See SAGE minutes:
17th Dec govt was told tier 3 was ineffective
Govt took legal action - to prevent schools closing (!)
22nd Dec strengthening of measures advised.
Most of country left in tier 2 or 3 over Christmas.
4th Jan Schools returned
5th January govt finally followed SAGE advice.
on 17th Dec, cases (7 day average) were 23,000
On 5th Jan, 56,000.
Keeping the country open over the period from being clearly advised their measures were ineffective to taking action - Christmas - caused cases to more than double.
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