Less than you might think is the simple answer.
Decision-makers being on holiday mean hiring requirements take a bit longer to get approved; interview processes take longer to be completed; decisions on offers get held up a bit.
But the main drivers for firms needing to hire (1. replacing lost staff and 2. plugging gaps in practices where new projects have been won) aren't impacted that much by the summer holidays and so nor therefore is hiring activity.
Hope this helps
Tony Restell
Top-Consultant.com