Novel H1N1 Flu

Novel H1N1 (referred to as “swine flu” early on) is a new influenza virus causing illness in people. This new virus was first detected in people in the United States in April 2009. Other countries, including Mexico and Canada, have reported people sick with this new virus. This virus is spreading from person-to-person, probably in much the same way that regular seasonal influenza viruses spread.

Why is novel H1N1 virus sometimes called “swine flu”?
This virus was originally referred to as “swine flu” because laboratory testing showed that many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. But further study has shown that this new virus is very different from what normally circulates in North American pigs. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia and avian genes and human genes. Scientists call this a "quadruple reassortant" virus.

Are there human infections with novel H1N1 virus in the U.S.?
Yes. Cases of human infection with novel H1N1 influenza virus were first confirmed in the U.S. in Southern California and near Guadalupe County, Texas. The outbreak intensified rapidly from that time and more and more states have been reporting cases of illness from this virus. An updated case count of confirmed novel H1N1 flu infections in the United States is kept at http://www.cdc.gov/h1n1flu/update.htm. CDC and local and state health agencies are working together to investigate this situation.

Is novel H1N1 virus contagious?
CDC has determined that novel H1N1 virus is contagious and is spreading from human to human. However, at this time, it is not known how easily the virus spreads between people.

What are the signs and symptoms of this virus in people?
The symptoms of novel H1N1 flu virus in people are similar to the symptoms of seasonal flu and include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. A significant number of people who have been infected with this virus also have reported diarrhea and vomiting. Also, like seasonal flu, severe illnesses and death has occurred as a result of illness associated with this virus.

How severe is illness associated with novel H1N1 flu virus?
It’s not known at this time how severe novel H1N1 flu virus will be in the general population. In seasonal flu, there are certain people that are at higher risk of serious flu-related complications. This includes people 65 years and older, children younger than five years old, pregnant women, and people of any age with certain chronic medical conditions. Early indications are that pregnancy and other previously recognized medical conditions that increase the risk of influenza-related complications, like asthma and diabetes, also appear to be associated with increased risk of complications from novel H1N1 virus infection as well.

One thing that appears to be different from seasonal influenza is that adults older than 64 years do not yet appear to be at increased risk of novel H1N1-related complications thus far in the outbreak. CDC is conducting laboratory studies to see if certain people might have natural immunity to this virus, depending on their age. Early reports indicate that no children and few adults younger than 60 years old have existing antibody to novel H1N1 flu virus; however, about one-third of adults older than 60 may have antibodies against this virus. It is unknown how much, if any, protection may be afforded against novel H1N1 flu by any existing antibody.

How does novel H1N1 flu compare to seasonal flu in terms of its severity and infection rates?
CDC is still learning about the severity of novel H1N1 flu virus. At this time, there is not enough information to predict how severe novel H1N1 flu outbreak will be in terms of illness and death or how it will compare with seasonal influenza.

With seasonal flu, we know that seasons vary in terms of timing, duration and severity. Seasonal influenza can cause mild to severe illness, and at times can lead to death. Each year, in the United States, on average 36,000 people die from flu-related complications and more than 200,000 people are hospitalized from flu-related causes. Of those hospitalized, 20,000 are children younger than 5 years old. Over 90% of deaths and about 60 percent of hospitalization occur in people older than 65.

So far, with novel H1N1 flu, the largest number of novel H1N1 flu confirmed and probable cases have occurred in people between the ages of 5 and 24-years-old. At this time, there are few cases and no deaths reported in people older than 64 years old, which is unusual when compared with seasonal flu. However, pregnancy and other previously recognized high risk medical conditions from seasonal influenza appear to be associated with increased risk of complications from this novel H1N1.

How does novel H1N1 virus spread?
Spread of novel H1N1 virus is thought to be happening in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing or sneezing by people with influenza. Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose.

How long can an infected person spread this virus to others?
At the current time, CDC believes that this virus has the same properties in terms of spread as seasonal flu viruses. With seasonal flu, studies have shown that people may be contagious from one day before they develop symptoms to up to 7 days after they get sick. Children, especially younger children, might potentially be contagious for longer periods. CDC is studying the virus and its capabilities to try to learn more and will provide more information as it becomes available.