What are the side effects of booster shots?

The immunization offered by COVID-19 vaccines decreases over time.

To extend those periods of immunity, additional doses, known as boosters, have been developed. However, and as with the first or second doses, some people may experience side effects after its application. Here we will see which ones usually occur with each vaccine.

What are booster shots?

First of all, it is necessary to differentiate the boosters from the third doses. The vaccines are the same, but the term is different depending on who receives it.

In certain people, vaccines may not produce enough antibodies, making them particularly vulnerable to infection, especially with more contagious variants circulating, such as Delta. In this higher risk group we can find:

  • People who received an organ through a transplant.
  • People who received stem cell transplants in the past two years.
  • Chemotherapy patients for tumors in the blood or for forms of cancer that affect the immune system.
  • People with severe immune deficiency
  • People with advanced or untreated HIV.
  • Patients under treatment with corticosteroids or other medications that impact the functioning of the immune system.

The third doses are intended for these groups, who are already vaccinated, but have compromised immunity. Experts recommend that the application be done 28 days after receiving the second dose (or the only one in the case of J&J).

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Instead, the booster works as a kind of reminder for the immune system to continue to recognize SARS-CoV-2 and be able to fight it more effectively.

Specialists note that boosters do not need to be given so close to the second dose, and are intended to:

  • Over 65 years old.
  • Long-term care facility residents.
  • Person aged 18 to 64 with underlying diseases.
  • People ages 18 to 64 who are at increased risk of exposure and transmission to COVID-19 due to their profession, such as healthcare workers, supermarket employees, or teachers.

What are the side effects?

Typically, side effects are believed to come directly from the vaccine, when in fact, experts explain, they are the result of a reaction from the immune system.

Following clinical trials, different vaccine manufacturers in the US submitted the following data to the US Food and Drug Administration (FDA):


Despite the fact that the Moderna booster consists of half the dose of the first series (50 micrograms), the side effects were similar in frequency to those of the first applications.

The symptoms that occurred (from most to least common) were:

  • Pain at the injection site.
  • Tiredness.
  • Headache.
  • Muscle pain.
  • Joint pain.

These symptoms were more noticeable in people aged 18 to 64 years, than in older adults. No serious adverse events were experienced.

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Unlike Moderna, the Pfizer booster is the same formula and has the same strength as the first few doses. As clinical trials show, the frequency of side effects was similar to that of the first applications.

The side effects that occurred (from most to least common) were:

  • Pain at the injection site.
  • Tiredness.
  • Headache.
  • Muscle pain.
  • Shaking chills.

No serious adverse events were reported, although lymph node swelling in the armpits was highlighted more frequently than in the first applications.

Johnson & Johnson

Unlike the vaccines mentioned above, the J&J booster is a second dose, since this vaccine originally consists of a single application.

Experts indicate that it can be administered approximately 2 months after the first application. The side effects that occurred (from most to least common) were:

  • Pain at the injection site.
  • Tiredness.
  • Headache.
  • Muscle pain.
  • Sickness.

The frequency of these symptoms was similar to that of the first dose.

Why should you get vaccinated?

If after the first few applications of the vaccines you experienced severe side effects, this does not necessarily mean that they will happen again with the booster.

Experts explain that after getting vaccinated for the first time, the body becomes familiar with the formula, so it can be better prepared for expected symptoms.

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The health authorities remind that the side effects are not worrisome, and that mass vaccination is very important to achieve herd immunity, that is, the more people vaccinated, the greater the protection against the virus.

Vaccination protects the vaccinated, protects others, and makes the circulation of the virus weakened, because it stops finding organisms to infect.

Available vaccines against COVID-19 protect up to 95% and more from contracting the coronavirus and developing the disease.

The information available so far, the product of scientific research, shows that if the person is vaccinated and is infected anyway, they will have a lower risk of developing a serious form of the disease.

This is because the vaccine contains substances that work by neutralizing the action of a protein that helps the coronavirus infect the human body.

Also, by getting immunized, the body is left with a supply of T lymphocytes and B lymphocytes that will remember how to fight this virus in the future.

Sources consulted: US Food and Drug Administration (FDA), Centers for Disease Control and Prevention (CDC), US National Library of Medicine, Mayo Clinic, World Health Organization (WHO).

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