key: cord-1007740-j518n7bx
authors: Lazaros, George; Klein, Allan L.; Hatziantoniou, Sophia; Tsioufis, Costas; Tsakris, Athanasios; Anastassopoulou, Cleo
title: The Novel Platform of mRNA COVID-19 Vaccines and Myocarditis: Clues into the Potential Association
date: 2021-07-13
journal: Vaccine
DOI: 10.1016/j.vaccine.2021.07.016
sha: 7b860e00dc8ff7abff20a79bfdcc0977f072a3ab
doc_id: 1007740
cord_uid: j518n7bx

nan

Keywords: COVID-19; mRNA vaccines; myocarditis; mRNA-LNP stability Title Acute myocarditis is the second most common inflammatory heart disease after pericarditis, with an estimated annual incidence of 22 cases per 100,000 subjects [1] .

The absence of specific pathognomonic features in conjunction with the wide spectrum of clinical manifestations that range from subclinical cases to sudden cardiac death, render myocarditis particularly challenging. Myocarditis is most often the result of viral infections. The currently pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged as an additional causal agent of myocarditis.

Several highly effective vaccines that avert COVID-19 hospitalizations and deaths were available just over a year after the emergence of SARS-CoV-2. This record in the history of vaccinology has allowed for the re-opening of societies and the return to a new normalcy after strict lockdowns. Two of the leading vaccines that were granted emergency use authorization, Pfizer-BioNTech's Tozinameran (Comirnaty, BNT162b2) and Moderna's CX-024414 (mRNA-1273), are based on the novel platform of messenger RNA (mRNA). As mass vaccination efforts intensify, the meticulous safety monitoring of COVID-19 vaccines continues. Recent reports on the potential link between mRNA vaccination and myocarditis raised concerns among healthcare workers, the public and social media.

In more detail, Israel, that is leading the vaccination race, reported in late April that it is examining cases of myocarditis that occurred days after receipt of the Pfizer-BioNTech vaccine. A total of 62 cases (updated on June 2 to 275 cases between December 2020 and May 2021) were recorded (56 after the second-and 6 after the first dose) out of 5 million vaccinated, or 1 in 100,000 who received a second dose (0.001%); notably, however, this percentage was five times higher in the 16-30 years age group (1/20,000 or 0.005%). Of the 62 cases, 60 were hospitalized but recovered and were discharged, while two young, previously healthy people died (a 22- who were prioritized for vaccination, and a few were among adolescents 12-17 years whose vaccination has just been approved (as of May 10). Although rare, the identified association can be serious as shown by the finding that the majority of cases did not recover and by the (albeit few) fatalities. The association of myocarditis with male sex and younger age could be attributed to sex hormones which may account for a more intense inflammatory response. As suggested by experimental studies on myocarditis in mice, testosterone may be implicated in the inhibition of antiinflammatory cells and the stimulation of immune responses mediated by Th1lymphocytes [3] . But what could be triggering the inflammatory response that perhaps for some prone individuals is localized in the cardiac muscle tissue in the first place?

The answer probably lies in the heart of mRNA vaccine technology. Two of the main hurdles that had to be overcome during the development of the platform, which had been known to work in principle since the 1990s (the injection of foreign mRNA into mouse muscle led to the expression of the encoded protein and the induction of cellular immunity), were the RNA's unwanted reactogenicity and fragility. Inflammatory reactions were mediated by innate immune sensors for both singlestranded (ss) and double-stranded (ds) RNA, e.g., by endosomal Toll-like receptor 3 (TLR3), TLR7 and TLR8 and cytoplasmic protein kinase R (PKR), RIG-I, MDA5, IFIT1, and 2′-5′-oligoadenylate synthetase, respectively. To minimize such undesired immune responses, the RNA is modified to contain N 1 -methylpseudouridine instead of uridine [4] . Additional modifications of the mRNA and its encapsulation in lipid nanoparticles (LNPs) help overcome, but not eliminate, the inherent instability of the mRNA molecule under physiological conditions. This intrinsic property of the mRNA and bio-and nano-similars-related issues that may arise during the production of vaccine components (that takes places at multiple sites across different states or countries) are important variables that could be of concern. Minor differences in biophysical characteristics, such as particle size, homogeneity, shape and liposome lamellarity, may affect the physicochemical stability, bioactive moiety uptake, distribution or circulation times and immunogenicity [5] .

Accordingly, some early commercial batches of the Pfizer-BioNTech's vaccine contained lower than expected levels of intact mRNA as shown by leaked EMA documents [6] . In an email dated November 23, an EMA official outlined a raft of issues on "truncated and modified mRNA species present in the finished product," with unknown consequences on vaccine safety and efficacy. Two "major objections" were filed with Pfizer by EMA regulators who were tasked with ensuring good manufacturing production (GMP) practices. EMA authorized Pfizer-BioNTech's vaccine on December 21, noting that "the quality of this medicinal product, submitted in the emergency context of the current (COVID-19) pandemic, is considered to be sufficiently consistent and acceptable." Approval of Moderna's vaccine, which is based on the same modality and subject to similar potential problems, followed.

Thankfully, both mRNA vaccines proved to be very safe and effective against the severe consequences of COVID-19, and, hopefully, this platform will be used to tackle other diseases in the near future. Minimal biosafety risks are entailed, as the mRNA offers transient gene expression and does not integrate into the host genome.

Nevertheless, as the span of vaccinations is increasing beyond adults to include children aged 6 months to 12 years and given the fact that the rare potential association with myocarditis is more frequent among male adolescents, we propose the following two-tiered strategy: 1) Medical care should be sought in the presence of symptoms compatible with myocarditis, such as chest pain, dyspnea and palpitations, after COVID-19 vaccination, according to CDC's preliminary instructions; and 2) the purification of the mRNA from in vitro transcription contaminants and the mRNA-LNP product stability need to be improved and carefully monitored for each vaccine batch. Further studies are warranted to understand additional aspects of the modality, including the biodistribution of LNPs after the intramuscular injection. 

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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