Chapter 10 Install and System Administration for FortiOS 5.0 : FortiGuard : Antivirus and IPS : Detection during update
  
Detection during update
During an update, the FortiGate unit will continue to detect to scan network traffic. Sessions occurring right before an update will be scanned using the current signatures. Sessions that occur during the update, when the signature database is reloading, will be on hold until the signatures load, at which point the new signatures are used to scan these sessions. Sessions occur i ng right after the update will also use the new signatures.
See Also
Antivirus and IPS Options
Manual updates
Automatic updates
Support Contract and FortiGuard Subscription Services
FortiCloud
FortiGuard Services
Web filtering
Email filtering
Security tools
Troubleshooting