Large Israeli study offers detailed look at safety profile of mRNA vaccine against SARS-CoV-2

A deep look into a nationwide mass vaccination setting in Israel revealed that the BNT162b2 (Pfizer–BioNTech) vaccine is not linked with an elevated risk of a majority of the adverse events under study, with the exception of myocarditis. However, even that potentially severe adverse event is much more pervasive following the infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), found a new study published in the New England Journal of Medicine (NEJM).

Before the messenger RNA (mRNA)–based vaccines against coronavirus disease 2019 (COVID-19) were distributed around the world, preapproval studies have shown that their safety profile is good. Still, these initial trials were not perfect due to their size and patient-mix limitations, which is inherent for phase 3 trials.

This is why thorough surveillance is needed to appraise the safety of newly introduced vaccines in real-world settings. Passive surveillance systems are suitable to capture such information, with the most notable example being Vaccine Adverse Event Reporting System (VAERS), albeit not optimal.

Likewise, active surveillance systems (embedded in large electronic health record databases) can help highlight suspicious trends even better. However, the lack of a rigorous control group hampers the ability to pinpoint any causal effects.

But still, one of the underlying trends of vaccine hesitancy in the population is the fear of side effects and adverse events. Although the literature published thus far confirms the acceptable safety profile of mRNA-based vaccines, high-quality safety data from a real-world setting is still sparse.

In this study, a research group led by Dr. Noam Barda from Clalit Health Services in Tel Aviv (Israel) aimed to place the increased risk of adverse events after vaccination with Pfizer–BioNTech mRNA vaccine in context by comparing them to adverse events after documented infection with SARS-CoV-2.

A gigantic, real-world data set

In order to pursue that goal, the researchers have used a huge data set from the largest health care organization in Israel that involved more than 2.4 million vaccinated individuals. Vaccinated people were individually matched with unvaccinated ones – in accordance with their sociodemographic traits and clinical variables.

To place these results into context, the researchers have also performed a parallel analysis involving more than 240 thousand SARS-CoV-2–infected persons matched to uninfected persons. It has to be emphasized that the same adverse events have been studied in both the vaccination and SARS-CoV-2 infection analyses.

Furthermore, it has to be noted that all the effect measures presented in this study were based only on a new incidence of a certain adverse event under study; therefore, there was a lack of information on the potential additional risk among individuals with a medical history of each of these adverse events.

Pinpointing potential adverse events

In short, the study has shown that vaccination was strongly associated with an increased risk of myocarditis (2.7 events per 100,000 persons), as well as lymphadenopathy (which occurred relatively frequent with 78.5 events per 100,000 persons), herpes zoster infection (which can translate to an increased risk of Bell’s palsy) and appendicitis.

On the other hand, SARS-CoV-2 infection was linked to a substantially elevated risk of myocarditis (11 events per 100,000 persons), alongside the risk of pericarditis, arrhythmia, pulmonary embolism, deep-vein thrombosis, myocardial infarction, thrombocytopenia, and intracranial hemorrhage.

The study has also shown that the risk of myocarditis is basically increased by a factor of three following vaccination. Furthermore, among the 21 persons with myocarditis in the group that received the vaccine, the median age was 25 years and there was a substantial male predominance.

Certain unexpected effects were also observed, as Pfizer–BioNTech vaccine appears to offers a degree of protection against conditions such as intracranial hemorrhage and anemia, possibly by halting SARS-CoV-2 infection, which is a known risk factor for such events.

Continuous surveillance needed

In a nutshell, this study has estimated that the Pfizer–BioNTech vaccine may come with a risk of an increased incidence of specific adverse events over a 42-day follow-up period. Even though most of these events were mild, some of them (such as myocarditis) are potentially serious.

“However, our results indicate that SARS-CoV-2 infection is itself a very strong risk factor for myocarditis, and it also substantially increases the risk of multiple other serious adverse events”, say study authors in this NEJM paper. “These findings help to shed light on the short- and medium-term risks of the vaccine and place them in clinical context,” they add.

Naturally, further studies will be necessary to precisely gauge the potential of long-term adverse events, while the findings of this study will have to be supplemented by similar publications from other countries in order to obtain the complete picture.

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university – University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.