The WHO article sayshttps://www.who.int/emergencies/disease-outbreak-news/item/2023-DON465
"This Disease Outbreak News was first published on 16 May 2023 and later updated on 17 May 2023 to correct the number of cases, deaths, and the number of cases with PCR testing.
Situation at a glance
On 5 April 2023, the National IHR Focal Point for the United Kingdom informed WHO of an increase in severe myocarditis in neonates..."
"Enterovirus PCR testing of all ten cases (with either blood, throat swab, nose swab, nasopharyngeal aspirate or cerebrospinal fluid samples) confirmed the presence of an enterovirus, subtyping (where available) to either coxsackie B3 or coxsackie B4."
Don't know if you've already seen this or not but here is more info about coxsackie B3 and B4.
"Coxsackie B virus belongs to the Picornavirus family and can cause a variety of diseases, including gastrointestinal illness, myocarditis, pneumonia, aseptic meningitis, encephalitis, and hepatitis."
https://www.ncbi.nlm.nih.gov/books/NBK560783/