CDC Says U.S. Seeing Latest Flu Season in Nearly Three Decades

Still Not too Late to Vaccinate

Three-dimensional rendering of influenza A virus
February 10, 2012 — According to the CDC influenza surveillance report “FluView,” flu activity in the United States increased for the week ending February 4, 2012. The percentage of respiratory specimens testing positive for influenza nationally rose to 10.5 percent from 7.6 percent the prior week. This is the first time this season that the percent of respiratory specimens testing positive for influenza has surpassed 10 percent, which is generally a marker to indicate that flu season is beginning.

By this measure, flu is off to a late start this season. In the past 29 years, the percent of respiratory samples testing positive for flu has remained below the 10 percent mark until February only once before (1987-1988).

CDC tracks influenza activity through 8 surveillance categories. At the moment, other influenza activity indicators are lagging as well. Influenza-like-illness (ILI) remains below baseline nationally, which is again late for this time of year. However, this week, two regions of the country – the central and northwest regions – are reporting ILI activity above baseline for the first time this season, and one state – California – is the first to report widespread geographic influenza activity this season. Additionally, the second pediatric death of the season was reported in this week’s FluView, which was associated with an unsubtyped influenza A virus.

These key flu indicators are expected to increase in the coming weeks. “The increases we are seeing in the number of respiratory samples testing positive for flu should forecast increases in other flu activity indicators in the coming weeks,” says Lyn Finelli, chief of domestic surveillance for CDC’s Influenza Division.

CDC recommends that if you haven’t gotten vaccinated yet, you should get your vaccine now. You need this season’s vaccine to protect against influenza this season. It takes about two weeks after vaccination for the body’s immune response to fully kick in. CDC recommends that everyone 6 months and older get an annual flu vaccine. Though still too early to tell for the season, so far, most of the U.S. influenza viruses tested have been well-matched to circulating influenza viruses. Most of these viruses have been influenza A (H3N2) viruses, but in recent weeks the proportion of 2009 H1N1 viruses have been increasing, particularly in states bordering Mexico. Recently Mexico has reported that 2009 H1N1 viruses are predominating during their ongoing flu season.

The timing of flu seasons is unpredictable and can vary in terms of when the season starts, when it peaks and when it ends. Flu season can begin as early as October and last as late as May. Typically flu season is said to begin when certain key flu indicators remain elevated for a number of consecutive weeks. One of these indicators is the percent of respiratory specimens testing positive for flu. When this exceeds 10 percent, this is one marker that flu season is beginning.

In addition to universal vaccination, CDC also recommends the use of influenza antiviral medications as a second line of defense against the flu. The CDC guidance in particular emphasizes early antiviral treatment as soon as possible for patients who are severely ill and for those who are at greatest risk for complications from influenza.

This includes:

  • hospitalized patients with suspected or confirmed influenza,
  • People with severe or progressive illness,
  • Outpatients who are at high risk for influenza complications (for example, young children, people 65 and older, pregnant women, and persons with certain underlying chronic medical conditions).
  • In addition, because consistent clinical benefit of early oseltamivir treatment has been found in reducing the risk of lower respiratory tract complications such as those requiring antibiotics, persons with uncomplicated influenza who are not in a high risk group and who present within 48 hours of illness onset can be treated with antiviral medications based upon clinical judgment.

CDC estimates that influenza virus infections in the United States result in an average of more than 200,000 related hospitalizations, and between 3,300 to 49,000 deaths each year, depending upon the severity of the influenza season.

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