What You Should Know for the 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 determine how well the vaccine protects against illness. These estimates provide more information about how well this season’s vaccine is working.

On January 11, 2013, CDC published interim early estimates of the 2012-2013 flu vaccine’s effectiveness at preventing medical visits due to laboratory-confirmed flu among people enrolled at five study sites across the United States. These estimates were adjusted for site, but were not adjusted for age or other potential confounders. CDC reported an overall VE of 62% with a 95% confidence interval of 51% to 71%. (Note: the confidence interval reflects how precise the VE estimate is. More information about confidence intervals is available below.)

On February 21, 2013, CDC published these updated and adjusted estimates in the Morbidity and Mortality Weekly Report entitled: “Interim Adjusted Estimates of Seasonal Influenza Vaccine Effectiveness—United Sates, February 2013”. Interim data from 2,697 children and adults enrolled through the U.S. Influenza Vaccine Effectiveness (Flu VE) Network during December 3, 2012-January 19, 2013, were used to estimate the overall effectiveness of seasonal flu vaccine for preventing laboratory-confirmed flu virus infection. Overall, the VE estimate was 56% (95% confidence interval [CI]: 47% to 63%), after adjustment for age, study site, race/ethnicity, self-rated health, and days from illness onset to enrollment. This finding is very similar to the earlier interim estimate and falls within the 95% confidence interval for the original estimate. The adjusted VE against laboratory-confirmed flu A (H3N2) and flu B was 47% (95% CI: 35% to 58%) and 67% (95% CI: 51% to 78%), respectively. These estimates also are comparable to early, unadjusted VE estimates against flu A and flu B, which were 55% (95% CI: 39% to 67%) and 70% (95% CI: 56% to 80%), respectively.

These results indicate the vaccination with the 2012-2013 flu season vaccine reduced the risk of flu-associated medical visits from influenza A (H3N2) by one half and from influenza B by two-thirds for most of the population. Overall VE estimates suggest that the 2012-2013 influenza vaccine has moderate effectiveness for most age groups against circulating flu viruses, similar to previously published reports. The one exception to this was the VE among people 65 and older against influenza A (H3N2) viruses. The adjusted VE against outpatient medical visits due to laboratory-confirmed influenza A (H3N2) in adults aged 65 and older was 9% (95% CI:-84% to 55%).

These overall estimates are within the range of what is expected during seasons when most circulating flu viruses characterized by CDC are like the viruses included in the vaccine, which is what we are seeing this season. These findings also are similar to those published in a recent meta-analysis (Osterholm et al., 2011), which summarized the benefits of flu vaccines using data from randomized controlled clinical trials. In addition, the estimates also are consistent with mid-season flu vaccine effectiveness (VE) estimates for preventing medically-attended flu in Canada and the United Kingdom published in the journal Eurosurveillance on January 31, 2013.

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

Why is the updated adjusted vaccine effectiveness (VE) estimate (56%) lower than CDC’s previous VE estimate (62%) for the 2012-2013 seasonal vaccine?

The second VE estimates published in February included an additional 3 weeks of data collected from the five study sites during the peak of the flu season and also was adjusted for age, race/ethnicity, self-rated health and days from illness onset to enrollment. However, the two estimates are not significantly different. In January, CDC published a point estimate for VE of 62% with a 95% confidence interval of 51% to 71% (i.e., There was a 95% chance that the same study, if repeated, would yield a finding within the confidence interval). A confidence interval provides an upper and lower boundary of the estimate and also gives an indication as to the precision of an estimate. The updated interim estimate is for a VE point estimate of 56%, which is well within the original confidence interval. Both estimates indicate moderate vaccine effectiveness in preventing outpatient medical visits due to circulating flu viruses in most of the population. CDC continues to recommend annual flu vaccination.

Do flu vaccines work in people 65 and older?

Some older people, including those with certain chronic illnesses, are at greater risk of serious flu complications, but also may respond less well to vaccination. Human immune defenses become weaker with age, which places some people 65 and older at greater risk of flu-related complications. People 65 and older account for about 65% of flu-related hospitalizations each year and about 90% of flu-related deaths. Aging and chronic health problems can also diminish the body’s ability to mount a protective immune response after flu vaccination, which can result in lower levels of vaccine effectiveness (VE) in some older people. Reported VE estimates in people 65 and older have varied. Some years, significant protection has been measured. In other years, little to no protection has been measured. In general, however, when overall VE results are stratified by age, the flu vaccine seems to work less well in people 65 and older.

