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According to new surveillance statistics released on Friday from the Centers for Disease Control and Prevention (CDC), forty-seven states in the U.S. are now reporting widespread influenza activity . The virus, which first appeared in the Southeast, has reached epidemic levels.
Forty-seven states have reported widespread geographic influenza activity (i.e. incidence of flu) for the week between December 30, 2012 and January 5, 2013, and twenty-four states and New York City have reported high influenza-like illness (ILI) activity (i.e. the proportion of outpatient visits to healthcare providers for influenza-like illness; see map above) .
Earlier this month, Dr. Joe Bresee, Chief of the Epidemiology and Prevention Branch in CDC’s Influenza Division, said that “reports of influenza-like-illness are nearing what have been peak levels during moderately severe seasons” .
Flu deaths and the epidemic
Deaths from flu are much more common than many people think. Thousands of adults and hundreds of children die each year from the flu. Because flu seasons are unpredictable and often fluctuate in length and severity, a single estimate can’t be used to summarize seasonal flu-related deaths. Instead, the CDC uses a range of estimated deaths to represent the variability of flu.
From the 1976-1977 flu season to the 2006-2007 flu season, flu-associated deaths ranged from a low of ~3,000 to a high of ~49,000 people . Death certificate data and weekly influenza virus surveillance information was used to estimate how many flu-related deaths occurred among people whose cause of death was listed as respiratory or circulatory disease on their death certificate.
For the flu to be an epidemic, it has to cause 7.2% or more of deaths as reported through the 122 Cities Mortality Reporting System in a given week. During the first week of January 2013, the percentage of deaths caused by flu was 7.3%, just above the epidemic threshold for the first time this season.
That in itself isn’t disconcerting: flu outbreaks normally reach epidemic level for one or two weeks every season.
Googling indicators of flu activity
What is alarming are search trends for the flu. Google Flu Trends, which uses aggregated Google search data to estimate current flu activity around the world in near real-time, suggests that this year’s outbreak is the worst in the last six years (see graph below).
In 2009, Google determined that there is a close relationship between how many people search for flu-related topics and how many people actually have flu symptoms . The Flu Trends system counts how often Google sees a flu-related search and estimates how much flu is circulating in regions around the world.
In comparison to the CDC’s numbers, which are based on after-the-fact reports from healthcare providers around the country, Google’s numbers are based on an algorithm that can detect outbreaks nearly two weeks before they show up in CDC reports. And if their model turns out to be accurate, this year could be the most widespread flu outbreak in decades.
Michael Jhung, a medical officer in the CDC’s influenza division, said that he wouldn’t be shocked if that was true . Jhung’s models suggest that the virus hasn’t yet reached its peak. We will have a better idea this week Friday when the CDC releases data on the number of people that visited the doctor with respiratory symptoms during the week of December 30th.
Current influenza vaccine 62% effective
What about the influenza vaccine? Researchers have determined that the current influenza vaccine is 62% effective in reducing symptoms of the disease . This means that vaccination has reduced the risk for influenza-associated medical visits by approximately 60%.
Dr. Joseph Bresee, chief of the Epidemiology and Prevention Branch in CDC’s Influenza Division, said that the number is consistent with what the CDC expected :
Sixty-two percent effectiveness of the vaccine in a population that’s a broad population that includes both healthy people and a lot of elderly and sick people is what we’d expect from influenza vaccine in a year in which the circulating strains look like the strains that were included in the vaccine. If you look back over the last few years at the studies that CDC has done, this is in line with what we found and also in line with some recent reviews of vaccine trials that have been done over the last several years.
For the first week of January, scientists tested almost 13,000 respiratory specimens . Thirty-two percent tested positive: 80% were positive for influenza A, primarily H3N2 (only 1% of samples tested positive for the 2009 influenza A H1N1 strain). Influenza B was found in 20% of samples that tested positive; two-thirds of specimens were positive for a B/Wisconsin/1/2010-like virus. A second B strain from the B/Victoria lineage was identified in 6.7% of specimens.
Dr. Tom Frieden, director of the CDC, told reporters on Friday that the current vaccine includes both influenza A strains and the predominant influenza B strain :
About 90 percent of all of the strains circulating are included in [this year’s] vaccine. In fact, they’re the most — the three most common strains and the current vaccines have only three — have space for only three strains. So the pick of vaccine strains was as good as it could have been this year. The other close to 10 percent are a second influenza B.
The CDC expects vaccine manufacturers to have vaccines on the market within a year or two that have space for four different vaccines.
Protect yourself from the flu
There’s a lot you can do to protect yourself against the flu. First and foremost: get the flu vaccine. For the 2012-2013 flu season, based on the numbers above, you’re 60% less likely to get the flu. If you do contract the virus after being vaccinated, you’re likely to have less severe symptoms.
Besides the flu vaccine, washing your hands is the best way to stay healthy and avoid getting sick. Coughing and sneezing can spread cold and flu germs and hand washing is the single most important thing you can do to prevent the spread of infection. The trick is to wash your hands for at least 20 seconds.
If you have symptoms of the flu, stay home and limit contact with others. See your doctor. Early treatment with antivirals such as Tamiflu can reduce the severity of illness and keep you out of the hospital.
- Seasonal Influenza (Flu) – Weekly Report: Influenza Summary Update. Centers for Disease Control and Prevention. Accessed 2013 Jan 13.
- FluView: ILINet State Activity Indicator Map. Centers for Disease Control and Prevention. Accessed 2013 Jan 13.
- Flu Activity Picks Up Nationwide. Centers for Disease Control and Prevention. 2013 Jan 4.
- Ginsberg et al. Detecting influenza epidemics using search engine query data. Nature. 2009 Feb 19;457(7232):1012-4. doi: 10.1038/nature07634.
- Thompson et al. Updated Estimates of Mortality Associated with Seasonal Influenza through the 2006-2007 Influenza Season. MMWR 2010 Aug 27; 59(33):1057-1062.
- Going Viral. Slate. 2013 Jan 9.
- Early Estimates of Seasonal Influenza Vaccine Effectiveness — United States, January 2013. MMWR 203; 62(Early Release);1-4. 2013 Jan 11.
- Press Briefing Transcript — CDC Update: Flu Season and Vaccine Effectiveness. Centers for Disease Control and Prevention. 2013 Jan 11.