COVID-19 poses a higher risk for heart inflammation and blood clot symptoms than vaccines

Two separate vaccine studies have found that COVID-19 poses a higher risk of symptoms than the risk of certain rare vaccine symptoms. 

One study out of Israel's Clalit Research Institute in Tel Aviv found that while the Pfizer/BioNTech slightly raises the risk of myocarditis, or heart inflammation, actually catching COVID-19 posed a higher risk for heart inflammation.

Researchers looked at nearly 2 million people — some vaccinated, others unvaccinated and some who had COVID-19 and others who didn't — and monitored vaccinated people for 42 days after their first injection. Similarly, they monitored people who had COVID-19 and compared the two groups.

The study found the mRNA vaccines did increase risk of myocarditis, with about 1 to 5 events per 100,000 persons. However, the presence of COVID-19 increased that risk even more, with 11 events per 100,000 persons.

"Myocarditis is inflammation of the heart muscle," Dr. Sean T. Liu, who was not a part of the study, told CBS News. Liu, an assistant professor of medicine and microbiology at the Icahn School of Medicine at Mount Sinai, pointed out that while the risk of myocarditis is higher after receiving the vaccine, "the risks of myocarditis are still under investigation and should be considered but not create worry."

The study also found COVID-19 "substantially increased" many other serious adverse events, including: acute kidney injury, with 125.4 events per 100,000 COVID-19 patients, pulmonary embolism, with 61.7 events per 100,000 COVID-19 patients, and deep-vein thrombosis, a serious blood clot condition, with 43 events per 100,000 COVID-19 patients. 

"COVID-19 is a terrible disease. I have watched it tear through families and kill my friends," Liu said. "There are risks and benefits to every medical intervention. The benefits of COVID-19 vaccination undoubtedly outweigh the risks."

The study, published in the New England Journal of Medicine, also looked at the risk for other symptoms such as lymphadenopathy, appendicitis and herpes zoster infection in the vaccines versus COVID-19 infection and found the vaccine was not associated with an elevated risk of most of the adverse events examined. 

In fact, the vaccine actually protected against some adverse events, Grace Lee, professor of pediatrics at the Stanford University School of Medicine, wrote in a commentary about the study.

"What is even more compelling about these data is the substantial protective effect of vaccines with respect to adverse events such as acute kidney injury, intracranial hemorrhage, and anemia, probably because infection was prevented," Lee wrote. "Furthermore, the persons with SARS-CoV-2 infection appeared to be at substantially higher risk for arrhythmia, myocardial infarction, deep-vein thrombosis, pulmonary embolism, pericarditis, intracerebral hemorrhage, and thrombocytopenia than those who received the BNT162b2 vaccine."

In an email to CBS News, Lee said their are two major benefits of mRNA vaccines that have been demonstrated by COVID-19. 

"[One,] vaccine development is a lot faster when working with mRNA vaccines vs. inactivated vaccines or other more traditional platforms," she said. "[Two,] mRNA vaccines have really produced incredibly robust immune responses."

Another study out of the U.K. also shows COVID-19 patients are at a higher risk of developing blood clots than people who have received the AstraZeneca or Pfizer vaccines. The AstraZeneca vaccine is available in the U.K. and unlike the mRNA Pfizer and Moderna shots used in the U.S., it is a viral vector vaccine.

This study, published in the British Medical Journal, monitored 29 million vaccinated people and about 1.7 million COVID-19 patients.

The study found an increased risk of blood clot syndromes after the first AstraZeneca and Pfizer shot. There was also an increased risk of cerebral venous sinus thrombosis (CVST or a blood clot in the brain's venous sinuses) after a first dose of both vaccines, "which might be a potential signal, although numbers were small and further confirmation is needed," the researcher say.

However, "the risks of these outcomes after vaccination were much lower than those associated with SARS-CoV-2 infection in the same population."