U.S. researchers analyzed tissue samples from the autopsies of 25 people, including 20 who were vaccinated.

Samples from the hearts of three patients, all of whom died within 30 days of a Pfizer shot, tested positive for messenger ribonucleic acid (mRNA).

Eight bilateral axillary lymph node samples, from people who died within 30 days of a Moderna or Pfizer vaccine, also tested positive. The companies' shots utilize mRNA.

The research shows "the vaccine can persist for up to 30 days, including in the heart," Dr. James Stone, with the departments of pathology at Massachusetts General Hospital and Harvard Medical School, told The Epoch Times via email.

The study was published by npj Vaccines. Authors declared no conflicts of interest. They said the research was supported by Massachusetts General Hospital, which is in Boston.