Flu seasons are unpredictable in a number of ways. Although epidemics of flu happen every year, the timing, severity, and length of the season varies from one year to another.
Flu viruses are constantly changing so it's not unusual for new flu virus strains to appear each year. For more information about how flu viruses change, visit How the Flu Virus Can Change.
The timing of flu is very unpredictable and can vary from season to season. Flu activity most commonly peaks in the U.S. in January or February. However, seasonal flu activity can begin as early as October and continue to occur as late as May.
CDC recommends a yearly flu vaccine for everyone 6 months of age and older as the first and most important step in protecting against this serious disease. While there are many different flu viruses, the flu vaccine is designed to protect against the three main flu strains that research indicates will cause the most illness during the flu season. Getting the flu vaccine as soon as it becomes available each year is always a good idea, and the protection you get from vaccination will last throughout the flu season.
In addition, you can take everyday preventive steps like staying away from sick people and washing your hands to reduce the spread of germs. If you are sick with flu, stay home from work or school to prevent spreading influenza to others.
Flu vaccines are offered in many locations, including doctor’s offices, clinics, health departments, pharmacies and college health centers, as well as by many employers, and even in some schools.
Even if you don’t have a regular doctor or nurse, you can get a flu vaccine somewhere else, like a health department, pharmacy, urgent care clinic, and often your school, college health center, or work.
Visit the HealthMap Vaccine Finder to locate where you can get a flu shot.
Vaccine manufacturers now estimate 138-145 million doses of influenza vaccine will be produced for the U.S. market this season. (This is an increase from their initial estimate of 135-139 million.) An estimated 30 million to 32 million of these doses will be quadrivalent flu vaccine. The rest will be trivalent flu vaccine.
There are several flu vaccine options for the 2013-2014 flu season.
Traditional flu vaccines made to protect against three different flu viruses (called “trivalent” vaccines) are available. In addition, this season flu vaccines made to protect against four different flu viruses (called “quadrivalent” vaccines) also are available.
The trivalent flu vaccine protects against two influenza A viruses and an influenza B virus. The following trivalent flu vaccines are available:
The quadrivalent flu vaccine protects against two influenza A viruses and two influenza B viruses. The following quadrivalent flu vaccines are available:
(*”Healthy” indicates persons who do not have an underlying medical condition that predisposes them to influenza complications.)
CDC does not recommend one flu vaccine over the other. The important thing is to get a flu vaccine every year.
Flu vaccines are designed to protect against the influenza viruses that experts predict will be the most common during the upcoming season. Three kinds of influenza viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses. Each year, these viruses are used to produce seasonal influenza vaccine.
The 2013-2014 trivalent influenza vaccine is made from the following three viruses:
It is recommended that the quadrivalent vaccine containing two influenza B viruses include the above three viruses and a B/Brisbane/60/2008-like virus.
More information about influenza vaccines is available at Preventing Seasonal Flu With Vaccination.
Inactivated 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?
Multiple studies conducted over different seasons and across vaccine types and influenza virus subtypes have shown that the body’s immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time. The decline in antibodies is influenced by several factors, including the antigen used in the vaccine, age of the person being vaccinated, and the person's general health (for example, certain chronic health conditions may have an impact on immunity). When most healthy people with regular immune systems are vaccinated, their bodies produce antibodies and they are protected throughout the flu season, even as antibody levels decline over time. People with weakened immune systems may not generate the same amount of antibodies after vaccination; further, their antibody levels may drop more quickly when compared to healthy people.
For everyone, getting vaccinated each year provides the best protection against influenza throughout flu season. It’s important to get a flu vaccine every year, even if you got vaccinated the season before and the viruses in the vaccine have not changed for the current season.
It's not possible to predict with certainty which flu viruses will predominate during a given season. Flu viruses are constantly changing (called 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.
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.
Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related viruses. A less than ideal 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, it's important to remember that the flu vaccine contains three virus viruses so that even when there is a less than ideal match or lower effectiveness against one virus, the vaccine may protect against the other viruses.
For these reasons, even during seasons when there is a less than ideal match, CDC continues to recommend flu vaccination. This is particularly important for people at high risk for serious flu complications, and their close contacts.
CDC carries out and collaborates with other partners within and outside CDC to assess how well flu vaccines work. During the 2013-2014 season, CDC is planning multiple studies on the effectiveness of both the flu shot and the nasal-spray flu vaccine. These studies will measure vaccine effectiveness in preventing laboratory confirmed influenza among persons aged 6 months and older, since beginning in the 2010-2011 season the Advisory Committee on Immunization Practices (ACIP) recommended annual vaccination for everyone in this age group.
Information about vaccine supply is available on the CDC influenza web site.
Yes. If you get sick, there are drugs that can treat flu illness. They are called antiviral drugs and they can make your illness milder and make you feel better faster. They also can prevent serious flu-related complications, like pneumonia. For more information about antiviral drugs, visit Treatment (Antiviral Drugs).
Antiviral resistance means that a virus has changed in such a way that the antiviral drug is less effective in treating or preventing illness. Samples of viruses collected from around the United States and worldwide are studied to determine if they are resistant to any of the FDA-approved influenza antiviral drugs.
CDC routinely collects viruses through a domestic and global surveillance system to monitor for changes in influenza viruses. CDC will continue ongoing surveillance and testing of influenza viruses. Additionally, CDC is working with the state public health departments and the World Health Organization to collect additional information on antiviral resistance in the United States and worldwide. The information collected will assist in making informed public health policy recommendations.
Seasonal influenza, H7N9 influenza, or MERS-CoV infection can cause similar respiratory symptoms. However, of these viruses, your symptoms are most likely caused by seasonal influenza. H7N9 and MERS-CoV are less common and have not been reported in the United States. At this time, H7N9 has only been detected in China. All MERS-CoV cases have been linked to countries in or near the Arabian Peninsula.
If you are hospitalized for a severe respiratory illness of unknown causes within 10 days of traveling to a country where H7N9 has been detected, or you if you have come in contact with a patient who is to confirmed to have H7N9 infection, you may be tested for this disease. If you have recently traveled to countries where MERS-CoV has been detected and developed a fever, cough or shortness of breath within 14 days after returning to the U.S., contact your doctor. (At this time, H7N9 has been detected only in China. All MERS-CoV cases have been linked to countries in or near the Arabian Peninsula.) It is not possible to determine whether a patient has seasonal influenza, H7N9 influenza, MERS-CoV infection or illness due to another pathogen based on symptoms alone. However, there are tests to detect seasonal influenza, H7N9 influenza, MERS-CoV infection. Your doctor will determine if you should be tested for any of these illnesses based on your symptoms, clinical presentation and recent travel history.
Seasonal influenza, H7N9 influenza and MERS-CoV infection can cause similar respiratory symptoms, including fever and cough. However, so far, symptoms of most reported cases of H7N9 and MERS-CoV have been more severe than is common with seasonal influenza. Of these viruses, doctors are most likely to encounter patients with seasonal influenza infection. Seasonal influenza viruses circulate each year in the United States, with most flu activity occurring between October and May.
At this time, no cases of H7N9 or MERS-CoV have been reported in the United States. However, CDC recognizes the potential for these viruses to spread globally, including to the United States. Most likely this would occur in a traveler who got infected while in a country where these viruses have been detected. (At this time, H7N9 has only been detected in China. All MERS-CoV cases have been linked to countries in or near the Arabian Peninsula.)
Check the CDC website for the latest guidance and situation updates on these viruses.