H1N1 / Swine Flu

This virus was originally referred to as “swine flu” because laboratory testing showed that many of the genes in the virus were very similar to influenza viruses that normally occur in pigs (swine) in North America. But further study has shown that the 2009 H1N1 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 bird (avian) genes and human genes. Scientists call this a "quadruple reassortant" virus.

Yes. While 2009 H1N1 activity declined after later October, 2009 human illness with 2009 H1N1 is ongoing in the United States. In fact, the 2009 H1N1 virus is the predominant influenza virus in circulation so far during the 2009-2010 flu season. The United States experienced its first wave of 2009 H1N1 activity in the spring of 2009, followed by a second wave in the fall, with the number of people infected peaking at the end of October. There are still uncertainties surrounding the rest of this flu season, including the possibility that seasonal influenza viruses will spread during the winter as they usually do while 2009 H1N1 viruses continue to cause illness. In past pandemics, flu activity has occurred in waves and it’s possible that the United States could experience another wave either later in the 2010 winter, or later. In the past, when new viruses have emerged to cause flu pandemics, the new virus has continued to spread among people. Experts believe it’s likely that the new 2009 H1N1 virus will continue to circulate among people for some time, perhaps as a typical winter flu. In fact, a 2009 H1N1 virus has been selected as the H1N1 vaccine component for the Southern Hemisphere’s upcoming seasonal flu vaccine. (More information about the selection of the viruses for the 2010-2011 Northern Hemisphere flu vaccine is available.).

CDC routinely works with states to collect, compile and analyze information about influenza, and has done the same for the 2009 H1N1 virus since the beginning of the outbreak. This information is presented in a weekly report, called FluView.

The 2009 H1N1 virus is contagious and is spreading from human to human.

Spread of the 2009 H1N1 virus is thought to occur in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing, sneezing or talking by people with influenza. Sometimes people may become infected by touching something - such as a surface or object - with flu viruses on it and then touching their mouth or nose.

(February 10, 2010)

Getting infected with any influenza virus, including 2009 H1N1, should cause your body to develop immune resistance to that virus so it's not likely that a person would be infected with the identical influenza virus more than once. (However, people with weakened immune systems might not develop full immunity after infection and might be more likely to get infected with the same influenza virus more than once.) However, it's also possible that a person could have a positive test result for flu infection more than once in an influenza season. This can occur for two reasons:

1. A person may be infected with different influenza viruses (for example, the first time with 2009 H1N1 and the second time with a regular seasonal flu virus. Most rapid tests cannot distinguish which influenza virus is responsible for the illness. And,
2. Influenza tests can occasionally give false positive and false negative results so it's possible that one of the test results were incorrect. This is more likely to happen when the diagnosis is made with the rapid flu tests.

The symptoms of 2009 H1N1 flu virus in people include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. Some people may have vomiting and diarrhea. People may be infected with the flu, including 2009 H1N1 and have respiratory symptoms without a fever. Severe illnesses and deaths have occurred as a result of illness associated with this virus.

Illness with 2009 H1N1 virus has ranged from mild to severe. While most people who have been sick have recovered without needing medical treatment, hospitalizations and deaths from infection with this virus have occurred.

In seasonal flu, certain people are at “high risk” of serious 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. More than 70 percent of adults who have been hospitalized with the 2009 H1N1 virus have had one or more medical conditions previously recognized as placing people at “higher risk” of serious seasonal flu-related complications. This includes pregnancy, diabetes, heart disease, asthma and kidney disease. In one study, fifty-seven percent of children who had been hospitalized as a result of 2009 H1N1 have had one or more “higher risk” medical conditions.

Young children are also at high risk of serious complications from 2009 H1N1, just as they are from seasonal flu. And while people 65 and older are less likely to be infected with 2009 H1N1 flu, if they get sick, they are also at “high risk” of developing serious complications from their illness.

CDC laboratory studies have shown that no children and very few adults younger than 60 years old have existing antibody to the 2009 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 2009 H1N1 flu by any existing antibody.

Most people who get the flu (either seasonal or 2009 H1N1) will have mild illness, will not need medical care or antiviral drugs, and will recover in less than two weeks. Some people, however, are more likely to get flu complications that result in being hospitalized and occasionally result in death. Pneumonia, bronchitis, sinus infections and ear infections are examples of flu-related complications. The flu can also make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu. The list below includes the groups of people more likely to get flu-related complications if they get sick from influenza.

People at High Risk for Developing Flu-Related Complications

• Children younger than 5, but especially children younger than 2 years old
• Adults 65 years of age and older
• Pregnant women

People who have medical conditions including:

• Asthma
• Neurological and neurodevelopmental conditions [including disorders of the brain, spinal cord, peripheral nerve, and muscle such as cerebral palsy, epilepsy (seizure disorders), stroke, intellectual disability (mental retardation), moderate to severe developmental delay, muscular dystrophy, or spinal cord injury].
• Chronic lung disease (such as chronic obstructive pulmonary disease? [COPD] and cystic fibrosis)
• Heart disease (such as congenital heart disease, congestive heart failure and coronary artery disease)
• Blood disorders (such as sickle cell disease)
• Endocrine disorders (such as diabetes mellitus)
• Kidney disorders
• Liver disorders
• Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
• Weakened immune system due to disease or medication (such as people with HIV or AIDS, or cancer, or those on chronic steroids)
• People younger than 19 years of age who are receiving long-term aspirin therapy

In addition, some studies have shown that obese persons (body mass index >=30) and particularly morbidly obese persons (body mass index >=40) are at higher risk, perhaps because they have one of the higher risk conditions above but do not realize it.

Flu 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. More than 90% of deaths and about 60 percent of hospitalization occur in people older than 65.

CDC estimates of the numbers of 2009 H1N1 cases, hospitalizations and deaths are that people younger than 65 years of age are more severely affected by 2009 H1N1 flu relative to people 65 and older compared with seasonal flu. CDC estimates that with 2009 H1N1, approximately 90% of hospitalizations and 88% of estimated deaths from April through December 12, 2009 occurred in people younger than 65 years old. However, because severe illness and deaths have occurred among people 65 and older and because supplies of 2009 H1N1 vaccine have increased dramatically, CDC is now encouraging all people 6 months and older, including people older than 65, to get vaccinated against 2009 H1N1.

People infected with seasonal and 2009 H1N1 flu shed virus and may be able to infect others from 1 day before getting sick to 5 to 7 days after. This can be longer in some people, especially children and people with weakened immune systems and in people infected with 2009 H1N1 viruses.