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2012-2013 Flu Season Drawing to a Close

March 29, 2013 – Flu activity is continuing to decline, according to this week’s FluView, which reports that influenza-like-illness (ILI) in the United States has fallen below baseline for the first time since early December. Other indicators are declining as well, signaling that the flu season is drawing to a close. The March 29, 2012 FluView covers influenza activity reported from March 17-23, 2013.

Flu seasons vary in their timing and severity.

This season, influenza activity started about 4 weeks early and was intense. Influenza-like-illness rose quickly to well above the baseline of expected activity and remained elevated for 15 consecutive weeks, making this season slightly longer than average. For the past 10 seasons, ILI has remained above baseline an average of 12 consecutive weeks with a range of one week to 16 weeks.

Surveillance systems that track flu-associated hospitalizations and deaths give insight into the severity of the season.

Flu-associated hospitalizations began to rise in mid-December with sharp increases seen among people 65 years and older. The cumulative hospitalization rates among seniors went from 17 per 100,000 people for the week ending December 15, 2012 to 182 per 100,000 during March 17-23, 2013. While hospitalization rates have leveled off, this is the highest proportion of persons 65 and older hospitalized for flu that has been measured since this kind of record-keeping began during the 2005-2006 season. People 65 and older accounted for 50 percent of all reported hospitalizations. The cumulative influenza-related hospitalization rate across all age groups is now 42 per 100,000 people.

Surveillance for deaths tells a similar story. CDC monitors flu-related deaths through the 122 Cities Mortality Reporting System, which reports the total number of death certificates processed and the number of those for which pneumonia or influenza is listed as the underlying or contributing cause of death in 122 U.S. cities. Pneumonia and influenza diagnoses (P & I) first rose above the epidemic threshold the week ending January 5, 2013 and peaked the week ending January 19, 2013 at 9.9 percent. This was the highest recorded P & I in nearly a decade, but it is comparable to recorded percentages for past severe seasons, including 2003-2004 when P&I reached 10 percent. P&I was above epidemic threshold for 11 weeks this season.

Pediatric deaths are the only nationally notifiable outcome for seasonal influenza. CDC reported an additional 5 pediatric deaths this week for a total of 110 to date for the 2012-2013 season. The number of pediatric deaths is likely to climb further as additional deaths are reported.

The 2012-2013 season will close out on September 28, 2013. The 2013-2014 season begins on September 29, 2013.

Last season, which is an example of a mild season, there was very little influenza activity and ILI was elevated only slightly for just one week. Comparatively, the 2012-2013 season is more like the 2003-2004 and 2007-2008 influenza seasons, which were characterized as “moderately severe” and associated with more flu illnesses, hospitalizations and deaths.

This season, an estimated 134.9 million doses of seasonal flu vaccine were distributed. Final vaccine uptake estimates are expected in the fall of 2013.

Just as flu seasons vary, how well the flu vaccine works each season can vary too. Each year CDC studies how well the seasonal flu vaccine protects against having to go to the doctor for influenza illness. This season, early flu activity allowed CDC’s U.S. Flu Vaccine Effectiveness Network to collect enough data across five sites to publish interim results of vaccine effectiveness (VE) twice; once in January 2013, and updated adjusted results on February 21, 2013.

February results showed the 2012-2013 flu vaccine reduced the risk of flu-associated medical visits from influenza A (H3N2) viruses by one half and from influenza B by two-thirds for most of the population. This means that vaccinated people reduced their risk of having to go to the doctor for influenza by more than half, thus providing significant benefit to the person getting vaccinated and to public health in general.

CDC’s estimates for influenza vaccine effectiveness against medically attended illness during 2012-2013 are consistent across age groups, with the exception of among people 65 and older. The overall VE against influenza A and B viruses among people 65 and older was 27 percent. While VE for people 65 years and older was similar to other age groups for influenza B viruses, it was lower for influenza A (H3N2) viruses. The reasons for this aren’t clear, but it’s possible that some people 65 and older did not mount a good immune response to H3N2 from vaccination.

Vaccination, particularly for people 65 and older, remains a priority. According to CDC, while it can vary in how well it works, flu vaccination is the best tool currently available to protect against influenza infection. CDC recommends that – with rare exception – everyone aged 6 months and older should get an annual flu vaccination to protect themselves and their loved ones against the flu.

For more information on this season’s VE, including the answer to the question “Should people 65 and older still get vaccinated?” see Vaccine Effectiveness for 2012-2013.