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2012-2013 Influenza Season

Questions & Answers

Vaccine Effectiveness for 2012-2013

How well is the vaccine working this season?

CDC conducts studies each year to estimate how well the flu vaccine protects against having to go to the doctor because of flu illness. This season CDC is publishing information about how well the flu vaccine is working in the United States at three different times during the season: the beginning, middle, and end of the flu season.  (Note: For the 2013-14 flu season CDC plans to publish a middle and end of season estimate, but not a beginning of season estimate. An early season estimate was done this season as a result of the high levels of early season flu activity). CDC’s estimates of the benefits of flu vaccine (also known as vaccine effectiveness or “VE” for short) are based on information CDC collects as the flu season progresses. Throughout the flu season, CDC collects data to determine how well the flu vaccine works in different age groups, and how well it works against the specific flu viruses that are spreading and causing illness. CDC’s estimates of vaccine effectiveness can change over time as more information is collected. CDC's publishes estimates of vaccine effectiveness to help inform prevention and treatment decisions made by doctors and other health care practitioners during the flu season.

What are CDC’s current estimates of flu vaccine effectiveness this season?

CDC’s mid-season VE estimates were published on February 21, 2013, in a Morbidity and Mortality Weekly Report entitled: “Interim Adjusted Estimates of Seasonal Influenza Vaccine Effectiveness—United Sates, February 2013”. Overall, the VE estimate for protecting against having to go to the doctor because of flu illness was 56% for all age groups (95% confidence interval: 47% to 63%). (For background information on understanding VE estimates and confidence intervals, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work? and go to the questions: “How does CDC present data on vaccine effectiveness” and “Why are confidence intervals important for understanding vaccine effectiveness?”) This VE estimate means that getting a flu vaccine this season reduced the vaccinated population’s risk of having to go to the doctor because of the flu by more than half. However, VE can vary across age groups and across different flu viruses, so CDC further analyzed the VE estimates to adjust for these factors. When broken down by different age groups, the VE against flu A and B viruses ranged from 27% in people 65 and older to 64% in children (aged 6 months to 17 years old).

When looking at flu virus specific VE, effectiveness against flu A (H3N2) virus – which was the main virus spreading this season – was estimated to be 47% (95% CI: 35% to 58%), while effectiveness against flu B was 67% (95% CI: 51% to 78%) for all ages. (Note: There were not enough flu A (H1N1) viruses detected at the beginning of the flu season to make an early estimate of how well the flu vaccine was specifically working against those viruses.)

These results indicate that vaccination with the 2012-2013 flu season vaccine reduced the risk of flu-associated medical visits from flu A (H3N2) viruses by one half and from flu B viruses by two-thirds for most of the population. Overall, VE estimates suggest that the 2012-2013 flu vaccine has moderate effectiveness for most people against the flu viruses spreading in the United States, similar to previously published reports. The one exception to this was the VE among people 65 and older against flu A (H3N2) viruses, which was lower. The single point estimate for VE in this age group was 9% (95% CI: -84% to 55%). Note that because the confidence interval crossed zero for the 65 and older age group, this estimate is not statistically significant, and therefore, the results should be interpreted with caution. Overall, this estimate means that vaccine effectiveness was lower than expected in this age group against flu A (H3N2) viruses. (For background information on understanding VE estimates and confidence intervals, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work? and go to the questions: “How does CDC present data on vaccine effectiveness” and “Why are confidence intervals important for understanding vaccine effectiveness?”)

These overall vaccine effectiveness estimates are within the range expected during flu seasons when most flu viruses spreading and causing illness are like the viruses the flu vaccine is designed to protect against, which is the case this season. These findings also are similar to those published in a recent review of VE studies (Osterholm et al., 2011), from randomized controlled trials and observational studies. In addition, the estimates also are consistent with mid-season flu VE estimates for preventing flu treated by a physician in Canada and the United Kingdom published in the journal Eurosurveillance on January 31, 2013.

Flu vaccination, even with moderate effectiveness of about 60%, can also reduce the following: flu-related illness, antibiotic use, time lost from work, hospitalizations, and deaths.

Is CDC’s mid-season (VE) estimate (56%) different than CDC’s previous early season VE estimate (62%) for the 2012-2013 seasonal vaccine?

The early season results and the mid-season results published by CDC are consistent with each other. CDC published its early season flu vaccine effectiveness (VE) estimates on January 11, 2013. This estimate was preliminary, but it provided an overall look at how well the flu vaccine was working against all flu viruses in the United States across the whole population. Unlike the mid-season VE estimate, this early season VE estimate did not look at how well the flu vaccine was working in different age groups or against specific subtypes of flu viruses. For this early season estimate, CDC reported VE of 62% (95% CI: 51%-71%). CDC’s mid-season VE estimates published in February included an additional 3 weeks of data collected during the peak of the flu season. These estimates were adjusted to control for characteristics of the study participants that can bias results. For example, CDC adjusted for the following characteristics: age, race/ethnicity, study site, self-rated health, and days from illness onset to enrollment in the study. Adjusting for these factors can change the overall estimate of VE, but it’s reassuring that CDC’s early season VE estimate and mid-season estimate are not significantly different. CDC’s end of season VE estimates will also adjust for medical conditions that are associated with increased risk of serious complications from the flu.

