never infected. This is because the cells have memory and the body is prepared to respond more robustly.
Younger adults may have more effects than older people
Older people’s bodies don’t fight the virus as much. Younger individuals have a much more robust immune response, which results in greater side effects. After a dose of Moderna, 57% of people under 65 developed side effects, compared with 48% of people over 65. After the second dose, almost 82 percent of people in the younger group developed effects, compared with almost 72 percent of older adults.
In the case of Pfizer, about 47% of people aged 18 to 55 years developed fatigue after the first dose, while 34% of people aged 56 and older reported the same effect. After the second dose, the numbers increased to 59% and 51%, respectively. After the Johnson & Johnson single dose vaccine, almost 62% of people aged 18 to 59 years developed effects, compared with 45% of people aged 60 and over.
Women suffer more side effects from vaccines
The US Center for Disease Control and Prevention says that about 79% of reported cases of side effects of the vaccine came from women. Women also tend to react more to other vaccines, such as influenza or measles. These effects are even more pronounced in pre-menopausal women.
Scientists suspect that the difference has to do with estrogen levels, as estrogen, unlike testosterone, tends to be an immune stimulant.
Risky medical conditions do not lead to stronger side effects
As the Bussiness Insider explains, it is possible that people with weakened immune systems, such as cancer patients, may not develop a strong immune response to the vaccine. And that response determines its side effects.
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