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H1N1 Differs From 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. Amidst all the noise a central question is lost: is H1N1 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.

Little Immunity Among the Population

As with all pandemics, the rapid spread of swine flu across the globe resulted from the emergence of an influenza strain that is "antigenically new" to much of the population: most of the world's population is immunologically naive, suggesting that infection rates could reach as high as 70 percent. In addition, the H1, the hemagglutinin of this virus, is genetically similar to the unusually deadly 1918 flu, raising concerns about its potential virulence, notes 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.

So Far, Less Virulent In People Than Expected

"Despite animal data suggesting that H1N1 is more virulent than seasonal flu, so far it appears to have relatively low virulence in people," Dr. Moscona said. Some researchers hypothesize that H1N1 does not express the polymerase protein PB2-F2, which was present in the viruses that caused the pandemics of 1918, 1957, and 1968, but this theory has recently been challenged by another analysis. "It's very interesting but difficult to figure out what genetic features of a virus are associated with its virulence," she said. "For this one, we don't really know yet."

Anne Moscona, MD
Anne Moscona, MD

To date H1N1's case-fatality ratio seems to be about the same as that of seasonal flu (~0.1 to 0.3 percent of medically attended cases, ~0.05 to 0.2 percent of all symptomatic cases). "However, the ratio is based on data from when the swine flu first appeared," Dr. Moscona said, "so we're just not really sure yet. The number of deaths from 2009-H1N1 may be substantially higher, but so far it does not look like it's a particularly more virulent virus."

Whether H1N1 will evolve and become more virulent is also unknown. "Viruses are constantly evolving, and will evolve in a certain direction if there is a selective pressure for features that become more transmissible or better adapted to causing disease in humans," she said. "Depending on the selective pressure on this virus, there could be mutations, but not necessarily in a direction of greater virulence. You can't really predict that."

Differences Between H1N1 and Seasonal Flu

Some differences between the two types of flu are clear: most dramatically, the confirmed cases of H1N1 are concentrated in people under 24, and 83 percent of the deaths and 71 percent of hospitalizations are in those between 5 and 64. With seasonal flu 90 percent of the deaths occur in people over 65. "The prevailing but controversial explanation for why younger people are more severely affected is that people over 65 have some immunity based on exposure to a similar virus during pandemics like the one in 1957. Another is that younger people generate more vigorous immune response, which is in itself detrimental. The truth is that in this case, we really don't know," she said.

Unlike seasonal flu, H1N1 is transmissible in both warm and cold weather, while seasonal flu is transmissible mainly through aerosol means and is thought to be really sensitive to warmth – it can only really occur when it's cold, between late December and early March in the US, Dr. Moscona noted.

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

Groups At Elevated Risk

Pregnant women are dramatically more affected by H1N1, and eight percent of all the deaths from this virus have been in pregnant women, who make up only one percent of the population, so they've been really disproportionately infected by it. "People with underlying conditions such as pregnancy should definitely be vaccinated because they are more vulnerable to disease," said Dr. Moscona.

Another underlying condition that comes with a real elevated risk of severe outcome – more so for this virus than we've ever seen before – is 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. A third of the fatal cases have been in people with neurological disorders.

Institutional 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.

Physicians can help mitigate the stress at points in the system such as emergency rooms by making sure that patients get clear guidance on what to do if they feel sick, what to not worry about, the right numbers to call, and the right places to go for care. "People are a little bit confused about what to do and need guidance. Doctors' offices can make available an information sheet in the office or by email about what patients should do so that they can avoid some of the panicked, confused phone calls," Dr. Moscona said.

For information on diagnostic tests, antiviral recommendations, specimen collection, and more visit the Centers for Disease Control's web site section dedicated to helping clinicians understand H1N1 and treat patients with the flu.

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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