Did flu vaccines work in people 65 and older this season?

CDC’s VE study measured lower VE among people 65 and older against flu A this season than it did among other age groups. However, VE against flu B was similar to what was seen in other age groups, while VE against flu A (H3N2) viruses in people 65 and older was significantly lower than in other age groups. One possible explanation for this is that some older people did not mount an effective immune response to the A (H3N2) virus component of this season’s flu vaccine; however, it’s not possible to say this for certain. For more general information about VE in people 65 and older see, Vaccine Effectiveness – How Well Does the Flu Vaccine Work? and see the question “How effective is the flu vaccine in the elderly?”

Based on this season’s VE estimates, should people 65 and older still get vaccinated?

Despite the fact that flu vaccines can work less well in people who are 65 and older, there are many reasons why people in that age group should be vaccinated each year.

  • First, people 65 and older are at high risk of getting seriously ill, being hospitalized and dying from the flu.
  • Second, while the effectiveness of the flu vaccine can be lower among older people, there are seasons when significant benefit can be observed in terms of averting illness that results in a doctor’s visit. Even if the vaccine provides less protection in older adults than it might in younger people, some protection is better than no protection at all, especially in this high risk group.
  • Third, current CDC studies look at how well the vaccine works in preventing flu illness that results in a doctor’s visit or admission to a hospital. This is just one outcome. There are other studies that look at the effects of flu vaccination on hospitalization rates as well as looking at death as on outcome. For example, one study concluded that one death was prevented for every 4,000 people vaccinated against the flu (Fireman et al, 2009).
  • In frail elderly adults, hospitalizations can mark the beginning of a significant decline in overall health and mobility, potentially resulting in loss of the ability to live independently or to complete basic activities of daily living. While the protection elderly adults obtain from flu vaccination can vary significantly, a yearly flu vaccination is still the best protection currently available against the flu.
  • There are limited data to suggest that flu vaccination may reduce flu illness severity; so while someone who is vaccinated may still get infected, their illness may be milder.
  • Fourth, it’s important to remember that people who are 65 and older are a diverse group and often are different from one another in terms of their overall health, level of activity and mobility, and behavior when it comes to seeking medical care. This group includes people who are healthy and active and have responsive immune systems, as well as those who have underlying medical conditions that may weaken their immune system, and therefore, their bodies’ ability to respond to vaccination. Therefore, when evaluating the benefits of flu vaccination, it’s important to look at a broader picture than what one study’s findings can present. Although flu vaccine is not perfect, the overall evidence supports the public health benefit of flu vaccination. Vaccination is particularly important for people 65 and older who are especially vulnerable to serious illness and death, despite the fact that the vaccine may not work as well in this age group.

Is the relatively low VE against flu A (H3N2) virus measured in this study among people 65 and older caused by changes in the virus?

Lower vaccine effectiveness can occur when flu viruses undergo small changes that trick the human immune system into not recognizing and fighting against them. These small changes occur over time in all flu viruses and are part of a process known as “antigenic drift.” For more information, see “How the Flu Virus Can Change.” CDC monitors for changes in flu viruses year-round. This season, most of the flu viruses analyzed by CDC continue to be “like” the viruses the flu vaccine is designed to protect against. While some changes in circulating flu viruses have been detected this season, this is normal since flu A viruses are constantly changing, and these changes have not been significant. Most importantly, the vast majority of circulating flu viruses have NOT shown changes expected to lower vaccine effectiveness.

What caused the low VE among people 65 and older against flu A (H3N2) viruses?

One possible explanation for this is that some older people did not mount an effective immune response to the flu A (H3N2) component of this season’s vaccine; however, it’s not possible to say this for certain. CDC recognizes the need for developing better flu vaccines in the elderly.

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Flu Activity During the 2012-2013 Season

When will flu activity peak?

The timing of flu activity is very unpredictable and can vary from season to season. Flu activity most commonly peaks in the United States in January or February. However, seasonal flu activity can begin as early as October and continue to occur as late as May. The 2012-2013 flu season began relatively early – in fact, 4 weeks earlier – compared to recent seasons (see Press Briefing Transcript: U.S. Influenza Activity and Vaccination Rates for Current Season). By January 11, 2013, flu activity was high across most of the United States, and flu activity remained elevated across the United States as of the week ending March 9, 2013. Although the timing of flu activity is not predictable, substantial flu activity can occur as late as May.

Who has been most severely impacted this flu season?

