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updated December 07, 2010

H1N1 flu (swine flu): What's happening now

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    James Steckelberg, M.D.

    The 2009 H1N1 flu pandemic is over, but the H1N1 virus continues to circulate and infect people who don't have immunity — a group that includes anyone who hasn't caught the virus or been vaccinated against it. In this interview, James Steckelberg, M.D., an infectious disease specialist at Mayo Clinic, answers questions about the 2009 virus and its likely role in upcoming flu seasons.

    What makes one flu virus different from another?

    There are two main types of flu — influenza A and influenza B. Influenza A is more common and generally more serious than influenza B.

    Influenza A viruses come in assorted strains, each named for the place and date of its identification. Formally, the 2009 pandemic flu virus is named A/California/7/2009 (H1N1), because its first documented appearance was in residents of California during 2009. The number 7 tells which laboratory identified the virus. H1N1 indicates the viral serotype; it's a kind of shorthand for characteristics that allow the virus to enter your cells.

    Flu viruses evolve quickly, which is the reason viruses of the same serotype, such as H1N1, aren't all the same.

    Is it possible to catch 2009 H1N1 from pigs?

    No. The 2009 pandemic virus, now considered a seasonal flu virus, is a human-adapted virus that does not spread among pigs. People occasionally catch swine viruses when they have close contact with pigs, but someone infected with a swine virus typically can't spread it to other people.

    What is the connection between 2009 H1N1 flu and swine flu?

    The 2009 pandemic flu is a human infection. Swine flu is an infection in pigs. The distinction may be confusing because other H1N1 viruses also occur in both species.

    The swine H1N1 virus and the 2009 human H1N1 virus share a notorious ancestor — the virus that caused a devastating global pandemic in 1918 and 1919. That virus infected humans and pigs at the same time, but it quickly made separate adaptations to each species. The human-adapted virus was deadlier than its swine counterpart.

    In pigs, a direct descendant of the 1918 swine flu virus still causes a disease called classical swine flu. In humans, the 1918 H1N1 virus caused seasonal flu epidemics until 1957, when a new pandemic flu virus temporarily pushed the H1N1 virus out of the picture. In 1968, another pandemic virus — H3N2 — emerged, replacing the 1957 pandemic virus. Then, in the late 1970s, human H1N1 returned — fortunately, without the lethal consequences it had in 1918. For the next 41 years, the revived H1N1 and the 1968 H3N2 viruses were relatively stable, both surviving as seasonal flu viruses.

    Among bird flu viruses, however, genetic stability is impossible. An unknown number of unique flu viruses are continuously mixing their genes and creating new viruses within hundreds of bird species. A few of these new viruses will be capable of infecting pigs, and fewer still may be able to infect people.

    Once it infects a pig, a bird flu virus may cross with a swine flu virus. It's also possible for bird and human-adapted flu viruses to meet up in swine. All this mingling of viruses adds variety to the flu virus gene pool in swine — and the more variety there is, the greater the chance that some new combination of genes will cross into humans and become the next pandemic virus. A similar process, most likely, created the 2009 pandemic virus.

    Why was H1N1 flu considered a serious threat in 2009?

    Any outbreak of a flu-like illness outside the normal flu season should be taken seriously. If the cause turns out to be a completely new flu virus — as it did in 2009 — virtually everyone is at high risk of infection; you can't be immune unless you've been vaccinated or exposed to the virus before. Also, past experience with new flu viruses has shown that they cause more severe illness and death than do recurring seasonal flu viruses — sometimes, substantially more, as was the case in 1918.

    So a red flag appeared in April 2009, when two children from neighboring California counties developed a flu-like illness well after the peak of the flu season. By the time experts at the Centers for Disease Control and Prevention (CDC) determined that the cause was a previously unknown — "novel" — influenza virus, the infection had already spread from its apparent site of origin in southern Mexico to several areas in the United States and Canada. It took only a few weeks to appear on every continent.

    Did all the announcements about the 2009 pandemic cause unnecessary public alarm?

    That's impossible to know. The effects of the pandemic fortunately fell far short of the worst-case projection that the death toll would be higher than that of the great 1918 pandemic. Research may eventually reveal the relative impact and lead to precise methods of predicting the course of the next flu pandemic.

    Also, although the 2009-2010 flu season wasn't catastrophic, in the United States, it lasted nearly twice as long as flu seasons in the previous several years and resulted in four to five times as many children's deaths. Overall, it confirmed the unpredictability of influenza A viruses as they evolve under the ever-changing constraints of host evolution and human civilization.

    If I get the seasonal flu shot in the fall, will I be safe from H1N1 flu?

    Yes. The seasonal flu shot targets three influenza viruses from previous years' flu epidemics. The targeted viruses change each year, based on recommendations from virologists, immunologists and public health experts. The 2009 H1N1 flu virus is one of the three targets of the 2010-2011 seasonal flu shot.

    What else can I do to protect myself, my family and my co-workers?

    Besides getting vaccinated, you can:

    • Wash your hands with soap and water frequently. Alcohol sanitizers also may be helpful.
    • Avoid close contact with anyone who has cold or flu symptoms.
    • Stay home if you're ill, and encourage others to do the same.
    ©1998-2013 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
    Read this article on Mayoclinic.com.


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