Study Largely Confirms Known, Rare COVID-19 Vaccine Side Effects

SciCheck Digest

An international study of around 99 million people confirmed known serious side effects of COVID-19 vaccination. It also identified a possible relationship between the first dose of the Moderna vaccine and a small risk of a neurological condition. Social media posts about the study left out information on the vaccines’ benefits and the rarity of the side effects.

More than half a billion doses of COVID-19 vaccines have now been administered in the U.S. and only a few, very rare, safety concerns have emerged. The vast majority of people experience only minor, temporary side effects such as pain at the injection site, fatigue, headache, or muscle pain — or no side effects at all. As the Centers for Disease Control and Prevention has said, these vaccines “have undergone and will continue to undergo the most intensive safety monitoring in U.S. history.”

A small number of severe allergic reactions known as anaphylaxis, which are expected with any vaccine, have occurred with the authorized and approved COVID-19 vaccines. Fortunately, these reactions are rare, typically occur within minutes of inoculation and can be treated. Approximately 5 per million people vaccinated have experienced anaphylaxis after a COVID-19 vaccine, according to the CDC.

To make sure serious allergic reactions can be identified and treated, all people receiving a vaccine should be observed for 15 minutes after getting a shot, and anyone who has experienced anaphylaxis or had any kind of immediate allergic reaction to any vaccine or injection in the past should be monitored for a half hour. People who have had a serious allergic reaction to a previous dose or one of the vaccine ingredients should not be immunized. Also, those who shouldn’t receive one type of COVID-19 vaccine should be monitored for 30 minutes after receiving a different type of vaccine.

There is evidence that the Pfizer/BioNTech and Moderna mRNA vaccines may rarely cause inflammation of the heart muscle (myocarditis) or of the surrounding lining (pericarditis), particularly in male adolescents and young adults.

Based on data collected through August 2021, the reporting rates of either condition in the U.S. are highest in males 16 to 17 years old after the second dose (105.9 cases per million doses of the Pfizer/BioNTech vaccine), followed by 12- to 15-year-old males (70.7 cases per million). The rate for 18- to 24-year-old males was 52.4 cases and 56.3 cases per million doses of Pfizer/BioNTech and Moderna vaccines, respectively.

Health officials have emphasized that vaccine-related myocarditis and pericarditis cases are rare and the benefits of vaccination still outweigh the risks. Early evidence suggests these myocarditis cases are less severe than typical ones. The CDC has also noted that most patients who were treated “responded well to medicine and rest and felt better quickly.”

The Johnson & Johnson vaccine has been linked to an increased risk of rare blood clots combined with low levels of blood platelets, especially in women ages 30 to 49. Early symptoms of the condition, which is known as thrombosis with thrombocytopenia syndrome, or TTS, can appear as late as three weeks after vaccination and include severe or persistent headaches or blurred vision, leg swelling, and easy bruising or tiny blood spots under the skin outside of the injection site.

According to the CDC, TTS has occurred in around 4 people per million doses administered. As of early April, the syndrome has been confirmed in 60 cases, including nine deaths, after more than 18.6 million doses of the J&J vaccine. Although TTS remains rare, because of the availability of mRNA vaccines, which are not associated with this serious side effect, the FDA on May 5 limited authorized use of the J&J vaccine to adults who either couldn’t get one of the other authorized or approved COVID-19 vaccines because of medical or access reasons, or only wanted a J&J vaccine for protection against the disease. Several months earlier, on Dec. 16, 2021, the CDC had recommended the Pfizer/BioNTech and Moderna shots over J&J’s.

The J&J vaccine has also been linked to an increased risk of Guillain-Barré Syndrome, a rare disorder in which the immune system attacks nerve cells. Most people who develop GBS fully recover, although some have permanent nerve damage and the condition can be fatal.

Safety surveillance data suggest that compared with the mRNA vaccines, which have not been linked to GBS, the J&J vaccine is associated with 15.5 additional GBS cases per million doses of vaccine in the three weeks following vaccination. Most reported cases following J&J vaccination have occurred in men 50 years old and older.

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COVID-19 vaccines — like all vaccines and other medical products — come with side effects, including serious side effects in rare cases. The vaccines were rolled out to protect people from a novel virus that has killed millions of people globally and would likely have killed millions more without the arrival of the vaccines. There is a broad consensus from experts and governmental health agencies that the benefits of COVID-19 vaccination outweigh the risks.

Researchers have scrutinized the COVID-19 vaccines’ safety and continue to do so. A study published Feb. 12 in the journal Vaccine reported on an international group of more than 99 million people who received COVID-19 vaccines, primarily finding links to known rare side effects. The study largely focused on the Pfizer/BioNTech and Moderna vaccines, which have been widely given in the U.S., as well as the AstraZeneca vaccine, which was never authorized in the U.S.

“What we take away, is that the Covid-19 vaccination campaigns have been very effective in preventing severe disease,” study co-author Anders Hviid, head of the department of epidemiology research at the Statens Serum Institut in Denmark, told us in an email. “The few serious side effects that we have observed in this and other studies have been rare.”

