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What's the Difference Between H1N1 and Seasonal Flu?

New York, NY (Oct 28, 2009)

Road sign for a hospital

H1N1 or swine flu is in the air these days, both literally and figuratively. New stories about the controversial H1N1 vaccine, how the virus is affecting interactions at schools, businesses, and social settings, and how it is being tracked are appearing on the internet, cable news, and in newspapers every few hours. But amidst all the noise, it's hard to find the answer to one simple question: is this virus any more dangerous or deadlier than its cousin, the seasonal flu?

Infectious disease specialists have been gathering information for many months in an effort to pin down some key features of this flu and to predict how it will act in the future. So far H1N1 appears to make people feel sick in a very similar way to the seasonal flu but it is affecting some people more severely than others.

Most People Lack Immunity

"The influenza that spread rapidly across the globe this past year resulted from the emergence of a flu strain that most humans alive today have never been exposed to," said Anne Moscona, MD, a Professor of Microbiology and Immunology, Professor of Pediatrics, and Vice Chair for Research of Pediatrics at Weill Cornell Medical College, and an Attending Pediatrician at NewYork-Presbyterian Hospital/Weill Cornell Medical Center. And the novelty of the virus suggests that a large number of people – as many as 70 percent of Americans – could come down with this flu because they lack any immunity to it.

The swine flu virus also contains a key protein that is genetically similar to the unusually deadly flu that swept across the globe in 1918, Dr. Moscona notes. So far H1N1 appears to have relatively low virulence, a measure of the severity of the disease a virus produces. "But viruses are constantly evolving," she said, "and will evolve in a certain direction if it is to their advantage to become more transmissible or better able to cause disease in humans. There could very well be selective pressure on this virus that causes it to change or mutate, but not necessarily in a direction of greater virulence. You can't really predict that."

Differences Between H1N1 and Seasonal Flu

Over the past months some clear differences between seasonal and swine flu have emerged: the most dramatic difference is that the confirmed cases of H1N1 are concentrated in people under 24, and that 83 percent of the deaths and 71 percent of hospitalizations are in those between 5 and 64.

Anne Moscona, MD
Anne Moscona, MD
With seasonal flu 90 percent of the deaths occur in people over 65. "While there are several theories about why younger people are more severely affected by H1N1, the truth is that in this case, we really don't know," Dr. Moscona said.

Unlike seasonal flu, swine flu can be passed from person to person in warm weather and cold weather. Seasonal flu, which peaks during late December through early March, is most efficiently passed from person to person through droplets in the air when the weather is cold. "When it warms up in the spring, transmission stops," said Dr. Moscona. Infectious disease specialists hypothesize that H1N1 can spread easily through direct contact during warm and cold weather.

Another significant way that the viruses differ is that in a third of the cases that have come to medical attention H1N1 affects the gastrointestinal tract as well as the respiratory system. "That's just something that we don't see in seasonal flu, which is really a respiratory virus," Dr. Moscona said.

Pregnant women are dramatically more affected by H1N1, and eight percent of all the deaths of this virus have been in pregnant women, who make up only one percent of the population. "They've been really disproportionately infected by it, and we do not know why that is," said Dr. Moscona. Another group with an elevated risk of severe outcome from swine flu is those with neurological disorders. "People with neurocognitive, neuromuscular disorders, seizure disorders really have an elevated risk not of getting sick to begin with but of having a severe outcome," mentions Dr. Moscona. A third of the fatal cases to date have been in people with neurological disorders. "People with underlying conditions such as these should be very concerned and should be vaccinated," she said.

Health-Care Response

Seasonal flu hasn't really started circulating yet so people who are coming down with the flu now are infected with H1N1. But when H1N1 and seasonal flu start to co-circulate this winter there is likely to be a big effect on the health-care system, notes Dr. Moscona. "There's been a lot of planning and preparation for the peak flu, and work to develop surge capacity. If we get the vaccine out, especially to those in really high-risk groups, and then to everybody else, we may be able to really lessen the H1N1's impact," she said.

For more information on what to do if you or someone you know feels sick, symptoms to look for, what to not worry about, the right numbers to call, and the right places to go for care, visit the Centers for Disease Control's flu web site.

Contributing faculty for this article:

Anne Moscona, MD, is a Professor of Microbiology and Immunology, Professor of Pediatrics, and Vice Chair for Research of Pediatrics at Weill Cornell Medical College, and an Attending Pediatrician at NewYork-Presbyterian Hospital/Weill Cornell Medical Center.

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