Table 1.

Similarities and differences between seasonal and pandemic influenza.

Seasonal influenza A and B	Pandemic influenza A
Occurrence
Annual (in temperate regions)	Four true pandemics in last 100 years Shortest inter-pandemic interval 11 years, longest interval 39 years
Predictability
Occurrence: predictable seasonality, but dominant antigenic type/subtypes vary Impact: difficult to predict until dominant type/subtype is known	Occurrence: difficult to predict when it will happen and what the subtype will be Impact: difficult to predict, although historical trend is for major impact, particularly for younger adults and children
Antigenic change
Antigenic drift (subtle changes in existing HA/NA)	Antigenic shift (major change in HA/NA resulting in new virus and subtype)
Immunity
Some naturally-acquired immunity is likely in adults, through previous infection and/or vaccination. Antigenic drift facilitates immune escape, leading to recurrent infections. Young unvaccinated children will lack immunity until infected or vaccinated	Specific antibody-mediated immunity is lacking and most of the population will not have significant cross-protective immunity from previous influenza infections The effect of T-cell mediated immunity is largely unknown but could potentially give some cross-reactive protection against severe disease (especially in the mucosa)
Risk groups for severe influenza
Elderly persons, infants, those with certain underlying health conditions (asthma, COPD, heart disease), obesity, pregnancy	As for seasonal influenza, but there may be over-representation of younger adults and children, and otherwise healthy individuals. Spread depends on absent or low herd immunity
Impact
Varies season-to-season WHO estimates between 3 and 5 million cases and 290,000 to 650,000 global annual deaths In wealthy countries, most deaths occur in those >65 years of age	Mortality varies between different pandemics and is difficult to predict in advance 1918 H1N1 pandemic believed to have caused at least 50 million deaths globally 2009 H1N1 pandemic is believed to have caused 250,000–500,000 deaths globally
Vaccines
Readily available in many countries before influenza season begins. Annual vaccine recommendations made for Northern and Southern hemispheres, dependent on predictive algorithms and epidemiology. Recently vaccine effectiveness poor in H3N2-dominated years	Strategic preparedness in some countries for viruses with pandemic potential e.g., avian influenza viruses Pandemic influenza viruses arise from diverse sources and are unpredictable Likely lag-time between a pandemic commencing and vaccine being available lessens the probability that vaccines will have a major impact
Antivirals
Predominantly neuraminidase inhibitors Other classes of antivirals are in development and may have additional impact alone or in combination	Sensitivity to existing antivirals cannot be guaranteed. Some countries stockpile existing antivirals as countermeasures, but demand may outstrip supply during a higher-impact pandemic. Resistant, highly transmissible pathogenic influenza variants could be devastating