Vaccines and Side Effects: What to Expect

Vaccines and Side Effects: What to Expect
Vaccines and Side Effects: What to Expect

Pain at the injection site, fatigue, headache, chills and fever are some of the most common side effects of anti-covid vaccines: common but not “mandatory” and – above all – transient (nothing that does not subside with a paracetamol tablet). The response to vaccination is personal and difficult to predict, because it is linked to the characteristics of the immune system. However, it is possible to identify some general trends in reactions based on the age, gender, health conditions of the vaccinee and the dose of vaccine (first or second) one is preparing to receive.

Do you feel worse after the first or second dose? The most common side effects of Pfizer and Moderna mRNA vaccines, given in double doses 21 or 28 days apart, respectively, are pain in the arm that was injected, fatigue, headache, muscle pain, chills and temperature. For both vaccines, the most intense reaction usually occurs after the second dose. While this malaise can create understandable discomfort, the fact that it manifests is actually a reassuring sign. It means that the immune system, having produced the Spike protein in response to the first dose, is now able to recognize it and activate itself in case it encounters it.

On the contrary, the vaccine from Oxford and AstraZeneca seems to cause a more intense immune reaction already after the first dose, as also specified by the EMA. The most frequent side effects are pain at the injection site, headache, fatigue, chills, joint pain, fever and nausea. Most of these adverse reactions it is moderate in character and resolves within 48 hours of vaccination. Effects reported after the second dose are generally milder and less frequent.

Recall that the AstraZeneca vaccine is already largely effective after the first dose (even if the duration of the protection offered is not known): indeed, the effectiveness increases by delaying the booster up to 12 weeks from the first injection.

Those who have already had covid have a strong reaction already at the first dose. The immune system of the recovered has already encountered the spike protein once (and in the “original version”). One dose of the vaccine is sufficient to awaken defenses and confer complete protection, in light of the fact that natural infection already confers a virus-specific immune response. The AIFA also acknowledged these indications and advised against administering a second vaccination dose to the recovered.

Young people have stronger immune reactions than the elderly. The reactivity of the immune system decreases with age, which is why young adults generally have more intense side effects to anti-covid vaccines. The lower sensitivity of the immune system of the elderly, the most at risk of severe forms of covid, has been a major concern during phase 3 trials of vaccines, those aimed at evaluating efficacy. Fortunately, older immune systems also seem to respond very well to available vaccines.

Women have stronger immune reactions than men. 79% of the adverse reactions reported to the American CDC after the first 14 million doses of Pfizer and Moderna vaccines were from women. In part, because women are more likely to report situations of physical discomfort to the doctor, but above all, because the female immune system triggers a more decisive reaction in response to infections (not surprisingly, 80% of autoimmune diseases occur in the female gender).

Women tend to have more pronounced adverse reactions to other vaccines as well, from the flu to the MMR vaccine (measles, mumps and rubella). In part it could be due to hormonal reasons, perhaps from it estrogen, as we have explained here.

Those with previous illnesses do not report more intense side effects. People with a weakened immune system from previous pathologies do not develop sufficient defenses in case of infection with CoViD-19 and are more likely than others to incur severe forms of the disease. For the same reasons, they may not react so strongly to anti-covid vaccines (precisely because the vigor of the reactions is in fact a signal of the triggered immune response).

Even those with autoimmune diseases do not seem to report worse than average adverse effects. At the same time, it is a priority that those affected by previous conditions that could lead to a worse outcome than covid be given priority in vaccination campaigns.

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