As the weather changes, I want to continue walking outdoors. When it rains, what is the safest way to keep the pram dry? I am concerned that many rain covers are PVC which I would imagine would increase heat and reduce airflow. My daughter is currently only 3 months old. Kind regards, Fiona

You are correct, there is concern for adequate ventilation & increasing temperature for a baby in a pram that has any type of covering.
While this article linked below does not apply to rain covers, it may have some useful tips.
https://rednose.org.au/article/the-dangers-of-covering-your-pram

I am not aware of any research that has looked at any ways of covering a pram to keep baby dry when out in the rain, other than what is available.
Maybe the company that manufacturers the pram you are using can provide other information about waterproofing.