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For added privacy, WhatsApp users can now send videos and photos that disappear from your WhatsApp chat immediately after the recipient must have opened them once.

The Media will not be saved to the recipient’s Gallery or Photos.
Once you send a view once photo or video, you will not be able to view it again.
There is no way you can forward, save, star, or share photos or videos that were sent or received with view once media feature enabled.
You may only see if a recipient has opened a view once photo or video if they have read receipts turned on.
If you do not open the photo or video after 14 days of it being sent, the media will expire from the chat and you may not view them again.
You must select view once media each time you want to send a view once photo or video.
View once media can be restored from your backup if the message is unopened at the time of the back up. If the photo or video has already been opened, the media will not be included in the backup and as such can’t be restored.
Note:

Only send your photos or videos with view once media enabled to trusted individuals. Lets take for example, it’s possible for someone to:
•Take a screenshot or screen recording of the media before it disappears. You won’t be notified if someone takes such action.