Fig. 1
From: Animal models for the risk assessment of viral pandemic potential
Timeline of influenza pandemics. Each pandemic event is marked by a bold arrowhead on the timeline, which is not to the scale. The open arrowhead designates the ‘aborted pandemic’. The animal symbols designate the host origins of the HAs of the pandemic strains. Lines with the diamond heads designate the duration and the end of the circulation. The arrowed lines designate being circulated currently. The dotted lines designate unknown