It’s been more than a year since Canada first started rolling out COVID-19 vaccine doses. Now, Canadians are rolling up their sleeves to get booster shots to combat the highly transmissible Omicron variant. But do boosters have the same side effects as the initial vaccine shots?
Canada’s National Advisory Committee on Immunization (NACI) recommends that booster shots of an mRNA vaccine – from Pfizer and Moderna – be offered to adults aged 18 and above at least six months after their second dose.
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In updated guidance released on Dec. 3, NACI also said immunocompromised Canadians may receive a second booster, or fourth shot, at least six months after their last dose.
Side effects are common when it comes to vaccines and booster shots are no different.
“Side effects often tell us that our immune systems are actually working and are responding as they should to the vaccine,” said Dr. Samir Sinha, director of geriatrics at Sinai Health and University Health Network hospitals in Toronto.
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Experts say while some people may experience a more intense reaction to the shots with headaches, body chills and fatigue, others just feel some soreness in the arm that was injected.
“The adverse effects of dose three should not differ all that much compared to dose two or previous injections,” said Dr. Ciriaco Piccirillo, an immunologist and senior scientist at the Research Institute of the McGill University Health Centre (MUHC).
Clinical trial data shows that a booster dose of an mRNA COVID-19 vaccine has a favourable safety profile comparable to the second dose of the primary series, according to NACI.
A survey conducted in Israel showed that most people who received a third dose of Pfizer COVID-19 vaccine felt similar or fewer side effects than they did after receiving the second shot.
Pfizer’s booster shot has exactly the same formula and dosage (30 micrograms) as the initial two doses.
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According to clinical trial data submitted to the U.S. Food and Drug Administration (FDA) by Pfizer, pain at the injection site was the most common adverse reaction – reported by 83 per cent of recipients.
Fatigue was the second most frequent side effect (63.8 per cent), followed by headache (48.4 per cent), muscle or joint pain (39.1 per cent) and chills (29.1 per cent). The side effects were mostly mild to moderate, lasting roughly two days after receiving the booster shot.
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“Of note, swollen lymph nodes in the underarm were observed more frequently following the booster dose than after the second dose of a two-dose primary series,” the FDA said.
No deaths or other adverse events of clinical interest – such as myocarditis, pericarditis, Bell’s Palsy, or appendicitis – were reported following the booster dose.
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Moderna’s booster is the same formula but half the original dose – 50 micrograms – is administered, except for immunocompromised individuals, long-term care residents and those aged 70 and above, who should receive the full dose of 100 micrograms, as indicated by NACI.
Adverse reactions reported in Moderna’s booster clinical trials included pain at the injection site, fatigue, headache, muscle pain, joint stiffness, chills, nausea/vomiting, axillary swelling/tenderness, fever, swelling at the injection site, redness at the injection site, and rash.
Anaphylaxis and other severe allergic reactions, myocarditis, pericarditis, and syncope (fainting) have also been reported outside of clinical trials, Moderna said.
A more intense reaction following a booster is sign of a stronger immune response, said Sinha.
“Some people are reporting that they’ve experienced side effects for the first time or a more significant level of side effects with their booster dose than they did with their first or second dose,” he said.
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Pfizer’s booster is preferred for those aged 18 to 29 years due to the lower risk of myocarditis and pericarditis with this vaccine, according to NACI’s guidelines.
However, those aged 30 and up can get either Moderna or Pfizer.
Sinha said there is no concrete data to suggest that one brand of vaccine would elicit more side effects compared to the other.
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According to Piccirillo, it’s not entirely clear why some people develop adverse reactions, either local or systemic, and others do not.
Age could be a contributing factor, with younger people having more effects than older individuals, Piccirillo said.
“This is thought to relate to the more robust and more vigorous immune responses in the young, particularly in the first line of defence, termed innate immunity.”
Other factors including pre-existing conditions, genetics and environmental factors are likely at play, he added.
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Canada has approved the mixing of mRNA vaccines, which means a subsequent booster dose can be different than the primary vaccine series.
There is no evidence of increased adverse effects if Pfizer and Moderna are combined, Piccirillo said.
“There is no difference in the number, type or frequency of adverse effects if one were to give combinations of prime boost.”
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