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What You Should Know for the 2013-2014 Influenza Season

When did flu activity peak?

The timing of flu is unpredictable and can vary from season to season. Seasonal flu activity can begin as early as October and continue to occur as late as May. Flu activity most commonly peaks in the U.S. in January or February.

For the 2013-2014 season, influenza-like-illness (ILI) in the United States began increasing in mid-November, and toward the end of December, ILI was high across most of the country. Flu activity peaked during the week ending December 28, 2013 for the 2013-2014 season and began a downward trend in early January.

Severity indicators usually lag behind ILI. More information about activity this season can be found in the latest FluView and the MMWR report Update: Influenza Activity — United States, September 29, 2013-February 8, 2014.

Who was most severely impacted by flu this season?

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 season to another. However, seasonal influenza contributes to substantial morbidity and mortality each year in the United States.

During the 2013-2014 season, CDC received several reports of severe flu illness among young and middle-aged adults, many of whom were infected with the 2009 H1N1 virus; hospitalizations and deaths also were reported. More than 60% of the hospitalizations reported to CDC’s influenza surveillance system were in people 18 to 64 years old. More commonly, most flu hospitalizations occur in people 65 and older.

Additional information regarding the pattern of severe flu illness among young and middle-aged adults this season is available in the MMWR report Update: Influenza Activity — United States, September 29, 2013-February 8, 2014.

How many children died from the flu this season?

From September 30, 2013 to March 15, 2014, 75 flu-related deaths in children were reported to CDC. More information about reported pediatric deaths is available at FluView: Influenza-Associated Pediatric Mortality.

Are new flu viruses circulating this season?

Flu viruses are constantly changing so it's not unusual for flu viruses that are slightly different to appear each year. For more information about how flu viruses change, visit How the Flu Virus Can Change

Each season, CDC analyzes flu viruses that are circulating to see whether they are like the viruses that the seasonal flu vaccine is designed to protect against. This so-called “antigenic characterization” data is published weekly in FluView. So far this season, most of the flu viruses that have been analyzed at CDC are like the viruses included in the 2013-2014 flu vaccine. Only a very small percentage of influenza A (H1N1) viruses analyzed by CDC did not match the H1N1 virus that the 2013-2014 vaccine is designed to protect against.

Flu Vaccine Information and Recommendations for the 2013-2014 Season

How well is the vaccine working this season?

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?

CDC conducts studies each year to estimate how well the flu vaccine protects against having to go to the doctor because of flu illness. CDC’s interim VE estimates, published in a February 21, 2014 Morbidity and Mortality Weekly Report Interim Estimates of 2013–14 Seasonal Influenza Vaccine Effectiveness — United States, February 2014, indicate that influenza vaccination offered substantial protection against the flu this season, reducing a vaccinated person’s risk of having to go to the doctor for flu illness by about 60 percent across all ages.

The study also looked at VE by age group and found that the vaccine provided similar levels of protection against influenza infection across all ages. VE point estimates against influenza A and B viruses by age group ranged from 52 percent for people 65 and older to 67 percent for children 6 months to 17 years. Protection against the predominant H1N1 virus was even slightly better among older people; VE against H1N1 was estimated to be 56 percent in people 65 and older and 62 percent in people 50 to 64 years of age. All findings were statistically significant. The interim VE estimates this season are comparable to results from studies during other seasons when the viruses in the vaccine have been well-matched with circulating influenza viruses.

Should I still get vaccinated since flu season has started?

Yes. CDC recommends that people get vaccinated against flu as long as flu viruses are circulating. Influenza seasons are unpredictable. They can begin as early as October and substantial activity can occur as late as May.

It takes about two weeks after vaccination for antibodies to develop in the body that provide protection against the flu.

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.

Is vaccine still available?

Flu vaccine is produced by private manufacturers. Information about the number of seasonal flu vaccine doses distributed this season is available at Seasonal Flu Vaccine & Total Doses Distributed.

