From the BBC
'What will happen from 2021?
The deal on the future relationship between the EU and the UK was announced on 24 December. It says that all EHIC cards issued before the end of 2020 will be valid until their expiry date.
After that, the UK will issue a new card. The UK government says the new card will be called the UK Global Health Insurance Card (GHIC), but there are no further details yet on how to obtain it.
Like EHIC, the new card will cover chronic or existing illnesses and routine maternity care as well as emergencies.
The Brexit agreement says any specialised treatment, such as dialysis or cancer treatment, "must be subject to a prior agreement between the insured person and the unit providing the treatment" to ensure the treatment is available.
The government's advice , however, still says you should buy travel insurance with healthcare cover before you go on holiday.'