Approximately 183 million Americans, or 59% of the country’s population are thought to be immune to the H1N1 virus that caused a pandemic in 2009, according to estimates by scientists at the National Institute of Allergy and Infectious Diseases, part of the NIH (National Institutes of Health). They are also attempting to speculate where *pH1N1 will go from here by reviewing the fate of some that took place in history.

*H1N1 refers to the one in 2009. pH1N1 refers to the post-pandemic virus – including its possible future courses.

The pH1N1 is still circulating around the world, but at much lower levels than a year ago.

Many are thought to be immune to pH1N1 because they have either been exposed to it or received a vaccine. In order to remain in circulation with so many immune individuals about, the virus will need to adapt – this means change, either gradually or suddenly.

In the journal mBio, the authors explain what happened to previous pandemics and outline the paths their viruses took. Some fizzled out, while others returned with a vengeance, such as those of 1889 (Asiatic Flu) and 1918 (Spanish Flu) – scientists are uncertain why.

pH1N1 is unlikely to come back in a big way, the authors believe. Too many of us are immune – the numbers of immune people can only go up as more receive the 2010/2011 seasonal flu shots, which protect against the pH1N1 strain.

The scientists believe pH1N1 will mimic what happened after the 1968 pandemic (Hong Kong Flu) – it will stay around for a while, causing relatively few deaths.

However, they stress that there is much we don’t know about how viruses adapt to growing immunity amongst humans. Complacency might be a big mistake, they add.

Maintaining high levels of immunity among humans worldwide in order to keep the virus in check makes sense, the researchers believe. This means vaccinating everyone over the age of six months.

A pandemic is an outbreak of global proportions. It occurs when a new type of virus emerges among humans – it causes serious illness and is easily human transmissible (spreads easily from person-to-person).

A pandemic is different from an epidemic or seasonal outbreak.

  • Put simply, a pandemic covers a much wider geographical area, often the whole planet. A pandemic also infects a larger number of people than an epidemic. An epidemic is specific to one city, region or country, while a pandemic goes much further than national borders.
  • An epidemic is when the number of people who become infected rises well beyond what is expected within a country or a part of a country. When the infection takes place in several countries at the same time it then starts turning into a pandemic.
  • A pandemic is usually caused by a new virus strain or subtype – a virus humans either have no immunity against, or very little immunity. If immunity is low or non-existent the virus is much more likely to spread around the world if it becomes easily human transmissible.
  • In the case of influenza, seasonal outbreaks (epidemics) are generally caused by subtypes of a virus that is already circulating among people. Pandemics, on the other hand, are generally caused by novel subtypes – these subtypes have not circulated among people before. Pandemics can also be caused by viruses, in the case of influenza, that perhaps have not circulated among people for a very long time.
  • Pandemics generally cause much higher numbers of deaths than epidemics. The social disruption, economic loss, and general hardship caused by a pandemic are much higher than what an epidemic can cause.

The word pandemic comes from the Greek pandemos meaning “pertaining to all people”. The Greek word pan means “all” and the Greek word demos means “people”.

“The 2009 H1N1 Pandemic Influenza Virus: What Next?”
David M. Morens, Jeffery K. Taubenberger, Anthony S. Fauci
mBio vol. 1 no. 4 e00211-10. 28 September 2010
doi: 10.1128/mBio.00211-10

Written by Christian Nordqvist