In May 2013, flu vaccine manufacturers projected about 138-145 million doses would be available for the U.S. market during the 2013-214 season. As of late February, more than 134 million doses of flu vaccine had been delivered in the United States.

At this time, some vaccine providers may have exhausted their vaccine supplies, while others may have remaining supplies of vaccine. People seeking vaccination may need to call more than one provider to locate vaccine. The flu vaccine locator may be helpful.

Where can I get a flu vaccine?

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.

Should I still get vaccinated even if I have already gotten sick with the flu?

Yes. There are a couple of reasons why you should be vaccinated even if you have already been sick with a flu-like illness this season. First, it’s possible that your illness was not caused by an influenza virus. There are other respiratory viruses circulating along with flu that can have similar flu symptoms. The only way to know for sure that a flu virus is making you sick is to have a sample taken and tested in a laboratory. Second, even if you were sick with one influenza virus, the seasonal flu vaccine protects against flu viruses that research suggests will be most common. This means the vaccine can offer protection against other influenza viruses you haven’t been exposed to yet.

What kind of vaccines will be available in the United States for 2013-2014?

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.

What flu viruses does this season’s vaccine protect against?

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:

  • an A/California/7/2009 (H1N1)pdm09-like virus;
  • an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011;
  • a B/Massachusetts/2/2012-like virus.

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.

How long does a flu vaccine protect me from getting the flu?

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.

Can I get vaccinated and still get the flu?

Yes. It’s possible to get sick with flu even if you have been vaccinated (although you won’t know for sure unless you get a flu test and it is positive). This is possible for the following reasons:

  • You may be exposed to a flu virus shortly before getting vaccinated or during the period that it takes the body to gain protection after getting vaccinated. This exposure may result in you becoming ill with flu before the vaccine begins to protect you. (About 2 weeks after vaccination, antibodies that provide protection develop in the body.)
  • You may be exposed to a flu virus that is not included in the seasonal flu vaccine. There are many different flu viruses that circulate every year. The composition of the flu shot is reviewed each season and updated if needed to protect against the three viruses that research suggests will be most common. Characterization of flu viruses collected this season in the United States indicates that most circulating viruses are like the vaccine viruses; however, there is a smaller percentage of viruses that the vaccine would not be expected to protect against.
  • Unfortunately, some people can get infected with a flu virus the flu vaccine is designed to protect against despite getting vaccinated. Protection provided by flu vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, the flu vaccine works best among young healthy adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. While vaccination offers the best protection against flu infection, it's still possible that some people may become ill after being vaccinated. Flu vaccination is not a perfect tool, but it is the best tool currently at our disposal to prevent flu.

Has CDC received reports of people who have gotten a flu vaccine and then tested positive for flu?

Yes. CDC has received reports of some people who became ill and tested positive for the flu even though they had been vaccinated. This occurs every season. There are a number of reasons why people who got a flu vaccine may still get the flu this season.

To estimate how well flu vaccines work each year, CDC has been working with researchers at universities and hospitals since the 2004-2005 flu season conducting observational studies using laboratory-confirmed flu as the outcome.

It’s important that health care providers and the public remember that flu antiviral medications are available to treat the flu. CDC has recommendations on the use of these medications (sold commercially as “Tamiflu®” and “Relenza®”). Antiviral treatment as early as possible is recommended for any patients with confirmed or suspected flu who are hospitalized, seriously ill, or ill and at high risk of serious flu-related complications, including young children, people 65 and older, people with certain underlying medical conditions and pregnant women. Treatment should begin as soon as flu is suspected, regardless of vaccination status or rapid test results and should not be delayed for confirmatory testing. A full list of people considered at high risk for serious flu-related complications is available at People at High Risk of Developing Flu–Related Complications.

For more information on antiviral drugs, see Antiviral Drugs.

Is this season's vaccine a good match for circulating viruses?

Over the course of a flu season, CDC studies samples of flu viruses circulating during that season to evaluate how much similarity there is between viruses used to make the vaccine and circulating viruses. Data are published in the weekly FluView.

