Dear Vaccine Smarts,

I am hoping that you can clarify the terms pandemic flu, seasonal flu, swine flu and avian flu.

From what I have read in the media, it appears that the pandemic H1N1 flu that was known as the swine flu is now the seasonal flu. Is that right?


Tiki Island

Dear Ana,

The terms surrounding influenza can be confusing but we will try to explain them.

Seasonal flu is the term used to refer to the flu outbreaks that occur yearly, beginning in the fall, running through the winter and ending in the spring.

No one knows exactly why flu outbreaks occur in the cooler months. It has been long believed that it is because people tend to congregate together indoors making it easier for sneezing and coughing to spread the virus.

Recently, researchers have found evidence that it is the temperature and humidity level of the environment that makes the virus more contagious and not just the crowding indoors.

Pandemic flu refers to particularly contagious strains that spread rapidly from person to person to create a worldwide epidemic.

Part of the reason that a pandemic flu spreads quickly is that it is a new strain for which the population doesn’t have any immunity.

Most people who come into contact with the virus become infected then spread it to others.

After being infected, people develop immunity. It becomes more difficult for the pandemic flu to continue to move through the population as the number of people with immunity rises.

As this occurs, the pandemic flu then becomes dependent on the seasonal factors associated with cooler weather that encourage spread. The pandemic flu then becomes the seasonal flu.

That is exactly what happened with the 2009 pandemic flu. It was a new flu strain that first appeared in the spring of 2009 and quickly spread throughout the world, aided by modern transportation.

It spread throughout the summer months in the Northern Hemisphere and caused a large outbreak in the fall, winter and spring.

The pandemic was declared over the summer of 2010 but the 2009 pandemic flu strain continues to circulate as one of the common strains of seasonal flu.

Animals have their own strains of flu. Flu in birds is called avian flu and flu in pigs is known as swine flu.

These flu strains on very rare occasions infect people who are in close contact with infected animals. The infected person may get very sick and even die.

A virus that comes from animals is likely to cause a pandemic if it develops the ability to spread easily from person to person because people will not have any immunity to it.

The 2009 pandemic flu came from pigs and that is why it was first known as the “swine flu.”

It was re-branded as the Novel H1N1 flu because of religious and commercial concerns surrounding pork.

Most of the severe flu cases including those that have caused deaths this season have been caused by the 2009 pandemic strain.

This strain is covered in flu vaccines. Protect yourself and your family. It isn’t too late to get vaccinated.

Dr. Richard Rupp is a pediatrician and member of UTMB’s Sealy Center for Vaccine Development. Bridget Hawkins, Ph.D. is the assistant director of the SCVD. This column is supported by a UTMB President’s Cabinet Award to provide information about vaccines. Visit or like us on Facebook.

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