The new strain of coronavirus in parts of England is also behind a rise in cases in Wales, the country's health minister has told Sky News.
Level 4 restrictions in Wales - in line with Tier 4 rules in England - came into effect on Sunday, while plans to allow five days of relaxed restrictions allowing up to two families to form a festive bubble have been cut short and will now only apply to Christmas Day.
Mr Gething also said that the decision to place the country immediately into lockdown ahead of Christmas was due to information that the new strain was leading to the virus spreading faster.
"It was a very difficult cabinet meeting yesterday because we understand the impact of these choices will be very real for many families across the country and it's an impact that won't be evenly spread," he said.
"But actually if we don't act, we'll see much greater harm."
Regarding the change in stance for Christmas, he added that he has "always said we may need to behave differently in the future", suggesting that it was difficult to predict.
He continued: "I've never given absolute guarantees there'll be no movement away from anything, because coronavirus will make a mug of you if you try to do that.
"The virus doesn't particularly care, it doesn't have feelings. It's only looking to reproduce and to continue and that will cause harm to each of us, so we all have choices to make.
"We have choices to make about what we do, what we do with other people.
"So I appeal to everyone, the government and the NHS can't do this on our own.
"We need every family in Wales to play their part."
In a statement on Saturday, the chief medical officer for Wales, Dr Frank Atherton, said that there was "rapidly emerging" evidence that the new variant appeared to be "significantly more transmissible than the wild-type virus".
He also said that despite a small sample size, an Office for National Statistics (ONS) study showed that the new variant could be responsible for up to 60% of new infections in Wales.
Dr Atherton added: "Emerging public health advice suggests that the new variant is at least contributing to, and possibly driving, this growth rate.
"Given the risk of further high growth continuing, to contain the harm of the pandemic and prevent already strained NHS resources from being overwhelmed, Level 4 measures will need to be brought forward as soon as possible."
When asked about when the toughest measures might be lifted, Mr Gething said they would be reviewed "after three weeks".
"I would be surprised if after three weeks, we're in a position where the whole of Wales could move out of level four, because our rates are extraordinarily high," he cautioned.
"There are over 600 cases per 100,000 across the whole of Wales, so we'll have to review where we are after three weeks, we'll make a choice.
"We'll be clear with the public about whatever that choice is, but I don't think people should plan on the whole country coming out of level four in three weeks' time."