Earliest Flu Season Since 2003-2004 Plus H3 Predominant Strains Point to Severe Flu Year

Influenza activity continues to increase in the United States across most key indicators over the threshold that suggests that the flu season has started. Generally, flu peaks in January. The increase in late November and early December indicates that the peak will be reached at least a month earlier than usual. This is the earliest regular flu season in nearly a decade, since the 2003-2004 flu season. That was an early and severe flu year. While flu is always unpredictable, the early nature of the cases as well as the specific strains suggest that this could be a bad flu year. The strains in the community are influenza "a," H3N2 predominant. H3 predominant years tend to be the worst years. Influenza is a serious disease. It causes hundreds of thousands of hospitalizations a year and thousands of deaths.


High levels of influenza-like-illness (ILI) activity continue to be concentrated in the south central and southeastern regions of the nation, but other parts of the country are seeing increases as well. About 2.2 percent of all visits to doctors are for illnesses that are like the flu, a significant proportion of which will end up being flu. In states that have reported high level activity through the influenza-like illness surveillance system as many as four percent or more of their visits are for flu-related symptoms. It is expected that it would spread throughout the country as the season progresses.


The proportion of visits to doctors for influenza-like illness (ILI) increased once again and is above the national baseline for the second week in a row. Twelve states are now reporting high ILI activity (Alabama, Georgia, Illinois, Louisiana, Mississippi, Missouri, North Carolina, South Carolina, Tennessee, Texas, Utah, and Virginia) which is an increase from the eight states that reported high ILI activity last week.


Overall, influenza activity is being reported widely across much of the United States. Twenty-nine states reported widespread geographic influenza activity for the week of December 9-15, 2012. During the prior week, only 18 states reported widespread activity.


Since October 1, 2012, 1,013 laboratory-confirmed influenza-associated hospitalizations have been reported; an increase of 336 hospitalizations from the previous week. This translates to a rate of 3.6 influenza-associated hospitalizations per 100,000 people in the United States.


The proportion of deaths attributed to pneumonia and influenza (P&I) based on the 122 Cities Mortality Reporting System was below the epidemic threshold.


Two influenza-related pediatric deaths were reported during the week of December 9-15. One death was associated with an influenza A virus of unknown subtype. The second death was associated with an influenza A(H3) virus. Eight influenza-associated pediatric deaths have been reported so far during the 2012-2013 season.


Nationally, the percentage of respiratory specimens testing positive for influenza viruses in the United States during the week of December 9-15 decreased slightly to 28.3%. For the three weeks from November 25 to December 15, the regional percentage of respiratory specimens testing positive for influenza viruses ranged from 10.3% to 50.8%.


Influenza A (H3N2), 2009 influenza A (H1N1), and influenza B viruses have all been identified in the U.S. this season. During the week of December 9-15, 2,138 of the 2,709 influenza positive tests reported to CDC were influenza A and 571 were influenza B viruses. Of the 2,138 influenza A viruses identified, approximately 56% were H3 viruses and 1% were 2009 H1N1 viruses; 44% were not subtyped.


This year's strains look to be a great match with this year's vaccine. Since October 1, 2012, CDC has antigenically characterized 351 influenza viruses, including 10 2009 influenza A (H1N1) viruses, 226 influenza A (H3N2) viruses and 115 influenza B viruses.

All 10 of the 2009 influenza A (H1N1) viruses were characterized as A/California/7/2009-like. This is the influenza A (H1N1) component of the Northern Hemisphere vaccine for the 2012-2013 season.


Of the 226 influenza A (H3N2) viruses, 224 (99%) were characterized as A/Victoria/361/2011-like. This is the influenza A (H3N2) component of the Northern Hemisphere influenza vaccine for the 2012-2013 season.


Approximately 69% of the 115 influenza B viruses belonged to the B/Yamagata lineage of viruses, and were characterized as B/Wisconsin/1/2010-like, the influenza B component for the 2012-2013 Northern Hemisphere influenza vaccine. The remaining 31% of the tested influenza B viruses belonged to the B/Victoria lineage of viruses.


The vaccine is widely available. There are already over 120 million doses released and distributed. About 112 million Americans have been vaccinated so far this season, about 37 percent of people six months of age and older. Forty percent of children and 35 percent of adults have been vaccinated. Last year by the end of flu season, 48 percent of the general population, six months of age and older had been vaccinated. Coverage is higher among older people.


Over the past several years flu vaccination rates have increased, including among children, among pregnant women, and among health care workers. These are three key groups because they are so important to the spread of flu, and they're so vulnerable to severe disease in the case of pregnant women. While nearly half of all pregnant women have been vaccinated already, the biggest predictor of whether a pregnant woman is vaccinated is whether her obstetrician will offer a vaccine in the office. Nearly three-quarters of pregnant women who are offered a vaccine in the office get vaccinated.


Health care workers get vaccinated at different rates depending on their field. Eighty to 90 percent of pharmacists, doctors, and nurses have already been vaccinated this season. Fewer allied health workers, aids, and health care providers in nursing homes are being vaccinated. Work sites and pharmacies are major sources of vaccination for adults with more than a third of the vaccines being given either at work sites or pharmacies. While it is important to wash your hands, cover your mouth and remain home if you're sick, the vaccine remains the best tool to prevent influenza.


Since October 1, 2012, CDC has tested 18 2009 influenza A (H1N1), 344 influenza A (H3N2), and 135 influenza B virus isolates for resistance to neuraminidase inhibitors this season. The tested viruses showed susceptibility to the antiviral drugs oseltamivir and zanamivir. High levels of resistance to the adamantanes (amantadine and rimantadine) persist among 2009 influenza A (H1N1) and A (H3N2) viruses. Adamantanes are not effective against influenza B viruses.


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