One of the ways that CDC evaluates the match between vaccine viruses and circulating viruses is through antigenic characterization. As of the week ending March 15, 2014, most (99.9%) of the flu viruses that have been analyzed at CDC are like the viruses included in the 2013-2014 flu vaccine. The match between the vaccine virus and circulating viruses is one factor that impacts how well the vaccine works.

How do we know if there is a good match between the vaccine viruses and those causing illness?

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.

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 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 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.

What will CDC do to monitor vaccine effectiveness for the 2013-2014 season?

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.

Where can I find information about vaccine supply?

Information about vaccine supply is available on the CDC influenza web site.

Is there treatment for the flu?

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).

Flu Antiviral Drugs Recommendations for the 2013-2014 Season

What is antiviral resistance?

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.

What will CDC do to monitor antiviral resistance in the United States during the 2013-2014 season?

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.

Has CDC received reports of antiviral resistant flu viruses during the 2013-2014 season?

CDC and its partner laboratories have detected a small number of 2009 H1N1 influenza viruses in the United States this season that are resistant to the antiviral drug oseltamivir (trade name Tamiflu).These findings are similar to those of other countries. Also, rare cases of oseltamivir-resistant influenza A (H3N2) viruses have been detected globally. However, the majority of currently circulating flu viruses in the United States and internationally are susceptible to the neuraminidase inhibitor class of antiviral drugs (oseltamivir and zanamivir). No flu viruses tested so far this season are resistant to zanamivir (trade name Relenza).

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among 2009 influenza A (H1N1) and A (H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, amantadine and rimantadine are not recommended for use this season.

CDC and its public health partners around the world are committed to informing the public of any significant increases in antiviral resistance among circulating influenza viruses. The latest national statistics on antiviral resistance are available in the weekly FluView report.

Has CDC recommended any changes to the current guidance on the use of antivirals for the 2013-2014 flu season?

No. At this time, the majority of circulating flu viruses remain susceptible to oseltamivir (trade name Tamiflu) and zanamivir (trade name Relenza). As a result, CDC has not changed its antiviral recommendations for the 2013-2014 season, which are available at Antiviral Drugs.

What can people do to protect themselves against antiviral resistant flu viruses this season?

Getting vaccination with the 2013-2014 seasonal flu vaccine is the first and most important step in preventing the flu. This season’s flu vaccine is formulated to protect against infection with 2009 H1N1 influenza viruses, including the oseltamivir-resistant 2009 H1N1 viruses identified in the United States this season. The vaccine also protects against an influenza A (H3N2) virus and one or two influenza B viruses (depending on the vaccine). Everyone 6 months of age and older who has not yet received a flu vaccine this season should get vaccinated. If you are in a group at high risk of serious flu-related complications, call your doctor if you develop flu symptoms you may benefit from early treatment. If you are not at high risk, if possible, stay home from work, school and errands when you are sick. This will help prevent you from spreading your illness to others.

What is CDC doing in response to antiviral resistant flu viruses detected this season?

CDC is working closely with its partners at the state level to monitor for antiviral resistant flu viruses this season. CDC has worked with states where antiviral resistant flu viruses have been found to enhance surveillance for these viruses and to inform public health professionals of the latest information and recommendations.

I am a U.S. resident experiencing flu-like symptoms (e.g. coughing, fever, sore throat, etc.). How do I know if I have seasonal influenza, H7N9 influenza, or MERS-CoV (Middle Eastern Respiratory Syndrome Coronavirus)?

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. The majority of H7N9 infections have occurred in China. The first and only case outside of China was in Malaysia and was reported on February 12, 2014. The case was detected in a traveler from an H7N9-affected area of 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 or travelers from 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, and 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.

What are the challenges doctors face in identifying and distinguishing between cases of seasonal flu, avian influenza A (H7N9) and MERS-CoV (Middle Eastern Respiratory Syndrome Coronavirus) this season?

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 or in travelers from 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.


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