What is a pandemic?

Q: What is a pandemic?

A: A "pandemic" was originally one of the World Health Organization (WHO)'s classifications for influenza. WHO categorized influenza into six levels, the highest of which was a "pandemic", which was defined as a community-level outbreak of an influenza virus in at least one country other than the place where the outbreak originated, indicating that the virus is spreading across countries.

Conceptually, the "pandemic" at the time referred to the scope of the influenza virus, not the severity and lethality of the epidemic. In 2009, the outbreak of H1N1 influenza from the United States and Mexico was classified as a "pandemic" by WHO, which stirred much controversy. Although this influenza has spread worldwide, its fatality rate is relatively low, which is inconsistent with people's understanding of the term.

Therefore, in 2010, the WHO website gave a simple definition of "pandemic", which is "a new disease spreading worldwide". For example, an influenza pandemic refers to the worldwide spread of a new influenza virus subtype, to which most people are not immune. The current novel coronavirus pneumonia (COVID-19) has become the first coronavirus infection to be known as a "pandemic."

At present, WHO no longer uses the above-mentioned six classifications to evaluate infectious diseases such as influenza, but uses four major stages instead. WHO spokesman Tarik Jasarevic said in an interview in late February that WHO no longer uses this old system, but people still use the term "pandemic" to describe the global spread of infectious diseases.

So "pandemic" is not a strict definition but a vague concept with operational flexibility. WHO Director-General Tedros Adhanom Ghebreyesus said on March 11 that describing the COVID-19 situation as a pandemic does not change the WHO's assessment of the threat posed by the novel coronavirus, nor does it change what the WHO is doing and countries should do.

Chris Whitty, chief medical officer for England and the Department of Health and Social Care's chief scientific adviser, said "this announcement by WHO reflects the changing global situation. There is ongoing transmission in multiple countries."