mRNA Vaccines Protect Against COVID-19 Mortality, Contrary to Misleading Posts

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SciCheck Digest

Numerous studies have shown the mRNA COVID-19 vaccines are safe and effective in preventing severe disease and death from COVID-19. But some social media posts are citing a criticized study that focuses on overall mortality to falsely imply the vaccines are harmful and don’t work. 



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Both mRNA COVID-19 vaccines, from Moderna and Pfizer/BioNTech, showed excellent results in the clinical trials. There were no serious safety concerns reported for either of them, and both were more than 90% effective in preventing symptomatic and severe COVID-19.

With the arrival of new viral variants and the passage of time, the vaccines no longer provide strong protection against mild disease, but many studies have shown that these vaccines work very well in preventing severe COVID-19 and death. One estimate suggests that COVID-19 vaccines prevented more than 3 million deaths in the U.S. 

Johnson & Johnson’s COVID-19 vaccine, which is designed differently and uses a harmless adenovirus to trigger an immune response, showed a slightly lower efficacy. Although no serious safety issues were detected in the clinical trials, rare cases of a dangerous blood clotting condition combined with low blood platelets known as thrombosis with thrombocytopenia syndrome were reported in the U.S. shortly after the vaccine was authorized for use. In 2022, the FDA limited its authorization. Now that the last doses expired in May, the vaccine is no longer available in the country. 

But some social media posts are citing a recent study that reanalyzed data from the original trials to compare the impact of both types of vaccines — the mRNA and adenovirus vector COVID-19 vaccines — on deaths from all causes, to incorrectly imply that the mRNA vaccines are dangerous or have no effect on mortality from COVID-19.

“Researchers inadvertently reveal mRNA covid vaccines are NOT ASSOCIATED WITH LOWER MORTALITY in reassessment of Random Control Trials (RCTs),” reads a post on Instagram, which also misleadingly implies that the original trial data were interpreted incorrectly. “27 of the 31 deaths in the RCTs that took either Moderna or Pfizer were blood, heart, or artery related, to the shock of no one who’s been paying attention,” the post adds.

“COVID Shots Had ZERO Impact on Reducing Deaths, Study Finds,” reads the headline of an item on a conservative website.

The posts do not specify overall mortality, leading many viewers to incorrectly assume the vaccines don’t protect against death from COVID-19.

Cases of inflammation of the heart muscle or its surrounding tissue, known as myocarditis and pericarditis, have occurred after mRNA COVID-19 vaccination, mostly in young men and after a second dose. But cases are rare and patients usually respond well to medicine and recover quickly.

The Moderna and Pfizer/BioNTech mRNA COVID-19 vaccines have not been linked to any other heart problems. Nor have they been shown to increase the risk of death. Study after study has confirmed the benefits outweigh the small risks.

A young man receiving the COVID-19 vaccine in a medical clinic. Photo by Ivan Pantic via Getty Images.

The cited study was published in April in the journal iScience by researchers from Denmark, the Netherlands, and Germany. The authors reanalyzed mortality data reported in the various clinical trials and found that the adenoviral vaccines appeared to reduce death from any cause, while the mRNA vaccines did not. They concluded that the adenoviral vaccines might have some beneficial effects.

Scientists have questioned the study since it was posted a year ago as a preprint. At that time, the study was also misinterpreted and used to misleadingly claim mRNA vaccines offer “no mortality benefit” at all, as our colleagues from PolitiFact wrote

The authors have acknowledged the limitations of the study and responded to some critics, but have not explicitly corrected claims stating the vaccines are dangerous or don’t reduce COVID-19 mortality. 

“It is not possible to say, based on our findings, that ‘mRNA COVID-19 vaccines are not effective or harmful,’” Dr. Christine Stabell Benn, the lead author, told us in an email. 

But Benn, who is a member of a controversial committee gathered by Florida Gov. Ron DeSantis to advise the state on public health issues, also told us the results from the clinical trials “do not exclude” the possibility that the vaccines “are associated with considerable harm.” As we’ve reported, DeSantis’ committee of contrarian experts held a roundtable in December claiming the COVID-19 vaccines were too risky.

The iScience Study

The iScience study’s goal was to determine if the COVID-19 vaccines had broad effects on the immune system, beyond the vaccine’s ability to prepare the immune system to better fight off the coronavirus. In particular, the authors wanted to use the available trial data to see if the vaccines had an effect on overall mortality.

Two of the authors of the study, Benn and Peter Aaby, both professors at the University of Southern Denmark, have been studying what are called non-specific effects of vaccines, or secondary vaccine effects, for decades.

Typically, vaccines targeting a particular disease are not expected to have any effects on other diseases, since the immune system is responding to specific pathogen. But some epidemiology studies suggest that some vaccines may have broader effects. The concept is still understudied and many questions remain, according to a report of a workshop on the topic held by the National Institute of Allergy and Infectious Diseases in July 2021.

