A second wave of coronavirus during the winter period ''would be much more sustained'' with the peak lasting longer, NHS Wales boss Dr Andrew Goodall has said.
Speaking to the Senedd's Health, Social Care and Sport Committee, Dr Goodall said a second wave could see infection levels rising higher than earlier in the year.
He said: ''We absolutely need to assume that there will be a second wave and we should expect that peak to be similar, if not worse, than our experience in March and April.
''Irrespective of the peak happening, although we would hope to mitigate it, it would probably be much more sustained during the winter period at a higher level because in winter the virus will act in a different manner.''
Dr Goodall said officials are studying the progress of the outbreak internationally, most notably in Melbourne where the city has had to go into another lockdown.
He said further lockdowns in Wales could be a possibility.
''In the cold with more people indoors the transmission is likely to be higher,'' he told the committee.
''Whether it is local lockdowns, or local outbreaks, this will translate into something much more significant over the winter months I'm afraid.''
Today, the chair of the Senedd's Health Committee said the warning of a second wave was "pretty stark" and warned that we must remain better prepared.
Dr Dai Lloyd told ITV News: "An overstretched service in February, March, was absolutely flawed, I think, at the start, with the lockdown.
"We didn't have enough PPE; testing. Everything to do with care homes and hospital services was in temporary chaos and the panic was rampant along hospitals wards and in care homes.
"We certainly are better prepared now and we need to make sure that we're good to go."
Dr Dai Lloyd interview on Wales at Six:
It comes as the Welsh Government announce that those who are shielding, thought to be 130,00 people, will no longer have to shield themselves from 16 August.
People most at risk were initially asked not to leave the house at all for a period of 12 weeks - and then from 1 June, people were told they can leave their homes to meet people from another household outdoors, as long as strictly follow to social distancing rules and good hygiene measures.