page_title: Influenza
section_title: Vaccination
hierarchical_section_title: Influenza / Prevention / Vaccination
caption_attribution_description: English: Petty Officer 3rd class Tiffany Long administers the influenza vaccination to Chief Petty Officer Romeo Mortel aboard the USS Kitty Hawk (CV 63) on Oct. 28, 2004. The immunizations are provided to all active-duty personnel to prevent the spread of the flu and maintain service members' health. Long serves as a hospital corpsman and is from San Diego, Calif. Mortel is assigned as a personnel man and is from Batangas, Philippines.
context_page_description: Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. The most common symptoms include: high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be diarrhea and vomiting, but these are not common in adults. Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as "stomach flu" or the "24-hour flu". Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.
  Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C. Type D has not been known to infect humans, but is believed to have the potential to do so. Usually, the virus is spread through the air from coughs or sneezes. This is believed to occur mostly over relatively short distances.
context_section_description: The influenza vaccine is recommended by the World Health Organization (WHO) for high-risk groups, such as pregnant women, children aged less than five years, the elderly, health care workers, and people who have chronic illnesses such as HIV/AIDS, asthma, diabetes, heart disease, or are immunocompromised among others. The United States Centers for Disease Control and Prevention (CDC) recommends the influenza vaccine for those aged six months or older who do not have contraindications. In healthy adults it is modestly effective in decreasing the amount of influenza-like symptoms in a population. In healthy children over the age of two years, the vaccine reduces the chances of getting influenza by around two-thirds, while it has not been well studied in children under two years. In those with chronic obstructive pulmonary disease vaccination reduces exacerbations, it is not clear if it reduces asthma exacerbations. Evidence supports a lower rate of influenza-like illness in many groups who are immunocompromised such as those with: HIV/AIDS, cancer, and post organ transplant. In those at high risk immunization may reduce the risk of heart disease. Whether immunizing health care workers affects patient outcomes is controversial with some reviews finding insufficient evidence and others finding tentative evidence.
  Due to the high mutation rate of the virus, a particular influenza vaccine usually confers protection for no more than a few years. Each year, the World Health Organization predicts which strains of the virus are most likely to be circulating in the next year (see Historical annual reformulations of the influenza vaccine), allowing pharmaceutical companies to develop vaccines that will provide the best immunity against these strains. The vaccine is reformulated each season for a few specific flu strains but does not include all the strains active in the world during that season. It takes about six months for the manufacturers to formulate and produce the millions of doses required to deal with the seasonal epidemics; occasionally, a new or overlooked strain becomes prominent during that time. It is also possible to get infected just before vaccination and get sick with the strain that the vaccine is supposed to prevent, as the vaccine takes about two weeks to become effective.
  Vaccines can cause the immune system to react as if the body were actually being infected, and general infection symptoms (many cold and flu symptoms are just general infection symptoms) can appear, though these symptoms are usually not as severe or long-lasting as influenza. The most dangerous adverse effect is a severe allergic reaction to either the virus material itself or residues from the hen eggs used to grow the influenza; however, these reactions are extremely rare.
  A 2018 Cochrane review of children in good general health found that the live immunization seemed to lower the risk of getting influenza for the season from 18% to 4%. The inactivated vaccine seemed to lower the risk of getting flu for the season from 30% to 11%. Not enough data was available to draw definite conclusions about serious complications such as pneumonia or hospitalization.
  For healthy adults, a 2018 Cochrane review showed that vaccines reduced the incidence of lab-confirmed influenza from 2.3% to 0.9%, which constitutes a reduction of risk of approximately 60%. However, for influenza-like illness which is defined as the same symptoms of cough, fever, headache, runny nose, and bodily aches and pains, vaccine reduced the risk from 21.5% to 18.1%. This constitutes a much more modest reduction of risk of approximately 16%. The difference is most probably explained by the fact that over 200 viruses cause the same or similar symptoms as the flu virus. Another review looked at the effect of short and long term exercise before the vaccine, however, no benefits or harms were recorded.
  The cost-effectiveness of seasonal influenza vaccination has been widely evaluated for different groups and in different settings. It has generally been found to be a cost-effective inter
caption: Giving an influenza vaccination
exif: {"Image ImageDescription": "041028-N-9864S-021\rPetty Officer 3rd class Tiffany Long administers the influenza vaccination to Chief Petty Officer Romeo Mortel aboard the USS Kitty Hawk (CV 63) on Oct. 28, 2004. The immunizations are provided to all active-duty personnel to prevent the spread of the flu and maintain service members' health.  Long serves as a hospital corpsman and is from San Diego, Calif.  Mortel is assigned as a personnel man and is from Batangas, Philippines.  DoD photo by Airman Joseph R Schmitt, U.S. Navy.  (Released)", "Image Make": "NIKON CORPORATION", "Image Model": "NIKON D1X", "Image Orientation": "Horizontal (normal)", "Image XResolution": "300", "Image YResolution": "300", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop 7.0", "Image DateTime": "2004:11:01 14:35:49", "Image Artist": "PHAN Joseph Schmitt", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "792", "GPS GPSVersionID": "[2, 0, 0, 0]", "Image GPSInfo": "1232", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1346", "Thumbnail JPEGInterchangeFormatLength": "10159", "EXIF ExposureTime": "1/80", "EXIF FNumber": "7/2", "EXIF ExposureProgram": "Manual", "EXIF ExifVersion": "0210", "EXIF DateTimeOriginal": "2004:10:28 09:59:07", "EXIF DateTimeDigitized": "2004:10:28 09:59:07", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "4", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "18/5", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF FocalLength": "28", "EXIF SubSecTime": "21", "EXIF SubSecTimeOriginal": "21", "EXIF SubSecTimeDigitized": "21", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "3008", "EXIF ExifImageLength": "1960", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CVAPattern": "[0, 2, 0, 2, 2, 1, 1, 0]"}