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- H1N1 flu (swine flu): What's happening now
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H1N1 flu (swine flu): What's happening now
By James M. 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.
Next page(1 of 2)
- World Health Organization. WHO recommendations for the post-pandemic period. http://www.who.int/csr/disease/swineflu/notes/briefing_20100810/en/index.html. Accessed Sept. 9, 2010.
- Key facts about influenza (flu) and flu vaccine. Centers for Disease Control and Prevention. http://www.cdc.gov/flu/keyfacts.htm. Accessed Aug. 2, 2010.
- Morens DM, et al. The persistent legacy of the 1918 influenza virus. New England Journal of Medicine. 2009;361:225.
- Zimmer SM, et al. Historical perspective — Emergence of influenza A (H1N1) viruses. New England Journal of medicine. 2010;361:279. http://www.nejm.org/doi/full/10.1056/NEJMra0904322. Accessed Aug. 25, 2010.
- Morens DM, et al. The 2009 H1N1 pandemic influenza virus: What next? mBio. 2010;1:e00211. http://mbio.asm.org/content/1/4/e00211-10.full.html. Accessed Sept. 30, 2010.
- The 2009 H1N1 Pandemic: Summary Highlights, April 2009 — April 2010. http://www.cdc.gov/h1n1flu/cdcresponse.htm. Accessed Sept. 30, 2010.
- Fiore AF, et al. Prevention and control of influenza with vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Early Release. 2010;59:1. http://www.cdc.gov/mmwr/pdf/rr/rr59e0729.pdf. Accessed Aug. 2, 2010.
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