Side effects of vaccination

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• If your child develops a fever, doctors and nurses will usually advise that they are given an appropriate weight-based dose of paracetamol to bring down the fever. It is then repeated 4–6 hours later as needed.

• If the fever lasts longer than 24 hours or is accompanied by other symptoms, you should consult a doctor.

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• Sometimes, the injection site shows redness or swelling. This is normal, and all traces will disappear on their own.

• If you are concerned about this, you should talk about it with a nurse or doctor at a health centre.

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• If you have any concerns, contact your nurse or doctor.

• If the child develops a high fever, cries abnormally or has seizures, contact a doctor as soon as possible.

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• The frequency of severe side effects is generally low for vaccines that are in use in Iceland but can vary from country to country, even though they are the same type of vaccine.

• If the risk of disease is significant, vaccines are often used that are relatively cheap to produce but may have more severe side effects than newer, more complex, and thus more expensive vaccines. Examples are live polio vaccines, where the disease is still active, and Japanese encephalitis vaccines produced in neuronal cell culture in Asia and used in routine vaccinations where the disease is endemic.

• The reasons why side effects of the same type of vaccine differ between countries may be related to different frequencies of genotypes that increase the risk of unusual immune system reactions. Consumption habits also differ between countries; for example, gelatin allergy varies by country.

• In general, abnormal immune system reactions to stimuli are often linked to specific genotypes of molecules that show antigens to the immune system (see HLA molecules) and can, therefore, differ by country and by the origin of individuals in each country. It is known that individuals with particular HLA genotypes sometimes get more severe symptoms or are more prone to complications from certain infections (see, e.g. here), and different responses of individuals to certain vaccines depending on the HLA type have been described, e.g. for influenza, measles, and hepatitis B vaccines.

• It is quite possible that such genotypes also affect the immune system's unwanted reactions to vaccines, but research on this has not yet been done, probably because such events are rare.

• Live vaccines can cause symptoms that resemble those of the disease being vaccinated against but are milder than those seen during disease transmission. However, more severe symptoms can occur if such vaccines are given to persons with a severely weakened immune system, and live vaccines should, therefore, be avoided in susceptible persons and during pregnancy.

• Serious side effects of vaccinations are infrequent but can be seen in approximately one in every 500,000–1,000,000 vaccinations. Therefore, the potential harm from vaccination is many times smaller than the harm resulting from the disease that the vaccination prevents.

• In many countries, all possible side effects of vaccinations are monitored very closely. In Iceland, healthcare workers are obliged by law to report unusual and severe side effects of vaccinations to the Icelandic Medicines Agency.

• The public must inform a healthcare professional, e.g., healthcare providers, of symptoms that occur after vaccination if they are severe (e.g., the whole arm is swollen) or are defined as rare, very rare, or frequently not known in the package leaflet.

• The Icelandic Medicines Agency marks COVID-19 vaccines for special control, and it is essential to gather information about side effects in this context.

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The prevalence of side effects of vaccines is classified as follows:

• Very common side effects - occur after more than 10% of vaccinations.

• Common side effects – occur after 1-<10% of vaccinations.

• Rare side effects – occur after 0.01%-<1% of vaccinations.

• Very rare side effects – occur after <0.01% of vaccinations.

• Frequency unknown—side effects have not occurred often enough to estimate the frequency. Here, they are called infrequent side effects together with those considered very rare according to the above.

Note that it is not always confirmed that there is a connection with a vaccine, even if a rare side effect is indicated in the patient information label. However, it is considered possible or even likely to be related to the vaccination due to the timing of the symptoms.

The most common side effects of vaccinations are due to the immune system's intended reaction to the vaccine, such as fever following vaccination with pneumonia bacteria.

• Local reactions are inflammation, redness and swelling at the injection site of vaccines, primarily due to special immune adjuvants that call immune cells (which here could be called inflammatory cells) to the injection site. Such side effects are part of the vaccine's primary effect, although they are not obvious and can be unpleasant. When the same vaccine is administered more than once, it is relatively common for side effects of this type to occur more quickly and be more pronounced during the second vaccination. This applies, for example, to annual influenza vaccination but is even more likely when there is a shorter interval between vaccinations, such as when rabies is vaccinated three times in one month or COVID-19 twice in one month.

• Enlarged lymph nodes, usually in the sinus on the side of the arm that was vaccinated, are also more common when there is a short interval between doses of the same vaccine and are caused by an immune reaction. In some cases, individuals with a history of the disease being vaccinated against are known to develop more symptoms after vaccination.

  • An example is if a person with a history of pneumonia receives a vaccine against a pneumonia bacterium that contains the same envelope type that caused the infection. It is not yet clear whether this applies to COVID-19 vaccinations, but such a history is rarely a contraindication to vaccination; rather, a scarce vaccine is saved by not giving it to individuals with a history of the disease.

• Many rare side effects are due to the unusual reaction of a person's immune system to the vaccine. Examples include allergic reactions to gelatin in the MMR vaccine and Guillain-Barré syndrome after influenza vaccination. In some cases, such as Guillain-Barré after influenza vaccination, some complications can also occur with the influenza disease itself. Guillain-Barré is much more common after influenza illness than after influenza vaccination, and in most places, it is especially recommended that individuals with a history of Guillain-Barré after influenza vaccination or illness receive an annual influenza vaccination. You can read more about Guillain-Barré, influenza and influenza vaccination here.

• Individuals with known or suspected anaphylaxis to polyethene glycol (PEG; Macrogol or Movicol, etc.) or a history of anaphylaxis of unknown aetiology should not receive vaccination with the Pfizer-BioNTech or Moderna vaccine for COVID-19.