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Should people 65 and older still get vaccinated?

Despite the fact that flu vaccines seem to 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 VE against medically attended flu infection can be lower among older people, there are seasons when significant benefit can be observed in terms of averting illness. Even if the vaccine provides less protection than it might to 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 outpatient and urgent care medical visits due to flu infection/illness. This is just one outcome. There are other studies that look at the effects of vaccination on severity of illness and hospitalization rates as well as looking at death as on outcome. There is also some data to suggest that vaccination can reduce illness severity; so while someone who is vaccinated may still get infected, their illness may be milder. There also are studies that show that flu vaccination can prevent significant numbers of hospitalizations and deaths. The authors of one study entitled, “Influenza Vaccination and Mortality: Differentiating Vaccine Effects From Bias,” (Fireman et al, 2009) concluded that vaccination prevented approximately 25 deaths per 100,000 people 65 and older who were vaccinated, or one death was prevented for every 4,000 people vaccinated.
• Fourth, it’s important to remember that people who are 65 and older are very heterogeneous. This group encompasses 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 one study finding can present. Although flu vaccine in not perfect, overall the evidence supports the public health benefit of 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 H3N2 measured in this study among people 65 and older caused by changes in the virus?

Lower vaccine effectiveness can result when circulating flu viruses change so that the immune system of the vaccinated person doesn’t recognize and/or respond to the altered virus (these are called antigenic changes). CDC monitors for genetic and antigenic changes in circulating flu viruses year-round. This season, most of the circulating flu viruses analyzed by CDC continue to be “like” the vaccine viruses. While some genetic changes have been detected, this is normal since flu A viruses are constantly changing, and these genetic changes have not been significant. Most importantly, the vast majority of circulating flu viruses have NOT shown antigenic changes that would signal potential problems with vaccine effectiveness.

What caused the low VE among people 65 and older against influenza A H3N2 viruses?

One possible explanation for this is that some older people did not mount an effective immune response to the H3N2 component of this season’s vaccine; however, it’s not possible to say that for certain.

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Flu Activity During the 2012-2013 Season

When will flu activity peak?

The timing of flu 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 – 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 February 9, 2013. During the most recent weeks, decreases have been observed in the South and East, while increases have continued in the West. Although the timing of flu activity is not predictable, substantial activity can occur as late as May. During the past 10 flu seasons, the proportion of people visiting doctors for influenza-like illness (ILI) remained at or above baseline for an average of 12 consecutive weeks, with a range of 1 week (2011-2012 season) to 16 weeks (2005-2006 season). During the pandemic, the proportion of visits to doctors for ILI remained above the national baseline for 19 consecutive weeks.

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, 2012, more than half of flu-associated hospitalizations were reported to have 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. The weekly percentage of deaths attributed to pneumonia and influenza (P&I) was above the epidemic threshold beginning early in January, with the majority of the P&I deaths occurring in adults 65 years of age and older. This is consistent with what has been observed previously, as researchers have estimated that 90% of flu-related deaths occur in people 65 and older, and the flu is a major contributor to hospitalizations in seniors. Data from 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 influenza A (H3N2) virus infection, 19 deaths were associated with influenza A virus infection that was not subtyped, and 29 deaths were associated with influenza 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 included in that season's vaccine. 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 influenza B viruses that have been analyzed by CDC do not match the influenza B virus included in the 2012-2013 vaccine.

(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 under 5 years but especially younger than 2 years, 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 a flu-like illness 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 three types of flu viruses that research suggests will be most common. This means the 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 vaccine would not be expected to protect against.
• Unfortunately, some people can get infected with the 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 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. This is an early season, with more flu activity being reported at this time than has been seen during recent flu seasons. CDC is watching the situation closely and will provide additional information as it becomes available. 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 suspension (liquid). The manufacturer has instructions for pharmacists on how to compound an oral 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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