To study the COVID-19 vaccines’ effects on overall mortality, the researchers counted the deaths reported in the two main mRNA trials and in five trials of three different adenovirus vector vaccines. They then combined the data for each vaccine type and compared it. 

For the mRNA vaccines, there were a total of 74,193 participants in both the Pfizer/BioNTech and the Moderna trials. Among them, there were 61 deaths: 31 in the vaccine groups and 30 in the placebo groups. Half of the non-COVID-19 deaths (27 of 54) were related to cardiovascular issues, the study found, and the vaccines had no beneficial effect on those deaths, according to the researchers. 

“There were no obvious benefits for these vaccines beyond preventing COVID mortality, which is not surprising,” said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, after reviewing the study for us. “And we know, for instance, that after vaccines became widely available in the United States, the benefits of mRNA vaccines in terms of preventing mortality from COVID-19 were overwhelming.”

In the adenovirus vector vaccines trials included in the study, there were a total of 122,164 participants, with 46 deaths among them: 16 in the vaccine groups, 30 in the placebo groups. Therefore, the researchers concluded, the data suggested these vaccines were associated with a reduction in overall mortality, which they attributed to a lower COVID-19 mortality and a lower cardiovascular mortality (there were no cardiovascular deaths in the vaccine groups and eight in the placebo ones). 

“The results suggest that adenovirus-vector vaccines compared with placebo have beneficial non-specific effects, reducing the risk of non-COVID-19 diseases. The most important cause of non-COVID-19 death was cardiovascular disease, against which the data for the current RCTs suggest that the adenovirus-vector vaccines provide at least some protection,” researchers said.

The authors speculate that the adenovirus vector vaccines “prime the immune system in a way similar to a ‘live’ vaccine,” which is the kind of vaccine that uses weakened virus and has most often been found in their previous studies to have non-specific effects.

But to other scientists, these results are being overinterpreted. The data does not allow for a meaningful analysis, they say, since the vaccine trials were not designed to assess mortality and the available data is minimal.  

“It’s hard to say anything very conclusive,” Hotez, who participated in the NIAID workshop on secondary vaccine effects, told us about the paper. If anything, he said, it can generate a hypothesis that would need much broader testing and research.

“The claims about mRNA vaccines, including the purported cardiovascular effects, are based on far too few events to make any realistic conclusions,” Gideon Meyerowitz-Katz, an epidemiologist from the University of Wollongong in Australia, told us in an email.

“Moreover, we have an enormous wealth of more recent evidence demonstrating that mRNA vaccines save lives. This particular analysis takes a handful of deaths from very different populations – the mRNA vaccine trials were conducted in a very different patient population to the adenovirus vector vaccine trials – and inappropriately compares them.” 

He also said it was concerning that the study used trial data for the Sputnik vaccine, a Russian adenovirus COVID-19 vaccine. Meyerowitz-Katz and others have found data inconsistencies with that trial, raising questions about how trustworthy it is.

Dr. David R. Boulware, a professor of medicine at the University of Minnesota Medical School, told us the fact that the trials were held in different locations is very significant.

“[T]he adenovirus vaccine trials … were run mostly in low and middle income countries where the overall case fatality rate was 2%. The mortality in the mRNA vaccine trials, mostly run in high-income countries, was 0.3%. Deaths were too few to show any reduction,” he told us in an email. 

Benn and her team responded to these concerns in May 2022 after the preprint was published, and again in May 2023. They say the study results should not be dismissed because of the limited data.

“Our key message is that yes, data is regrettably limited, but it does show differences in mortality effect between the two major vaccine types that if true would have major global health implications,” Benn wrote on Twitter.

Regardless of whether the adenoviral vaccines have beneficial non-specific effects, it’s incorrect to suggest that the mRNA vaccines don’t protect against COVID-19 mortality, or to imply that they’re dangerous.

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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Benn, Christine Stabell (@StabellBenn). “Many people have commented on our recently published analysis of the overall mortality effect of the mRNA and adenovirus-vector vaccines. We addressed the comments in this article when the preprint was published: https://linkedin.com/pulse/response-comments-our-recent-preprint-paper-christine-stabell-benn/ 1/5.” Twitter. 16 May 2023.

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Governor Ron DeSantis Petitions Florida Supreme Court for Statewide Grand Jury on COVID-19 Vaccines and Announces Creation of the Public Health Integrity Committee.” Ron DeSantis website. Press release. 13 Dec 2022. 

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Boulware, David R. Professor of medicine at the University of Minnesota Medical School. Email to FactCheck.org. 17 May 2023. Benn, Christine Stabell (@StabellBenn). “Our key message is that yes, data is regrettably limited, but it does show differences in mortality effect between the two major vaccine types that if true would have major global health implications. 4/5.” Twitter. 16 May 2023.

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