| A pandemic virus strain was first detected in North America |
Emergence in a less developed setting |
| Immediate virus sharing following virus isolation to promote rapid development and deployment of available diagnostics and vaccine candidates |
Delayed virus sharing |
| Apparent lower global impact in terms of disease severity on an overall population basis (i.e., all ages) compared to other pandemics (e.g., 1918 H1N1 virus), and to what was feared (e.g., HPAI H5N1 virus) |
A more pathogenic pandemic virus, or rapid evolution of more virulent circulating strains |
| Residual immunity among a proportion of the population (i.e., older adults) |
Total population susceptibility |
| Circulating pandemic virus strains susceptible to neuraminidase inhibitor antivirals stockpiled for pandemic planning and available in some countries |
Widespread neuraminidase inhibitor resistance in circulating strains; no antiviral medications available |
| Rapid dissemination of epidemiological, clinical, and virological data from North America and the Southern Hemisphere to inform the global community |
Poor or incomplete data, or lack of transparency and information sharing |
| Emergence during the end of a seasonal influenza epidemic |
Emergence during the peak of a seasonal influenza epidemic to complicate disease and virus identification and increase pressures on health services |
| Mild uncomplicated illness in most people with pandemic influenza virus infection |
A more pathogenic virus causing a high frequency of severe complications |
| Basic reproductive number (R
o) on a similar scale to seasonal influenza virus transmission |
A higher R
o
|
| Modes of transmission similar to seasonal influenza A and B virus spread |
Different modes of transmission (e.g., substantial contact/fomites transmission, conjunctival/ocular infection) |
| A vaccine that is highly immunogenic usually requiring only a single injection |
Poor immunogenicity and requiring multiple injections |
| A vaccine that appears to be safe and with similar safety profile to seasonal influenza vaccine, with very low frequency of severe complications |
Frequent and severe vaccine-associated adverse events |
