Wales' chief medical officer has warned of a third wave in May if coronavirus restrictions are lifted too soon.
Doctor Frank Atherton said he agreed with the latest changes to the coronavirus restrictions in Wales which reflects a "cautious, incremental approach" but ministers should be "willing to intervene" if cases rise again.
Modelling by scientists has shown that increased mixing and the more infectious variants of the virus could see cases spike again by May and into June.
On Friday, the First Minister Mark Drakeford confirmed the 'stay home' rule would be replaced with 'stay local' from Saturday, March 13. Four people from two different households are also now able to meet in gardens while some outdoor sports facilities can reopen.
From March 15, hairdressers and barbers will be allowed to open their doors and all primary school pupils will return to school.
Looking ahead to March 22, non-essential retail will be allowed to reopen in a phased approach while self-contained accommodation will also be open in readiness for Easter.
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Mr Drakeford said these dates will depend on coronavirus rates remaining suppressed in Wales. He said: "We are trying to reopen Welsh society and the Welsh economy carefully, cautiously and step-by-step. This means you've got to do the least risky things first. You have to assess whether they're having any adverse effect on the circulation of the virus. Even staying local will mean there are more risks to people than when they had to stay at home."
He added: "We have to do it in stages with the safest things first and if we can make a success of that we can do more."
Dr Atherton said the lockdown up to now had been effective in driving down infection rates and the aim now should be to maintain community transmission at the lowest achievable levels.
The seven-day average case rate has fallen below 50 cases per 100,000 population for the first time since September and the positivity rate for testing is now less than 4.5%, he said.
Dr Atherton added: "Keeping the prevalence of Covid-19 low is an important preventative strategy both for the avoidance of direct harms and to guard against the emergence of new variants. The maintenance of low prevalence requires the slow release of lockdown restrictions; lifting them too early or too quickly will lead to an epidemic resurgence.
"Our modelling scenarios suggest that overly-rapid relaxation combined with increased transmissibility of the now dominant variant and low public adherence to restrictions could lead to a third wave of virus circulation in late spring (May/June).
"If we are unable to avoid this scenario then it is likely that, despite the success of our vaccination programme, we would see a return in Wales to a period of high viral transmission with increased hospitalisations and deaths."
Restricting international travel is key to importing infection, especially new variants of the disease, into the UK, he added. He warned that continued restrictions on international travel may be warranted if case rates begin to increase again.
He also said legislation should be used to ensure that indoor mixing between households does not increase.
"The importance of public adherence to this restriction cannot be overemphasised," he added.
"Our modelling shows household mixing to be the single greatest determinant of whether we are likely to experience a substantive third wave. Implementing the personal, procedural, engineering and societal mitigations to reduce transmission of SARS-CoV-2 virus remains a vital part of our approach, but the new variant, being more transmissible, requires a step change in the rigour of application given the increased risk of spread.
"Most importantly, our public messaging should explain why continued restrictions on meeting up with family and friends inside our homes, caravan parks and holiday lets, continue to be necessary."
"Finally, we should continue to carefully monitor the impact of the proposed easement of restrictions and be willing to intervene again if we see evidence of increasing transmission."
The Welsh Government will review the latest restrictions on April 2. If there have been dramatic changes in the rates of infection or other indicators like hospital admission they may tweak future plans or reverse restrictions that have been lifted.
