Vaccines undergo particularly rigorous safety tests before receiving approval for use in the general population. However, no drug is guaranteed to be free from side effects, so what are the side effects that children receiving the COVID-19 vaccine may face?

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A nurse gives a boy a dose of the Pfizer vaccine at a COVID-19 vaccine clinic at Lyman High School in Longwood, FL, on the day before classes begin for the 2021–22 school year. SOPA Images/Getty Images

The Food and Drug Administration (FDA) approved the Pfizer vaccine for children aged 5–11 years on October 29, 2021. According to a recent survey by KFF COVID-19 Vaccine Monitor, just over one-quarter of parents of children in this age group are eager for their child to be vaccinated as soon as possible.

Although this may be surprising, other survey findings shed some light on the reasons behind this hesitancy: a fear of side effects.

Two-thirds of parents of children of this age said that they were worried that the COVID-19 vaccine would affect their children’s future fertility. So significant are these concerns that the American Academy of Pediatrics recently put out a video disputing any potential impact of the vaccine on puberty or fertility.

Vaccine hesitancy is not just over fears of an impact on fertility, however. Over three-quarters of parents reported that they were “very” or “somewhat” concerned that their child might experience serious side effects or that not enough is known about long-term effects from the COVID-19 vaccine.

So, what are the possible side effects?

In the same way that pharmaceutical companies raced to get the vaccine approved for adults, those companies have carried out trials to see if their vaccines are safe and effective in adolescents and children.

Speaking at an Independent SAGE briefing on November 5, 2021, Prof. Deenan Pillay — a professor of virology at University College London (UCL) in the United Kingdom — said:

“There have been a number of trials. We are always concerned about the untoward effect of all medicines in children, and, of course, we can’t just extrapolate from data that [come] from adults to children. We have got to wait to ensure there is safety in children. And now that has happened.”

So far, mRNA vaccines from Pfizer and Moderna have received approval in the U.S. for children over 12 years of age, with the Pfizer vaccine approved for 5–12-year-olds at the end of October 2021.

The European Medicines Agency announced that it would start investigating the safety of the vaccine in that age group on October 18, 2021.

Most countries offering vaccination to those over 12 years of age are offering the Pfizer or Moderna vaccine, Reuters recently reported. A single dose of the Pfizer vaccine is available for those over 12 years old in the U.K., where the Moderna vaccine also has approval for this age group.

Pfizer has plans to trial the vaccine in children aged 6 months to 5 years, and Moderna has ongoing trials to test the safety and efficacy of vaccines in children under 12 years old.

Novavax is about to start a study of up to 3,000 adolescents aged 12–17 years across up to 75 sites in the United States. Johnson & Johnson has enrolled children as young as 12 years into existing trials, and AstraZeneca is planning on conducting trials of its vaccine in children as young as 6 years.

The vaccine with the most evidence to support its use so far is Pfizer’s mRNA vaccine against SARS-CoV-2, which is the virus that causes COVID-19.

However, the vaccine doses given to children over the age of 12 years and those under the age of 12 years differ. Pfizer released data from its phase 2 and 3 trials at the end of September 2021. The data suggest that the vaccine was safe in children aged 5–11 years.

Children under the age of 12 years will be offered 10 micrograms (mcg) of the vaccine. This is compared with 30 mcg of the vaccine, which is the amount given to children over the age of 12 years and adults. Experts hope that this lower dose could result in fewer side effects, as lower doses generally should.

Senior vice president of Pfizer Vaccine Clinical Research and Development, Dr. Bill Gruber, broke down the drug company’s data from its phase 2 and 3 trials at the FDA’s Vaccines and Related Biological Products Advisory Committee Meeting on October 26, 2021.

He revealed that there were very few serious adverse events and no deaths during the phase 2 and 3 trials of children aged 5–12 years. He also explained that the side effects were similar to those that adults experience.

The most common side effects in children after their second dose of the vaccine — first dose reactions were less frequent — were fatigue and headache, with 39.4% and 28% of 5–12-year-olds experiencing those symptoms, respectively.

This is compared with 65.6% and 60.9% of adults. Of note were data showing that fever and chills experienced after the vaccine were lower among 5–12-year-olds than among 12–65-year-olds.

Just 6.5% of children aged 5–12 years experienced fever after vaccination, compared with 17.2% of those over 12 years. Also, just 9.8% of those aged 5–12 years experienced chills, compared with 40% of those over 12 years.

