Simultaneous global transmission of influenza is sufficient to define an influenza pandemic and is consistent with the traditional definition of a “global epidemic”. There will then be ample opportunity to describe in more detail the potential range of influenza pandemics in terms of transmissibility and severity of the disease. The new evidence for the A(H1N1) virus is that communicability, estimated by the effective reproduction number (R or average number of people infected by a single infectious person), was between 1.2 and 1.3 for the general population, but was about 1.5 in children (Kathryn Glass, Australian National University, personal communication). Some initial R estimates for pandemic (H1N1) 2009 may have been overestimated.3 However, as societies evolved, disease patterns and scientific understanding of disease spread also evolved. The Industrial Revolution brought millions of people to urban centers, while clippers and steam locomotives scattered more and more people far and even around the world. The cholera pandemic of 1831-1832 was the first time that the global spread of an infectious disease was widely described day after day in the popular press for more than a year, as it inexorably progressed from Asia to Europe via travel and trade routes. The discovery of the microbial causes of diseases has led to the development of vaccines and antisera against them, as well as widespread diagnostic tests to study and monitor diseases at their source. Under the umbrella of epidemics, the idea of a pandemic took shape before a specific meaning of the term languishing was associated with it. When the 1889 flu pandemic appeared, the concept of a pandemic already existed. The previously vague, imprecise and rarely used term was saved from obscurity for some reason – perhaps due to the remarkable explosiveness of influenza and the close monitoring of its rapid global spread in 1889 [11] – and associated with the remarkable global occurrence of influenza. Soon after, the term pandemic became common; By 1918, it had become a virtually common word. How is pandemic risk affected by regional differences in transmission? Although pandemics have often been described in partially immunized populations (e.g., evidence of a modest level of protection in individuals >60 years of age in the 1918 influenza pandemic [21]), it is clear that population immunity can be a powerful anti-pandemic force in limiting infections and microbial transmissions. However, immunity is a relative concept that does not necessarily imply complete protection against infection [22], as is the case with pandemics as diverse as cholera and influenza associated with new subtypes or drifting strains [1].
To illustrate this concept, let`s look at the contrasting examples of Ebola and COVID-19. The Ebola outbreak in 2014 followed the pattern of high incidence in low-income countries and low incidence in high-income countries. The virus has spread to several low-income African countries, but was effectively contained when introduced into high-income countries.55,56,57 In this case, high-income countries had the capacity to prevent a pandemic and were able to isolate and treat symptomatic people quickly. This resulted in high heterogeneity in transmission, which corresponds to the right-hand side of Figure 6a, with low-income countries at high risk and high-income countries at low risk. In contrast, high-income countries have not been able to escape the COVID-19 pandemic, in part because of asymptomatic and presymptomatic transmission of SARS-CoV-2, which allows them to escape public health surveillance and interventions.58,59 This has led to more similar transmission rates between different countries, according to the left side of Figure 6a, where the risk is more uniform across regions and therefore between definitions of the pandemic. There have been many flu pandemics in the past. Pandemic influenza often originates from animal influenza viruses and is not the same as seasonal influenza. Few, if any, people will be immune to a pandemic influenza virus, even if they have had seasonal flu or a seasonal flu vaccine. “A pandemic is fundamentally a global epidemic — an epidemic that spreads to more than one continent,” says Dan Epstein, spokesman for the Pan American Health Organization, a regional office of the World Health Organization. An epidemic is an epidemic of diseases that spread rapidly and affect many people at the same time. A pandemic is a type of epidemic: an epidemic that has spread over a wider geographical area than an epidemic and has affected a significant part of the population.
Graphs of pandemic probability versus inter-regional travel rates for a range of pandemic definitions. The difference in probability of different definitions of the pandemic changes as travel prices rise. (a) Representation of pandemic probability for a star array. (b) Presentation of the pandemic probability for a complete network. For a fully connected network, all regions are adjacent, so no lines are displayed for the interregional definition, which requires non-neighboring regions to experience outbreaks.
