Professor Linda Bauld was speaking as the latest figures showed the number of Scots believed to have Covid increased to one in 30 last week – up from one in 40 the previous week.
The Office for National Statistics estimates in the week ending June 11, 176,900 people in Scotland had the virus – about 3.36 per cent of the population.
The estimated Covid rate in Scotland is higher than in England, where it is believed one in 50 people had coronavirus in the week ending June 11. One in 45 people are estimated to have had the virus in both Wales and Northern Ireland.
Prof Bauld said the Omicron sub-lineages BA.4 and BA.5. were part of the reason for the increase in cases and a “small, but not significant” rise in the number of people in hospital.
However, she said vaccines were working well in the vast majority of people and were making a difference between the risk from infection and the risk of severe disease.
Professor Bauld, a professor of Public Health at the University of Edinburgh, told BBC Radio Scotland’s Good Morning Scotland programme: “I think it’s pretty clear to me from some of the data we’ve seen that we are certainly in another wave of infection, if not already it will be soon and that certainly seems to be the case in a number of countries. And that is not a big surprise because we did anticipate we would have a rising level of infections every three or four months potentially, that’s what international colleagues have said.
“It might be surprising to people though because it’s the summer, and they’re used to hearing from us the weather is better so people are more outside and therefore we reduce the risks through our behaviour.
“That’s true, but what we’ve got is Omicron and a different type of it, BA.4 and 5 – there’s these Omicron sub-lineages which seem to be rising in number in the UK and I think that’s part of the explanation why we’re beginning to see more cases, and we have seen a small, but not significant rise in the number of people in hospital as well.”
Prof Bauld said vaccines offered good protection against severe disease, but she thought it likely the offer of a booster jab may be extended to a wider range of groups in the autumn, beyond what the Joint Committee on Vaccination and Immunisation (JCVI) has recommended.
The JCVI is recommending care home residents and staff, frontline care workers, those aged 65 and over, and adults aged between 16 and 64 who are clinically vulnerable should be given another dose of Covid-19 vaccine in the autumn.
Prof Bauld said: “Your protection doesn’t fall off a cliff, it wanes gradually. Your protection against getting infected actually wanes pretty quickly and the vaccines are not sterilising, so they are not protecting us against the risk of infection.
“Your risk of severe disease … I think even beyond months after your dose, you may not be having such a robust antibody response, but other bits of the immune system, B and T cells, are still working against getting very unwell.”