Influenza Pandemics of the 20th Century

During the 20th century, three influenza pandemics occurred in succession. In 1918–1919, the so-called “Spanish flu” pandemic caused by the A(H1N1) virus affected the entire world. Estimates available on the World Health Organization (WHO) website indicate that at least 40 million people died as a result. The subsequent pandemics were much less severe: in 1957–58, the “Asian flu” linked to the A(H2N2) virus, and in 1968–69, the “Hong Kong flu” caused by the A(H3N2) virus.
In 2009, a new pandemic occurred, caused by a new A(H1N1) pdm09 virus resulting from a combination of various influenza viruses of avian, swine, and human origin.

A Regular Occurrence

These pandemics are part of the normal cycle of influenza virus circulation in humans. They begin when a new influenza virus is capable of infecting humans and is easily transmitted from person to person, because the population has little or no protection against this virus.
This virus generally replaces one of the viruses previously responsible for seasonal flu outbreaks during winter, and it continues to do so until the next pandemic. This is how the A(H1N1) virus, responsible for the 1918 pandemic, circulated among humans until 1957–1958, when it was replaced by the A(H2N2) virus during the Asian flu.
For 10 years, the latter caused seasonal flu epidemics, until the 1968 pandemic when the A(H3N2) virus emerged and took its place. Since the last pandemic in 2009, the A(H1N1) pdm09 virus has completely replaced the A(H1N1) virus that had been co-circulating with the A(H3N2) virus since 1977.

Variable scope and severity depending on the pandemic

Influenza viruses, whether seasonal or pandemic, generally have specific characteristics that distinguish them from one another. The profile of those most affected is not always the same as for seasonal influenza. For example, pregnant women were particularly at risk of severe illness during previous influenza pandemics. The Spanish flu was notably characterized by its high severity among young adults.
Similarly, the scale and severity can vary considerably from one pandemic to another. Thus, the Spanish flu claimed millions of lives, while the 2009 pandemic was, in France, on a scale comparable to that of seasonal flu.

Control measures are outlined in the national pandemic response plan, which was updated in 2011 and is available on the website of the Ministry of Health.

Pandemic surveillance builds upon routine surveillance, to which specific tools may be added. For the 2009 pandemic, surveillance of severe cases admitted to intensive care units was implemented, along with a special system to track flu-related deaths in real time.