Professor Linda Bauld, chair of public health at Edinburgh University, said mass events like the climate conference are still “risky” despite declining rates of Covid-19.
Up to 30,000 people are expected to travel to Glasgow for the summit in November.
Activists are expected to stay in the homes of people who live in and around the city.
There was a rise in Covid-19 cases in Cornwall in June, though the Government denied the G7 meeting there was to blame.
Professor Bauld told Times Radio: “Yes, I am worried about a Glaswegian spike, and I’m not worried just about Glasgow. The central belt is interconnected.
“And we’re seeing declines here but we may not be able to hang on to those gains.
“We’re trying to open up and everything’s trying to operate more normally.
“Everybody’s delighted to be able to go to the cinema or a concert or have more people in their homes.
“This is hard-won progress thanks to vaccines and other things.
“But having huge mass events at the moment, I think it is risky.”
The conference has set out its coronavirus safety measures for delegates, including daily lateral flow tests, but those attending will not be required to use Scotland’s vaccine passport system.
A Scottish Government spokesman told the Times: “Covid-19 continues to be closely monitored by all relevant agencies and we are working with the UK Government, Public Health Scotland and our partners in Glasgow to achieve our priority of a safe and secure Cop26.
“As part of the code of conduct, delegates will be asked to adhere to enhanced Covid measures in the Cop26 Blue Zone, such as one-metre physical distancing and the wearing of face masks unless seated.
“All other national and local restrictions will continue to apply outside the Blue Zone.”
Today it was announced a ‘climate train’ with about 500 travellers from all over Europe will head from Amsterdam to Glasgow on October 30th, under the banner ‘Rail to the COP.’