Free Spike Protein of mRNA COVID-19 Vaccines Implicated in Myocarditis

Myocarditis that arises after receipt of an mRNA-based COVID-19 vaccine among adolescents and young adults does not appear to result from a heightened overall immune response or from autoantibodies, new data suggest. Instead, the complication seems related to how some individuals process spike protein, produced by the body using the vaccine’s mRNA as a template. Levels of the antigen were unusually high among those with myocarditis.

Prior studies have provided insight into the key clinical features of myocarditis cases, rare but real, seen with Pfizer/BioNTech’s BNT162b2 and Moderna’s mRNA-1273 vaccines.

For their paper, published online in Circulation, Lael M. Yonker, MD (Massachusetts General Hospital, Boston), and colleagues turned to immunoprofiling as a way to better understand the underlying mechanisms.

“We discovered that individuals who developed postvaccine myocarditis uniquely exhibit elevated levels of free spike protein in circulation, unbound by antispike antibodies, which appear to correlate with cardiac troponin T levels and innate immune activation with cytokine release,” they explain, adding, “Although these findings might provide insight into the immunophenotype of vaccine-related myocarditis, they do not alter the risk-benefit ratio strongly favoring vaccination to protect against severe COVID-19-related complications.”

Yonker told TCTMD that, going into their study, the researchers had anticipated they would find a link between postvaccine myocarditis and autoantibodies that might be inadvertently targeting cardiac tissue. Another possible idea had been that the mRNA COVID-19 shots trigger an overly vigorous immune response, thanks to either a genetic predisposition or testosterone.

“We were expecting to see this hyper, revved up autoantibody response to the vaccine. . . . We really didn’t see that at all, which surprised us,” she said.

Interestingly, said Yonker, a known feature of multisystem inflammatory syndrome in children (MIS-C), which can develop after the acute phase of COVID-19, is the persistence of viral particles in the body. Lingering SARS-CoV-2 particles have been proposed as one mechanism for long COVID as well.

With myocarditis, the spike protein most likely arises downstream from the vaccine itself, not leftover virus, she explained. Typically, though, these particles don’t stick around but get cleared as the body processes the vaccine. Exactly why individuals with myocarditis don’t clear it the same way isn’t yet known, said Yonker.

What the results don’t do, she stressed, is change the fact that the data strongly support getting vaccinated against COVID-19. “Clearly, developing postvaccine myocarditis is much more rare than developing any of the complications of infection with SARS-CoV-2,” she noted.

Biykem Bozkurt, MD, PhD (Baylor College of Medicine, Houston TX), who wrote an editorial accompanying the paper, said the study “is providing further information about the why and the how” of postvaccine myocarditis.

Regardless of the mechanism, an important message is that myocarditis is extremely uncommon after mRNA vaccines, occurring at an incidence of around one to five cases per 100,000 in the general population, she emphasized. “Even in the higher-risk group, which is age 16 to 30, it’s one in 20,000.” On the other hand, the incidence of myocarditis is “markedly higher” following COVID-19 infection than it is following vaccination, Bozkurt said, and people with the infection see increases in MI, stroke, arrhythmia, deep vein thrombosis, pulmonary embolism, and other cardiovascular events as well.

“The benefit of vaccination far exceeds any of these risks,” she said.

Proteins Linger in Some People

Yonker et al analyzed blood samples from 16 patients ages 12 to 21 years (average age 16; 81% male) who had been hospitalized between January 2021 and February 2022 at Massachusetts General Hospital for Children or Boston Children’s Hospital with myocarditis after receiving an mRNA-based COVID-19 vaccine.

All presented with chest pain and elevated cardiac troponin T (median 260 ng/L; IQR 215-1,114 ng/L) and C-reactive protein (median 29.75 mg/L; IQR 15.53–50.58 mg/L). Median time between vaccination and symptom onset was 4 days (range 1-19 days). Three-quarters developed myocarditis after their second dose, while the rest had their symptoms occur after the first (12.5%) and third (booster) dose (12.5%).

