Are adverse vaccine reactions expected?
Text updated on 2021-02-17
Yes, it is quite normal to sometimes observe adverse effects after a vaccine injection!
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 events (within hours or days of vaccination) and long-term adverse events (within weeks, months or years). Immediate adverse reactions to Covid-19 are classic and not serious: discomfort at the injection site, sometimes fatigue and moderate fever, sometimes transient flu syndrome, exceptionally allergic reaction. Their frequency decreases slightly with age. Possible long-term adverse effects remain to be identified during Phase 4 of pharmacovigilance (see question How do you know if a vaccine is safe and protects against COVID-19 ?).
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.
From December 14-23, 2020, surveillance by the U.S. Center for Disease Control and Prevention (CDC) to track vaccine-related adverse events detected 21 cases of anaphylaxis after the first 1,893,360 reported doses of Pfizer-BioNTech® vaccine were administered. COVID-19 (11.1 cases per million doses), 71% of which occurred within 15 minutes of vaccination.Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Pfizer-BioNTech COVID-19 Vaccine - United States, December 14-23, 2020. MMWR Morb Mortal Wkly Rep 2021;70:46-51. DOI: http://dx.doi.org/10.15585/mmwr.mm7002e1
From December 21, 2020 to January 10, 2021, surveillance by the Center for Disease Control and Prevention (CDC) in the United States to track vaccine-related adverse events detected 10 cases of anaphylaxis following the administration of the first 4,041,396 doses of Moderna COVID-19 (2.5 cases per million doses administered). In nine cases, the onset occurred within 15 minutes of vaccination. No deaths related to anaphylaxis were reported.Allergic Reactions Including Anaphylaxis After Receipt of the First Dose of Moderna COVID-19 Vaccine - United States, December 21, 2020-January 10, 2021. MMWR Morb Mortal Wkly Rep 2021;70:125-129. DOI: http://dx.doi.org/10.15585/mmwr.mm7004e1
Interim results of the clinical trial with AstraZeneca vaccine in 11,636 participants.Voysey, M., Clemens, S. A. C., Madhi, S. A., Weckx, L. Y., Folegatti, P. M., Aley, P. K., ... & Bijker, E. (2020). Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. The Lancet, 397(10269), 99-111.
FDA report on the results of the clinical trial conducted with the ModeRNA vaccine in 30,418 participants.US Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee meeting-December 17, 2020-FDA briefing document-Moderna COVID-19 vaccine.
Results of the clinical trial conducted with Pfizer-BioNTech vaccine in 43,548 participants: 21,720 who received the vaccine and 21,728 who received placebo.Polack, F. P., Thomas, S. J., Kitchin, N., Absalon, J., Gurtman, A., Lockhart, S., ... & Gruber, W. C. (2020). Safety and efficacy of the BNT162b2 mRNA Covid-19 vaccinia. New England Journal of Medicine, 383(27), 2603-2615.