83 Soldiers, One Cardiac Signal: How to Read the Czech mRNA Study

SharpPost  |  May 9, 2026  |  Public Health

In late March 2026, the journal Vaccine shared a small study from the Czech Republic. It involved eighty-three healthy military personnel stationed at an air force base near Prague, all of whom had received two doses of an mRNA COVID-19 vaccine (either Pfizer or Moderna) back in 2021. These individuals donated blood up to nine times for researchers who were monitoring three key cardiac markers: NT-proBNP, which indicates how hard the heart is working; troponin, which shows if there's any damage to heart muscle cells; and SIRI, which measures systemic inflammation. Remarkably, within two weeks after the second dose, 49% of the participants experienced a rise in NT-proBNP to at least 1.5 times their baseline levels. However, troponin levels remained unchanged. The authors were careful in their conclusions, noting that while the heart was indeed working harder, it wasn't actually injured. Yet, within a week of the study's release, vaccine skeptics twisted the findings into a claim that "half the soldiers had heart damage." It's important to clarify that the paper does not support that assertion.

Blood draws took place from April to December 2021, with participants providing up to nine samples during the weeks leading up to and following each dose. Just two weeks after the first dose, NT-proBNP levels increased from the baseline to an average of 28.7 pg/mL. After the second dose, this level jumped to 36.2 pg/mL. The second dose seemed to boost the likelihood of producing this signal by about 13 times compared to the first. Notably, men, individuals without a history of COVID-19, and those who received the Moderna vaccine experienced more significant increases. However, with only 83 participants, these subgroup trends aren't definitive.

On the flip side, there's troponin, the protein that hospitals rely on to diagnose heart attacks. When heart muscle cells are genuinely damaged, troponin leaks into the bloodstream almost right away. Throughout the study, troponin levels remained unchanged. Similarly, SIRI, another inflammation marker, didn't correlate with NT-proBNP, which suggests that a general inflammatory response isn't at play here. As a result, the authors conclude that "the heart worked harder" but stop short of saying "the heart was injured." They also point out that no ECGs, echocardiograms, cardiac MRIs, or exercise tests were conducted. Whether the changes in biomarkers correspond to any visible heart issues is something that will need to be explored in future research.

When NT-proBNP levels exceed 450 pg/mL, doctors start to raise an eyebrow about potential heart issues. That's the point where they begin to think about taking action. After the second dose, the average reading was 36.2, which is a whopping 12 times lower than the clinical threshold. Typically, heart-failure patients have readings over 1,000, and older adults admitted for heart failure often hit above 2,000. The researchers referred to their 36.2 measurement as "subclinical" for this reason.

The mRNA vaccines and China's Sinovac inactivated vaccine operate on different principles. With mRNA vaccines, a genetic blueprint for the spike protein is sent into your cells. This prompts your immune system to recognize the protein and start producing antibodies. The downside? You might experience more fever and fatigue after the shot, and there's a rare risk of myocarditis in young men. On the other hand, inactivated vaccines introduce a whole, killed virus. This allows the immune system to see the complete pathogen and respond in a more gentle manner. The side effects tend to be mild, and this technology has a solid track record, having been used in polio, hepatitis A, and seasonal flu vaccines. However, the trade-off here is that antibody levels may be lower and can noticeably decrease after about six months.

Sources: NEJM 2020 / 2021; Lancet 2021; JACC 2022; Annals of Internal Medicine 2022; WHO SAGE 2021 evidence assessment.

Take a look at this table sorted by population. After receiving two doses of Moderna, young men are about 11 times more likely to experience myocarditis compared to those who get Sinovac. While the headline might sound alarming, the actual numbers tell a different story. We're talking about a rate of 35 cases per million, which is pretty small when you consider that there's over a 95% chance these young men will steer clear of severe illness. For older adults and those with weakened immune systems, the situation flips. The milder side effects of Sinovac make them more inclined to get vaccinated and return for the second dose. Starting with a series of inactivated doses followed by an mRNA booster can help make up for the lower antibody levels, leading to outcomes that can be on par with the all-mRNA approach.

What to Watch Next

Eighty-three subjects is just the beginning, not the final word. The next steps involve larger studies: the U.S. CDC and FDA's VAERS reporting system, the Nordic national health registries, and groups of vaccinated individuals in China. If things keep moving at this pace, we should see the first independent replication by the latter half of 2026.