Pfizer and Moderna have launched trials to determine whether there are any long-term negative health impacts associated with their Covid vaccines.
The studies will involve monitoring the small number of Americans who suffered rare side effects after receiving the shots over the past two years.
Both firms are required to carry out this long-term research by the Food and Drug Administration (FDA) as a condition of approval earlier this year.
Inflammation of the heart has been the most common serious adverse effect reported from the shots - though it is still very rare.
A study by British Columbia Centre for Disease Control, in Canada, found that 58 of every million recipients of Moderna's two-shot vaccine developed the condition.
The same study found that 21 of every million recipients of the original two-dose Pfizer vaccine also suffered the heart issue.
Cases were most common among men under the age of 30, affecting more than 250 per every one million men aged 18 to 29.
The Centers for Disease Control and Prevention (CDC) said it has recorded aronud 1,000 cases of heart inflammation among under-18s who received Covid shots.
While these cases usually resolve themselves without medical intervention, some fear there could be long term heart damage.
Moderna, based in Cambridge, Massachusetts, has already launched two trials tracking its shot's adverse affects, the most recent in September.
