H1N1 flu (swine flu)

Overview

The H1N1 flu, sometimes called swine flu, is a type of influenza A virus.

During the 2009-10 flu season, a new H1N1 virus began causing illness in humans. It was often called swine flu and was a new combination of influenza viruses that infect pigs, birds and humans.

The World Health Organization (WHO) declared the H1N1 flu to be a pandemic, meaning it had spread widely around the world, in 2009. That year the virus caused about 284,400 deaths worldwide. In August 2010, WHO declared the pandemic over. But the H1N1 flu strain from the pandemic became one of the strains that cause seasonal flu.

People without flu complications often get better on their own.

But flu and its complications can be deadly, especially for people at high risk. The seasonal flu vaccine helps protect against the H1N1 flu and other seasonal flu viruses.

Symptoms

The symptoms of flu caused by H1N1, commonly called swine flu, are like those of other flu viruses. Flu symptoms start about 1 to 2 days after you're exposed to the virus. Symptoms tend to start quickly.

The first symptoms often include:

• Fever, but not always.
• Chills.
• Headache.
• Muscle pain.
• Loss of appetite.
• Painful or watery eyes.

Symptoms that come on over time include:

• Cough.
• Sore throat.
• Runny or stuffy nose.
• Tiredness and weakness.
• Wheezing or shortness of breath.
• Diarrhea.
• Feeling sick to the stomach or vomiting, which is more common in children than adults.

In young children, these symptoms may show up more as being fussy or irritable. Children with flu also are more likely than adults to have ear pain, feel sick to the stomach, vomit or have diarrhea.

When to see a doctor

If you're healthy and get the flu, you may not need to see a healthcare professional. But some people are at higher risk of flu complications.

Call your healthcare professional if you have flu symptoms and you're pregnant or have a chronic disease. Some examples are asthma, emphysema, diabetes or a heart condition.

If you have emergency symptoms of the flu, get medical care right away. For adults, emergency symptoms can include:

• Trouble breathing or shortness of breath.
• Chest pain.
• Signs of dehydration such as not urinating often enough.
• Dizziness that doesn't go away.
• Seizures.
• Worsening of existing medical conditions.
• Severe weakness or muscle pain.

Emergency symptoms in children can include:

• Trouble breathing.
• Pale, gray or blue-colored skin, lips or nail beds depending on skin color.
• Chest pain.
• Dehydration.
• Severe muscle pain.
• Seizures.
• Worsening of existing medical conditions.

Causes

Influenza viruses such as H1N1 infect the cells that line your nose, throat and lungs.

The viruses spread through the air in droplets released when someone with the virus coughs, sneezes, breathes or talks. The virus enters your body when you breathe in contaminated droplets. It also can enter your body if you touch a contaminated surface and then touch your eyes, nose or mouth.

You can't get swine flu from eating pork.

You can spread the virus from about a day before symptoms start until about five days after they start. Children and people with weakened immune systems may be able to spread the virus for a slightly longer time.

Risk factors

Factors that may raise your risk of serious illness from H1N1 or seasonal flu complications include:

• Age. Influenza tends to have worse outcomes in children under age 2 and adults older than age 65.
• Living or working conditions. People who live or work in facilities with many other residents are more likely to get the flu. Some examples are nursing homes or military barracks. People staying in the hospital also are at higher risk.
• Weakened immune system. Cancer treatments, antirejection medicines, long-term use of steroids, organ transplant, blood cancer or HIV/AIDS can weaken the immune system. This can make it easier to catch the flu and may raise the risk of complications.
• Chronic illnesses. Chronic conditions may increase the risk of influenza complications. Examples include asthma and other lung diseases, diabetes, heart disease, and nervous system diseases. Other examples are metabolic disorders, problems with an airway, and kidney, liver or blood diseases.
• Race. Native Americans or Alaska Natives may have a higher risk of influenza complications.
• Pregnancy. Pregnant people are more likely to have influenza complications, especially in the second and third trimesters. This risk stays higher for up to two weeks after the baby is born.
• Obesity. People with a body mass index (BMI) of 40 or higher have a higher risk of flu complications.

Complications

Influenza complications include:

• Worsening of chronic conditions, such as heart disease and asthma.
• Pneumonia.
• Neurological symptoms, ranging from confusion to seizures.
• Respiratory failure.
• Bronchitis.
• Muscle damage, called rhabdomyolysis, or muscle swelling, called myositis.
• Bacterial infections.