Many popular posts on social media have shared results from the study, some lacking the context that the identified health problems are rare, that most aren’t new and that the vaccines have proven benefits. Various posts made unfounded claims, stating or implying that people should not have received the vaccines, that the risks outweigh the benefits or that the risk of the rare side effects is greater than was reported in the study.

“Hundreds of millions of people were used as lab rats and now the truth that WE ALL ALREADY KNEW can no longer be denied,” said one popular post, referring to the vaccines as “experimental” and “UNTESTED.” The post shared a screenshot of the headline of a New York Post article about the new study, which read, “COVID vaccines linked to slight increases in heart, brain, blood disorders: study.”

“This thing was forced on people who faced almost no risk from Covid,” said another widely read post. “It is completely unacceptable.” The post shared statistics from the paper without making it clear that serious health problems after vaccination were rare and that risk varied by vaccine type and dose.

The Vaccine study confirmed that the Moderna and Pfizer/BioNTech vaccines are linked in rare cases to myocarditis and pericarditis, conditions involving inflammation of the heart muscle and lining. The rate of myocarditis was most elevated after the second dose of the Moderna vaccine. Myocarditis risk — which is greatest in men in their late teens and early twenties — was identified via vaccine safety monitoring and first reported in 2021. Based on the current evidence, the CDC says, the benefit of vaccination outweighs the risk of these conditions, which improve for most people after medical treatment and rest.

The study confirmed neurological and blood clotting conditions associated with the AstraZeneca vaccine. In the U.S., these problems were linked to the Johnson and Johnson vaccine, contributing to this vaccine no longer being recommended or available.

The study also identified a new possible safety signal indicating a potential link between the first dose of the Moderna and AstraZeneca vaccines and rare neurological conditions. This included an association between the first doses of the vaccines and acute disseminated encephalomyelitis, or ADEM, an autoimmune condition that causes inflammation of the brain and spinal cord.

Hviid emphasized that the researchers only saw these neurological events after first doses of the two vaccines. “We did not see these signals following further doses of these two Covid-19 vaccines, nor did we see them after any dose of the Pfizer/BioNTech vaccine which has been more widely used,” he said.

“We are also talking about very rare events,” Hviid continued. “As an example, the association between the first dose of Moderna and acute inflammation of the brain and spine would, if causal, correspond to 1 case per 1.75 million vaccinated. It is only due to the sheer scale of our study, that we have been able to identify this minute potential risk.”

Study Bolsters the Evidence Serious COVID-19 Vaccine Side Effects Are Rare

The Vaccine study drew on national or regional health records from eight countries with institutions participating in the Global Vaccine Data Network, an international group that studies vaccine safety. The researchers analyzed health outcomes after around 184 million doses of the Pfizer/BioNTech vaccine, 36 million doses of the Moderna vaccine and 23 million doses of the AstraZeneca vaccine.

The researchers focused on 13 health problems that either had a known association with vaccination or for which there was some rationale to investigate whether there was an association. To determine whether the health problems were associated with vaccination, they compared the expected rates of the health problems — or the number of health events that should occur based on background rates in the regions studied — with the number of events they observed in the 42 days after vaccination.

“This study confirms the primary already detected and validated side effects established by previous literature,” Jeffrey S. Morris, director of the division of biostatistics at the University of Pennsylvania’s Perelman School of Medicine, told us via email, referring to the rare heart conditions associated with the Moderna and Pfizer/BioNTech vaccines, as well as the rare conditions associated with the AstraZeneca and Johnson and Johnson vaccines.

Morris said that findings on ADEM — the rare autoimmune neurological condition linked to first doses of the Moderna and AstraZeneca vaccines — “might be a new safety signal.”

ADEM involves inflammation to the brain and spinal cord, arising most often in children following an infectious illness. It has a sudden onset and typically eventually improves, with a full recovery in many, although not all, cases.

After the first dose of the Moderna vaccine, researchers observed seven ADEM cases, when they expected two. As we’ve said, Hviid calculated the rate of this side effect — if ultimately shown to be related to vaccination — to be 1 in 1.75 million following the first dose of the Moderna vaccine.

The data show “this was indeed an EXTREMELY rare adverse event,” Morris said, referring to ADEM. “It is understandable at this incidence rate why it may not have been detected before now, and why a study with 99 million participants like this is important to find even the most rare serious adverse events that are potential minority harm risks of these vaccines.”

The authors of the study wrote that more research is needed into ADEM following COVID-19 vaccination, saying that “the number of cases of this rare event were small and the confidence interval wide, so results should be interpreted with caution and confirmed in future studies.” The authors also wrote that neurological events have been found to occur at a much higher rate after COVID-19 than after COVID-19 vaccination.

The study means that “early warning systems are solid,” said Marc Veldhoen, an immunologist at the Instituto de Medicina Molecular João Lobo Antunes in Portugal, in a post on X, formerly known as Twitter. “To avoid any adverse reaction is not possible, but, identifying those at higher risk may be possible.”

Identifying those at greater risk of side effects can help guide decisions on which vaccines to recommend and what problems doctors should watch for in their patients.

Editor’s note: SciCheck’s articles providing accurate health information and correcting health misinformation are made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over FactCheck.org’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation.

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