An additional 45 age-matched individuals (average age 15; 40% male) who were symptomatic up to 3 weeks after their second mRNA shot served as controls, also providing blood samples.

The researchers tested for humoral and T-cell responses to SARS-CoV-2, antibody responses to common pathogens, cytokine levels, and SARS-CoV-2 antigens.

What they found suggests “that individuals who developed myocarditis have a humoral immune response comparable to that of asymptomatic adolescents and young adults” who comprised the control group, the paper notes. There was “no indication that a specific antibody response is associated with myocarditis, but instead, all adolescents and young adults mounted a substantial immune response, conferring protection against SARS-CoV-2 after vaccination.”

T cells showed “minimal differences” between the myocarditis and control groups. Cytokine profiles, however, were distinct and “reminiscent of the profile seen in MIS-C, suggesting likely innate inflammatory activation,” the researchers note. In the myocarditis patients, there were elevations in interleukin (IL)-8, IL-6, tumor necrosis factor-α, IL-10, interferon-γ, and IL-1β but a reduction in IL-4. Additionally, they had higher total leukocyte levels and lower platelet counts compared with the controls.

Most notably, the plasma of patients with postvaccine myocarditis had “markedly elevated levels” of free spike antigen (33.9 ± 22.4 pg/mL), with the protein circulating in the body unbound by antibodies. “Although postvaccine myocarditis clinically occurs more commonly in males, [this] was seen equally in both affected females and males,” the investigators point out.

For controls, no free spike protein was detected.

The Question Is: Why?

Yonker pointed out that only three of the people with postvaccine myocarditis were known to have had acute SARS-CoV-2 infection beforehand. “Most did not,” she said. “So it would be extremely unlikely that these people had any other source of spike [apart from the vaccine].”

In their paper, the researchers caution that the relationship may not be causative, especially since myocarditis isn’t unique to mRNA vaccines but can occur after influenza and smallpox vaccines, among others.

However, it appears that, in some individuals, the spike protein antigen is able to evade antibody recognition and contribute to myocarditis, they suggest. “There is growing in vitro evidence that spike protein itself can stimulate cardiac pericytes dysfunction or inflame the endothelium, potentially by downregulating angiotensin-converting enzyme 2 expression, by impairing endothelial nitric oxide bioavailability, or by activating integrin-mediated inflammation with hyperpermeability of the endothelial cell layer.”

Speaking with TCTMD, Bozkurt highlighted three hypotheses for why the spike protein might be elevated.

When the vaccines are created, the very steps that are taken to ensure mRNA lasts long enough to do its job (lipid encapsulation and nucleoside modifications) may be the very qualities that, in some people, allow the spike protein to evade destruction that would normally occur. “Usually, the mRNA after injection into the deltoid muscle is degraded within 15 minutes,” she explained.

Another contributor could be inadvertent high-dose mRNA, since the vaccine doses aren’t adjusted for body weight. “The mRNA gets translated by the ribosomes in the cell to protein, and then it goes to the surface of the cell to be recognized as an antigen for which the antibody is produced,” said Bozkurt. But the endoplasmic reticulum, where the ribosomes are located, has a threshold for how much it can handle. When this capacity to process substrate is exceeded, “unfolded protein response” can occur and trigger apoptosis.

Finally, said Bozkurt, it could relate to anti-idiotype antibodies—these bind to the antibodies intended to target the original antigen (in this case, the spike protein) and thus prevent it from being neutralized.

Yonker said the precise reasons for the elevations in spike protein, and why certain patients are affected in this way, merit further research. Their study also lays the groundwork for improving the design and dose of future vaccines. “These findings also suggest,” the researchers note in their paper, “that administration of antispike antibodies, if spike antigenemia is detected, could potentially prevent or reverse postvaccine myocarditis.”