Due to existing concerns about the risk of myocarditis and pericarditis among adolescent and young adult males, the scientists took specific precautions during this trial, Dr. Gruber explained to the committee.

He said: “To enhance possible detection of the rare events of myocarditis in adolescents and young adults, should [they] occur, specific instructions were provided to be vigilant with symptoms and signs of myocarditis […]. No anaphylaxis, no myocarditis, and no appendicitis were reported.”

The Centers for Disease Control and Prevention (CDC) and others are currently monitoring rates of myocarditis, which is inflammation of the heart muscle. This comes following reports in July 2021 that some teenage boys had received diagnoses of this condition after receiving the Pfizer vaccine, which those over 12 years of age have been able to receive since May 2021.

The CDC reports that males aged 12–29 years are most at risk of developing myocarditis.

It also states that although 687 cases of myocarditis following vaccination had been reported in under-30s in the U.S. between December 29, 2020, and June 11, 2021, healthcare professionals had given more than 52 million doses of the vaccine to people aged 12–30 years in total. So, this represents a very small risk.

However, there was still a discussion to be had over whether or not the risks of the vaccine, which were very small, outweighed the risks of developing COVID-19, which were also smaller for this section of the population than older adults.

One study, which has not yet undergone peer review, claimed that the risk of experiencing an adverse cardiac event following mRNA vaccination in males aged 16–17 years without any comorbidities was actually 3.5 times higher than the risk of hospitalization due to COVID-19. This was widely reported in August 2021.

Conversely, a study in the New England Journal of Medicine from October 6, 2021, reported findings from the Israeli Ministry of Health surveillance of the issue that seemed to prove a link between receipt of the Pfizer vaccine and myocarditis.

Data collected between December 20, 2020, and May 31, 2021, confirmed 136 cases of myocarditis after receipt of the Pfizer vaccine out of 5.12 million Israelis who had received two doses. Analysis suggests that the risk is highest after the second dose in male recipients aged 16–19 years, with a risk ratio of 1 in 6,637.

Study co-author Prof. Manfred Green, from the Department of Epidemiology at the University of Haifa in Israel, told Medical News Today in an interview:

“Myocarditis is more common in males and females, [and] there are all kinds of theories why […]. [In the study, we found that] it generally was a mild illness, a mild infection and a minor event requiring hospitalization, as people with myocarditis are almost always hospitalized for observation. They required […] very basic anti-inflammatory treatment to treat the inflammation. All were fine, they recovered well.”

Dr. Green sits on the committee due to decide whether or not to proceed with licensing vaccines for 5–12-year-olds in Israel on November 10, 2021. He pointed out that as the dose given to 5–12-year-olds is one-third that of the dose given to older children and adults, “it is expected [that] there will be [fewer] side effects and [fewer] adverse events.”

Much of the concern voiced about side effects from COVID-19 vaccines is over the risk they may pose to healthy children.

Meanwhile, children who have some preexisting conditions will be particularly vulnerable to COVID-19 and will benefit more from vaccination. So, what about them?

There have been few studies into these children, as children in these groups are few in number. However, one study that appears in the journal Archives of Disease in Childhood found no problematic side effects in a group of 20 adolescents aged 12–15 years with neurological conditions.

When evaluating the risks posed by possible side effects of the COVID-19 vaccine, it is impossible to do so without considering the possible benefits — though these may be hard to discern for the individual.

An analysis that appeared in the Journal of the Royal Society of Medicine on November 1, 2021, suggests that the vaccination of 12–17-year-olds is most beneficial while infection rates remain high — which, of course, they do in many parts of the world.

The analysis suggests that if SARS-CoV-2 infections are as high as 1,000 per 100,000 people per week over 16 weeks, vaccination could avert 4,430 hospital admissions and 36 deaths over 16 weeks. It also suggests that thousands of cases of long COVID could be avoided, even if the rate of long COVID was as low as 4% in teenagers.

Speaking at an Independent SAGE briefing on November 5, 2021, author Prof. Christina Pagel — a professor of operational research at UCL — said:

“It became really clear that if you got to really high case rates, then it is massively beneficial, and so even though children of 5–11 are less likely than adolescents to get really sick at the current rates of infection, with 6% [with the infection] at the end of October [in the U.K.], it is almost certain to be beneficial… I would be really surprised if there [weren’t] a benefit to vaccinating 5–11-year-olds, and I am a bit concerned that some members of [Joint Committee on Vaccination and Immunisation] are already saying ‘it’s too soon.'”

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