People 65 and older have been most severely impacted by the 2012-2013 flu season. As of February 9, 2013, more than half of reported flu-associated hospitalizations occurred in adults 65 years of age and older. Rates of flu-associated hospitalization among adults 65 years of age and older increased sharply from late December through January. CDC data has shown that the majority of deaths reported this season due to pneumonia and influenza (P&I) have occurred in people 65 years of age and older, and these deaths reached epidemic levels in early January. This is consistent with previous observations of the burden of flu illness in the elderly. Researchers have estimated that in most years 90% of flu-related deaths occur in people 65 and older, and the flu is a major contributor to hospitalizations in this age group. Data from statistical modeling studies looking at flu seasons from 1979 to 2001 estimate that as many as 60% of flu-related hospitalizations occur among people 65 and older.

How many children have died from the flu this season?

From September 30, 2012, to February 9, 2013, 64 flu-related deaths in children were reported to CDC. Sixteen deaths in children were associated with flu A H3N2 virus infection, 19 deaths were associated with flu A virus infection that was not subtyped, and 29 deaths were associated with flu B virus infection.

Are new flu viruses circulating this season?

Flu viruses are constantly changing, so it's not unusual for new flu viruses to appear. For more information about how flu viruses change, visit How the Flu Virus Can Change. CDC analyzes flu viruses that are circulating each season to see whether they are like the viruses the seasonal flu vaccine is designed to protect against. This so-called “antigenic characterization” data is published weekly in FluView. So far, most of the flu viruses that have been analyzed at CDC are like the viruses included in the 2012-2013 flu vaccine. However, some flu B viruses that have been analyzed by CDC do not match the flu B virus the 2012-2013 vaccine is designed to protect against.

(See FluView for more information).

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Flu Vaccine Information and Recommendations for the 2012-2013 Season

Is vaccine still available?

Flu vaccine is produced by private manufacturers, so availability depends on when production is completed. Information about the number of seasonal flu vaccine doses distributed this season is available at Seasonal Flu Vaccine & Total Doses Distributed.

In May and September 2012, flu vaccine manufacturers originally projected about 135 million doses would be available for the U.S. market during the 2012-2013 season. Recent updates from manufacturers to CDC indicate that more doses of flu vaccine were actually produced, totaling 145 million doses.

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.

Does CDC recommend prioritizing remaining supplies of flu vaccine?

No, CDC does not have a recommendation to prioritize remaining supplies of flu vaccine at this time. CDC continues to recommend flu vaccination for all people 6 months and older. It also continues to be especially important that people at high risk of flu complications get vaccinated, including pregnant women, children younger than 5 years old but especially younger than 2 years old, older adults 65 years and older, and people with chronic conditions like asthma, diabetes, and heart disease.

Does CDC recommend ongoing vaccination at this time?

CDC routinely recommends ongoing vaccination as long as flu viruses are circulating. While the 2012-2013 season has likely peaked, flu activity is ongoing and could continue for some time. During past seasons, significant flu activity has been observed as late as May.

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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 symptoms of flu this season. First, it’s possible that your illness was not caused by a flu 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 flu virus, the seasonal flu vaccine protects against the three flu viruses that research suggests will be most common. This means the flu vaccine can offer protection against other flu viruses you haven’t been exposed to yet.

Can I get vaccinated and still get the flu?

Yes. It’s possible to get sick with the flu even if you have been vaccinated (although you won’t know for sure unless you get a positive flu test). 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 flu vaccine would not be expected to protect against.
  • Unfortunately, some people can become 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 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 the flu.

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

Yes. CDC has received reports of some people who were vaccinated against the flu becoming ill and testing positive for the flu. This occurs every season. The 2012-2013 flu season was an early season, with more flu activity reported in early weeks than was seen during recent previous flu seasons. There are, however, a number of reasons why people who got a flu vaccine may still get the flu this season, see Can I get vaccinated and still get the flu.

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.

For the latest interim data on effectiveness of this year’s vaccine, see How well is the vaccine working this season?

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. More information about antiviral drugs and CDC’s recommendations are available at 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 close a match there is between viruses used to make the vaccine and circulating viruses. Data are published in the weekly FluView.

As of the week ending February 9th, most (91%) of the flu viruses that have been analyzed at CDC are like the viruses included in the 2012-2013 flu vaccine. The match between the vaccine virus and circulating viruses is one factor that impacts how well the vaccine works.

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Flu Antiviral Drugs Recommendations for the 2012-2013 Season

Are there supply concerns with antiviral drugs this season?

On January 10, 2013, the U.S. Food and Drug Administration (FDA) released information indicating there may currently be intermittent shortages of Oseltamivir Phosphate (Tamiflu) for Oral Suspension (6mg/mL 60 mL), due to increased demand for the drug. This is the pediatric (children’s) suspension (liquid). The manufacturer has instructions for pharmacists on how to compound an oral (by mouth) suspension from Tamiflu 75 mg (adult) capsules. These instructions provide for an alternative oral suspension when commercially manufactured oral suspension formulation is not readily available.

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