Experts say there are a variety of reasons for the U.K.’s steep Covid numbers — ranging from the half-hearted mask adoption (even when masks are required, such as on public transport, the rule is rarely enforced) to large indoor gatherings that have allowed the virus to spread.

The U.K.‘s hesitation in vaccinating younger teenagers, something that other countries in Europe and the U.S. did much earlier, and the return to schools in September, have also been cited as reasons for the sharp rise in cases, although the boom in infections among 0-18 year olds is now ebbing as infections rise in their parents’ generation, data shows.

Perhaps most ironically, the U.K.’s early vaccination rollout — which began in December 2020 and was one of the first in the world — is also seen as contributing to its high case rate now.

That’s because we now know — due to an increasing body of data — that immunity in vaccinated people wanes after about six months. The spread of the much more infectious delta Covid variant in the spring and summer is also seen as a factor that has diminished vaccine efficacy.