People on long-term aspirin therapy and those who are younger than 19 years of age are at risk of Reye's syndrome if infected with influenza.

Serious illness with flu may cause pregnancy complications, including the need for hospital care, pregnancy loss or low birth weight.

Prevention

H1N1 flu began causing illness in humans in 2009. Now it is a strain of the seasonal flu. Seasonal flu vaccines are available in the U.S. for people age 6 months and older. Seasonal flu vaccines are often recommended for everyone who doesn't have a medical reason to avoid the vaccine. If you have an egg allergy, you can often still get a flu vaccine.

Getting a seasonal flu vaccine lowers:

• The risk of getting the flu.
• The risk of having serious illness from the flu and needing to stay in the hospital due to the flu.
• The risk of dying of the flu.

If the vaccine is given later in pregnancy, the flu vaccine helps protect a newborn from the flu too.

Each year's seasonal flu vaccine protects against the three or four influenza viruses expected to be the most common during that year's flu season.

The first time children between the ages of 6 months and 8 years get a flu vaccine, they may need two doses given at least four weeks apart. After that, they can usually receive single annual doses of the flu vaccine. Check with your child's healthcare professional.

Also, check with your healthcare team before receiving a flu vaccine if you had a serious reaction to a previous flu vaccine. People who have had Guillain-Barre syndrome also may want to check with a healthcare professional before getting the flu vaccine. If you have a cold or don't feel well, you may still be able to get a flu shot. But if you are seriously ill, check with your healthcare team to see if you should delay getting the vaccine.

The flu vaccine may be available as an injection, jet injector or a nasal spray.

Seasonal flu shots

Flu shots are usually offered based on your age. For older children and adults, the flu shot is usually given in a muscle in the arm. Younger children may get the flu shot in a thigh muscle.

Some types of flu vaccines may be limited to certain groups. High-dose flu shots and shots with an ingredient called an adjuvant that boosts immune response may help some people avoid the need for care in the hospital due to influenza. People over age 65 can get these vaccines. These vaccines also may be available to people age 18 and older who have a solid organ transplant and take medicine to weaken their immune response.

Jet injector for seasonal flu shot

In the U.S., adults younger than 65 may be able to get the flu vaccine using a device called a jet injector. Instead of a needle, this device uses a high-pressure, narrow stream of fluid to give the vaccine through the skin.

Seasonal flu nasal spray

In the U.S., a seasonal flu nasal spray is approved for people ages 2 through 49.

The nasal spray flu vaccine isn't recommended for some groups, such as:

• People who have severe asthma or are currently wheezing.
• People who had a severe allergic reaction to a flu vaccine in the past.
• Pregnant people.
• Young people who take aspirin or a medicine that has salicylate.
• People with weakened immune systems and caregivers or close contacts of people with weakened immune systems.
• Children between ages 2 to 4 years old diagnosed with asthma or wheezing in the past 12 months.
• People who recently took antiviral medicine for the flu.
• People with a cerebrospinal fluid leak or the potential for a leak, as with a cochlear implant.

Check with your healthcare team to see if you need to be cautious about getting a nasal spray flu vaccine. If you can take the nasal spray flu vaccine, you may be able to take it yourself or give it to an eligible child at home, without seeing a healthcare professional.

Controlling the spread of H1N1

To help slow the spread of H1N1 or seasonal flu in general, follow good hygiene practices. This includes times when you are near animals that could have a flu virus, such as pigs and birds. Other ways to help stop viruses from spreading are:

• Wash your hands often. Use soap and water if you can, and wash for at least 20 seconds. Or use a hand sanitizer that has at least 60% alcohol.
• Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Don't touch your face. Try not to touch your eyes, nose and mouth.
• Clean and disinfect surfaces. Regularly clean surfaces that people touch often. This helps stop the virus from spreading from a contaminated surface to your body.
• Stay away from the virus. Try to avoid people who are sick or have flu symptoms. And if you have symptoms, stay home if you can. When flu is spreading, try to keep space between yourself and others while indoors, especially in areas with poor air flow. If you're at high risk of complications from the flu, consider avoiding swine barns at seasonal fairs and elsewhere.