Key Facts about Influenza (Flu) & Flu Vaccine
What is influenza (also called flu)?
The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccine each year.
Signs and symptoms of flu
People who have the flu often feel some or all of these signs and symptoms:
- Fever* or feeling feverish/chills
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Fatigue (very tired)
- Some people may have vomiting and diarrhea, though this is more common in children than adults.
*It's important to note that not everyone with flu will have a fever.
How flu spreads
Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth, eyes or possibly their nose.
Period of contagiousness
You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time.
How serious is the flu?
Flu is unpredictable and how severe it is can vary widely from one season to the next depending on many things, including:
- what flu viruses are spreading,
- how much flu vaccine is available
- when vaccine is available
- how many people get vaccinated, and
- how well the flu vaccine is matched to flu viruses that are causing illness.
Certain people are at greater risk for serious complications if they get the flu. This includes older people, young children, pregnant women and people with certain health conditions (such as asthma, diabetes, or heart disease).
Flu seasons are unpredictable and can be severe. Over a period of 30 years, between 1976 and 2006, estimates of flu-associated deaths in the United States range from a low of about 3,000 to a high of about 49,000 people.
Complications of flu
Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.
Prevent seasonal flu: Get vaccinated
The single best way to prevent the flu is to get a flu vaccine each season.
There are several flu vaccine options for the 2015-2016 flu season.
Traditional flu vaccines made to protect against three different flu viruses (called "trivalent" vaccines) are available. In addition, flu vaccines made to protect against four different flu viruses (called "quadrivalent" vaccines) also are available.
Trivalent flu vaccine protects against two influenza A viruses (an H1N1 and an H3N2) and an influenza B virus. The following trivalent flu vaccines are available:
- Standard-dose trivalent shots (IIV3) that are manufactured using virus grown in eggs. Different flu shots are approved for people of different ages, but there are flu shots that are approved for use in people as young as 6 months of age and up.
- An intradermal trivalent shot, which is injected into the skin instead of the muscle and uses a much smaller needle than the regular flu shot. It is approved for people 18 through 64 years of age.
- A high-dose trivalent shot, approved for people 65 and older.
- A trivalent shot containing virus grown in cell culture, which is approved for people 18 and older.
- A recombinant trivalent shot that is egg-free, approved for people 18 through 49 years of age.
The quadrivalent flu vaccine protects against two influenza A viruses and two influenza B viruses. The following quadrivalent flu vaccines are available:
When to get vaccinated against seasonal flu
Yearly flu vaccination should begin soon after flu vaccine is available, and ideally by October. However, getting vaccinated even later can be protective, as long as flu viruses are circulating. While seasonal influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later. Since it takes about two weeks after vaccination for antibodies to develop in the body that protect against influenza virus infection, it is best that people get vaccinated so they are protected before influenza begins spreading in their community.
Who Should Get Vaccinated This Season?
Everyone six months of age and older should get a flu vaccine every season. This recommendation has been in place since February 24, 2010 when CDC's Advisory Committee on Immunization Practices (ACIP) voted for "universal" flu vaccination in the United States to expand protection against the flu to more people.
Vaccination to prevent influenza is particularly important for people who are at high risk of serious complications from influenza. See People at High Risk of Developing Flu-Related Complications for a full list of age and health factors that confer increased risk.
More information is available at Who Should Get Vaccinated Against flu.
Who should not be vaccinated against seasonal flu?
Different flu vaccines are approved for use in different groups of people. Factors that can determine a person's suitability for vaccination, or vaccination with a particular vaccine, include a person's age, health (current and past) and any relevant allergies, including an egg allergy.
For more about preventing the flu, see the following: