As few as 2-3% of people in Wales have had coronavirus and developed antibodies, a study suggests.
Analysis of 1,006 adult samples from the Welsh Blood Service found the likely number of people with antibodies is 2.6%. Figures from Public Health England indicate that between 13% and 17.4% of people in London have antibodies for Covid-19.
A summary of advice from the Welsh Government's tactical advisory cell (TAC) said further sampling was needed in Wales. The document was published ahead of changes to the country's lockdown measures, which come into effect on Monday.
From then, people from two households will be able to meet outdoors as long as they do not travel more than five miles and observe social distancing.
"There is some data to show that around 2-3% of people living in Wales have had coronavirus and developed antibodies," the TAC document said. "This is based on a small sample and we need more people to offer samples through the Welsh Blood Service and wider NHS in Wales in order to get a better picture.
"There is evidence to show that as many as 35% of people who get Covid-19 don't notice any symptoms at all.
"They will still be infectious, and this means that hand washing and social distancing will continue to be very important to break chains of infection."
It said being outdoors in the sunshine carried a "very low risk" of becoming infected, as long as people stay two metres apart.
Evidence shows that the virus lives for only a few minutes in direct sunlight, though it can survive for many hours on surfaces indoors.
The TAC said children seem to have coronavirus "as much" as adults and can still be infectious, but generally suffer fewer symptoms
Addressing the issue of indoor and outdoor contact with people, it warned: "Significant changes should not be made to social distancing measures unless a robust contact tracing system is in place."
