According to the World Health Organization (WHO), a pandemic is defined as the “worldwide spread of a new disease.”12 When a new disease initially appears, most individuals lack the natural immunity to defend against it. This can cause a sudden, sometimes rapid, spread of the disease between people, across communities, and around the world. Without a natural immunity to fight off an illness, many people can become ill as it spreads. This is the case with Coronavirus Disease 2019 (COVID‑19) pandemic.
Pandemics are not automatically defined by their growth rate but rather by the spread of the disease. Understanding the growth rate of a pandemic can still help health officials prepare for an outbreak. The WHO is responsible for announcing the emergence of a new pandemic based on how the spread of the disease fits into the Pandemic Influenza Preparedness and Response.13
Phase 1. Viruses circulating among animal populations have not been shown to transmit to human beings. They are not considered a threat and there’s little risk of a pandemic.
Phase 2. A new animal virus circulating among animal populations has been shown to transmit to human beings. This new virus is considered a threat and signals the potential risk of a pandemic.
Phase 3. The animal virus has caused disease in a small cluster of human beings through animal to human transmission. However, human to human transmission is too low to cause community outbreaks. This means that the virus places humans at risk but is unlikely to cause a pandemic.
Phase 4. There has been human-to-human transmission of the new virus in considerable enough numbers to lead to community outbreaks. This kind of transmission among humans signals a high risk of a pandemic developing.
Phase 5. There has been transmission of the new virus in at least two countries within the WHO Region Trusted Source. Even though only two countries have been affected by the new virus at this point, a global pandemic is inevitable.
Phase 6. There has been transmission of the new virus in at least one additional country within the WHO region. This is known as the pandemic phase and signals that a global pandemic is currently occurring.
Many disease outbreaks follow a growth or spread pattern defined as exponential growth. This means they spread at a rapid rate over a specific period of days, weeks, or months.14 Past pandemics include (but are not limited to): 1) the 1918 flu pandemic (H1N1 virus) from 1918-1920, which took the lives of anywhere from 50 to 100 million people around the world,15 2) the 1957 flu pandemic (H2N2 virus), which occurred from 1957–1958 and took the lives of roughly 1.1 million people worldwide,16 and 3) and the 2002 SARS pandemic, which was associated with another coronavirus—SARS-CoV.17 The 2002 SARS coronavirus outbreak was a viral pneumonia epidemic that took the lives of over 770 people worldwide17.