An out-of-office reply is automated and can be sent for various reasons that wouldn’t count as a bounce or a reply. A bounce occurs when the email couldn’t be delivered for whatever reason – an out-of-office is actually a confirmation that someone has received the email; they just haven't been able to read it.
Keep tabs on these auto-replies and take note of when they occur most often. An out-of-office isn’t a stop sign, though! In fact, it would be a good idea to channel these replies into a follow-up flow, allowing you a second (and third) chance of reaching someone who hasn’t seen your email yet and is likely returning to an overflowing inbox after their mid-year break.