Chat about Novel Coronavirus - 2019-nCoV - COVID-19

Because a lot of people still aren’t vaccinated or have only had one jab, probably, it already is.

Covid: Unvaccinated most at risk from Delta variant https://www.bbc.co.uk/news/health-57441677

1) How come the UK is seeing such a rapid rise in cases despite a successful vaccination campaign?

Easing of lockdown + the new variant being more transmittable.

Specifically, it seems to be finding its way to people who aren't vaccinated at all mostly. It is also infecting people who have one vaccine (but very few with two). I've not seen any data on those who have one vaccine that says how soon AFTER their vaccine they got the virus. As it was already known a single vaccine 0-3 weeks in isn't ideal, it's not clear how well a single vaccine 2/3/4 weeks in is doing against the new variant.

2) And will mainland Europe meet the same fate as the delta strain becomes the dominant strain here?

We don't know what the UK's fate is yet, so it's hard to answer.

It's not clear that the increase in cases here will result in more hospitalizations and deaths (or more than can be managed by the NHS). If these severe cases remain manageable, than it will have been the UKs current level of vaccination which managed this. If that happens, that's good news for the rest of Europe (and elsewhere - Canada for example, which has smashed vaccinations into people - surpassing the UK for single doses I think - but remains quite far behind on second doses).

If the UK gets hammered, though, everywhere is also likely to get hammered, unfortunately.

3) Is it inevitable that the delta strain becomes the dominant strain here just like the alpha strain did half a year ago?

Who knows. There could be a new strain out of Utrecht (or Lyon, or Tallinn, or Zaragoza, or...) tomorrow that's worse.