Pfizer and Modern 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.

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