Identifying Read status of emails in Exchange Online / Office 365

You cannot get this information from the message trace logs, they only cover routing details. If you need the total numbers of read/unread messages for a period, you can find them in the Usage reports in the O365 Admin portal. If you need those details on a per-message basis, you will have to use some EWS based script (example: https://blogs.technet.microsoft.com/santhse/get-readstatus/) or resort to other workarounds such as using the Search-Mailbox cmdlets with the -LogLevel switch.