- metadata: - source: https://www.wsj.com/articles/researchers-probe-link-between-covid-19-vaccines-and-myocarditis-11636290002 --- # Covid-19 Vaccines and Myocarditis Link Probed by Researchers - WSJ > ## Excerpt > Several theories are under investigation by researchers seeking to explain heart-inflammation conditions among small numbers of vaccinated people. --- ## Several theories are under investigation seeking to explain heart-inflammation conditions among small numbers of vaccinated Covid Vaccine Efficacy Numbers, Explained Your browser does not support HTML5 video. Sound OnCreated with sketchtool. 0:00 / 6:00 1:36![](https://m.wsj.net/video/20210903/efficacy090321/thumbstrip.jpg) ![Covid Vaccine Efficacy Numbers, Explained](https://m.wsj.net/video/20210903/efficacy090321/efficacy090321_1280x720.jpg) Covid Vaccine Efficacy Numbers, Explained Recent studies have shown that the effectiveness of Covid-19 vaccines is decreasing, though experts say the shots still work well. WSJ explains what the numbers mean and why they don’t tell the full story. Photo illustration: Jacob Reynolds/WSJ Your browser does not support the audio tag. This article is in your queue. As U.S. health authorities expand use of the leading Covid-19 vaccines, researchers investigating [heart-related risks](https://www.wsj.com/articles/cdc-advisory-group-says-likely-association-between-mrna-covid-19-vaccines-and-rare-myocarditis-cases-11624479115?mod=article_inline) linked to the shots are exploring several emerging theories, including one centered on the spike protein made in response to vaccination. Researchers aren’t certain why the messenger RNA vaccines, one from Pfizer Inc. and partner BioNTech SE and the other from Moderna Inc., are likely causing the inflammatory heart conditions myocarditis and pericarditis [in a small number of cases](https://www.wsj.com/articles/some-adolescents-arent-getting-vaccinated-as-wary-parents-weigh-risks-11627291801?mod=article_inline). ## Resume Subscription We are delighted that you'd like to resume your subscription. You will be charged $ + tax (if applicable) for The Wall Street Journal. You may change your billing preferences at any time in the Customer Center or call [Customer Service](https://customercenter.wsj.com/contact). You will be notified in advance of any changes in rate or terms. You may cancel your subscription at anytime by calling [Customer Service](https://customercenter.wsj.com/contact). Please click confirm to resume now.