The combination of vaccination and natural SARS-CoV2 infection was associated with the development of severe heart failure and cardiogenic shock in patients with STEMI" – ST-elevation myocardial infarction, or complete artery blockage – "possibly related to an increased serological response" to vaccination, according to the paper in Vaccine.

The March 2020-March 2023 study does not appear to have drawn notice except among doctors, researchers and journalists who have faced suppression for pointing to worrisome connections between COVID vaccines and severe adverse events in low-risk populations such as youth.

"Hybrid exposure causes very high levels of spike antibodies!" MIT professor Retsef Levi, lead author of a 2022 study finding vaccination "significantly associated" with a spike in emergency heart problems in 16-39 year-olds in Israel, wrote on X after reading the Spanish paper.

Two days after Just the News reported on Levi's study, the Nature journal Scientific Reports added a scarlet letter to the paper in response to unspecified "criticisms" of its conclusions. The journal had still not posted its promised "full editorial response" six months later, and finally published the authors' minor corrections to several sections in August 2023.

Twitter, formerly X, similarly dissuaded users from reading a study – deeming the link "unsafe" when they clicked it – by the Centers for Disease Control and Prevention in the Journal of the American Medical Association. It found Pfizer's vaccine for ages 12-15 can't stop symptomatic infection by four months and turns negative by seven months, making infection more likely.

Four in five patients in the Spanish study were men, with an average age of 64, but U.K. diagnostic pathologist Clare Craig, co-chair of the nonprofit Health Advisory and Recovery Team, emphasized the Madrid-based researchers' findings for vaccinated patients under 65.

Either severe heart failure or cardiogenic shock, formally known as heart-attack categories Killip III/IV, was found in 7.9% of unvaccinated and 13.7% of vaccinated patients within 30 days of heart attack, and in 13.3% of vaccinated more than 30 days before hospital admission.

The gap was wider between unvaccinated and vaccinated in the previous 30 days under age 65 – 4.8% and 14.3%, respectively – while the proportion (11.8%) was lower in those vaccinated more than 30 days before admission.

The "significantly higher S-specific IgG levels" were more pronounced among younger patients who were vaccinated and previously infected.