Per the Center for Disease Control (CDC) August 2015
Why should people get vaccinated against the flu?
Influenza is a serious disease that can lead to hospitalization and sometimes even death. Every flu season is different, and influenza infection can affect people differently. Even healthy people can get very sick from the flu and spread it to others. Over a period of 31 seasons between 1976 and 2007,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. During a regular flu season, about 90 percent of deaths occur in people 65 years and older. “Flu season” in the United States can begin as early as October and last as late as May.
During this time, flu viruses are circulating at higher levels in the U.S. population. An annual seasonal flu vaccine (either the flu shot or the nasal spray flu vaccine) is the best way to reduce the chances that you will get seasonal flu and spread it to others. When more people get vaccinated against the flu, less flu can spread through that community.
How do flu vaccines work?
Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with the viruses that are in the vaccine.
The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. Traditional flu vaccines (called “trivalent” vaccines) are made to protect against three flu viruses; an influenza A (H1N1) virus, an influenza A (H3N2) virus, and an influenza B virus. There are also flu vaccines made to protect against four flu viruses (called “quadrivalent” vaccines). These vaccines protect against the same viruses as the trivalent vaccine and an additional B virus.
Who should get vaccinated this season?
Everyone 6 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 Influenza.
When should I get vaccinated?
Flu vaccination should begin soon after vaccine becomes available, ideally by October. However, as long as flu viruses are circulating, vaccination should continue to be offered throughout the flu season, even in January or later. While seasonal influenza outbreaks can happen as early as October, during most seasons 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.
Flu vaccine is produced by private manufacturers, and the timing of availability depends on when production is completed. Shipments began in August and will continue throughout October and November until all vaccine is distributed.
Why do I need a flu vaccine every year?
A flu vaccine is needed every season for two reasons. First, the body’s immune response from vaccination declines over time, so an annual vaccine is needed for optimal protection. Second, because flu viruses are constantly changing, the formulation of the flu vaccine is reviewed each year and sometimes updated to keep up with changing flu viruses. For the best protection, everyone 6 months and older should get vaccinated annually.
Does flu vaccine work right away?
No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. That’s why it’s better to get vaccinated early in the fall, before the flu season really gets under way.
Can I get seasonal flu even though I got a flu vaccine this year?
Yes. There is still a possibility you could get the flu even if you got vaccinated. The ability of flu vaccine to protect a person depends on various factors, including the age and health status of the person being vaccinated, and also the similarity or “match” between the viruses used to make the vaccine and those circulating in the community. If the viruses in the vaccine and the influenza viruses circulating in the community are closely matched, vaccine effectiveness is higher. If they are not closely matched, vaccine effectiveness can be reduced. However, it’s important to remember that even when the viruses are not closely matched, the vaccine can still protect many people and prevent flu-related complications. Such protection is possible because antibodies made in response to the vaccine can provide some protection (called cross-protection) against different but related influenza viruses. For more information about vaccine effectiveness, visit How Well Does the Seasonal Flu Vaccine Work?
Influenza vaccine effectiveness (VE) can vary from year to year and among different age and risk groups. For more information about vaccine effectiveness, visit How Well Does the Seasonal Flu Vaccine Work? For information specific to this season, visit About the Current Flu Season.
Will this season’s vaccine be a good match for circulating viruses?
It’s not possible to predict with certainty which flu viruses will predominate during a given season. Over the course of a flu season, CDC studies samples of flu viruses circulating during that season to evaluate how close a match there is between viruses used to make the vaccine and circulating viruses. Data are published in the weekly FluView. In addition, CDC conducts studies each year to determine how well the vaccine protects against illness during that season. The results of these studies are typically published following the conclusion of the flu season and take into consideration all of the data collected during the season. Interim preliminary estimates of the vaccine’s benefits that season using data available at that time also may be provided. For more information, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work?
Flu viruses are constantly changing (called “antigenic drift”) – they can change from one season to the next or they can even change within the course of one flu season. Experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered on time. (For more information about the vaccine virus selection process visit Selecting the Viruses in the Influenza (Flu) Vaccine.) Because of these factors, there is always the possibility of a less than optimal match between circulating viruses and the viruses in the vaccine.
Can the vaccine provide protection even if the vaccine is not a “good” match?
Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related viruses. A less than optimal match may result in reduced vaccine effectiveness against the virus that is different from what is in the vaccine, but it can still provide some protection against influenza illness.
In addition, even when there is a less than optimal match or lower effectiveness against one virus, it’s important to remember that the flu vaccine is designed to protect against three or four flu viruses, depending on the vaccine.
For these reasons, even during seasons when there is a less than optimal match, CDC continues to recommend flu vaccination. This is particularly important for people at high risk for serious flu complications, and their close contacts.
Vaccine Side Effects (What to Expect)
No, a flu vaccine cannot cause flu illness. Flu vaccines that are administered with a needle are currently made in two ways: the vaccine is made either with a) flu vaccine viruses that have been ‘inactivated’ and are therefore not infectious, or b) with no flu vaccine viruses at all (which is the case for recombinant influenza vaccine). The nasal spray flu vaccine does contain live viruses. However, the viruses are attenuated (weakened), and therefore cannot cause flu illness. The weakened viruses are cold-adapted, which means they are designed to only cause infection at the cooler temperatures found within the nose. The viruses cannot infect the lungs or other areas where warmer temperatures exist.
While a flu vaccine cannot give you flu illness, there are different side effects that may be associated with getting a flu shot or a nasal spray flu vaccine. These side effects are mild and short-lasting, especially when compared to symptoms of bad case of flu.
The flu shot: The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. Some minor side effects that may occur are:
- Soreness, redness, or swelling where the shot was given
- Fever (low grade)