Are adverse vaccine reactions expected?

The principle of a vaccine is to inject a substance (protein, RNA, virus...) into the human body that will mimic a virus or a bacterium without being dangerous. The injected substance is a foreign body that will be recognized by the immune system. The immune system will activate itself to eliminate the foreign body by producing antibodies and specialized cells (see the question What are the different types of COVID-19 ?). Depending on the individual, the onset of the immune system may produce more or less effects, which are usual signals of the immune system's fight against the foreign body (as we experience at the beginning of an infection). Adverse vaccine reactions are not symptoms of a disease, but are completely normal!

How long after the injection of the vaccine do we observe adverse effects?

A distinction is made between "immediate" adverse effects (in the hours or days following vaccination) and long-term adverse effects (in the weeks or months following vaccination). The side effects of vaccines against COVID-19 vaccines are classic and not serious: discomfort at the injection site, sometimes fatigue and moderate fever, sometimes a temporary flu syndrome, exceptionally an allergic reaction. Their frequency decreases slightly with age. 

Are all adverse events attributable to the vaccine?

In many cases, immediate adverse effects are also observed, although to a lesser extent, with the injection of a placebo (a neutral substance) because humans tend to be much more careful about how they feel after taking a drug or injecting a vaccine. This focus of attention leads to the perception of effects that would have gone unnoticed in everyday life. Therefore, it is necessary to compare the adverse effects that are observed with the vaccine injection and the placebo injection to determine which effects are actually attributable to the vaccine itself.

What are the different types of side effects?

A distinction is made between mild side effects that have little or no impact in everyday life and do not require special medical management, and serious side effects that will require special medical management. In addition, in the case of a serious reaction, it is important to distinguish between "serious events", when a causal link with the vaccine is not established, and "serious adverse reactions", when this link is proven (see question Why have elderly people died after being vaccinated?). Among the serious adverse reactions, cases of allergic reactions (anaphylaxis) requiring medical management have been observed in rare cases after the injection of certain COVID-19 vaccines. Two cases out of a million for Moderna vaccine and eleven cases out of a million for Pfizer vaccine. These severe effects were observed mainly in people with known severe allergies and occurred mostly within minutes of the injection.