Flu Still Flexing Its Muscle in Western States

February 01, 2013

Influenza continues to wane on a nationwide basis, but don't tell that to residents of the Western United states.

Of the 24 states reporting high levels of influenza-like illness (ILI), 19 were west of the Mississippi River during week 4 of 2013 ending January 26, according to the latest "Flu View" report on the 2012-2013 influenza season released Friday by the Centers for Disease Control and Prevention (CDC).

In the last week of 2012, 18 of the 29 states reporting high ILI levels lay east of the Mississippi.

This east-to-west migration of influenza is a long-standing pattern, according to the CDC.

Although the percentage of outpatient visits for ILI was declining throughout most of the country, it was rising steeply in week four of 2013 in 8 Western states: Alaska, Arizona, California, Hawaii, Idaho, Nevada, Oregon, and Washington. In the Midwestern states of Iowa, Kansas, Missouri and Nebraska, the ILI rate crept up slightly.

Nationwide, the ILI rate for week 4 was 4.2%, down slightly from week 3, according to the new Flu View report. This rate has been decreasing since it reached 5.6% in the last week of 2012.

Another key indicator of influenza activity — the percentage of respiratory specimens that test positive for the virus — also continued its gradual descent, going from 26.1% in week 3 of 2013 to 25.5% in week 4. And the number of states reporting widespread influenza fell to 42.

The rate of hospitalizations per 100,000 population attributed to influenza and pneumonia kept climbing, however, reaching a cumulative 25.9 for the influenza season so far.

Both the hospitalization and mortality rates for influenza typically lag behind those for influenza activity, because individuals infected with the virus may not become seriously ill right away, or promptly seek medical care.

Throughout the first 3 weeks of January, the percentage of deaths attributed to influenza in 122 benchmark cities was on the rise, but this rate decreased slightly to 9.4% in week 4.

The CDC issues its weekly Flu View report every Friday.

Table. Influenza Death Rate Decreases Slightly

Characteristics of the 2012-2013 Influenza Season

Week 52, 2012 (ending December 29)

Week 1, 2013 (ending January 5)

Week 2, 2013 (ending January 12)

Week 3, 2013 (ending January 19)

Week 4, 2013 (ending January 19)

Respiratory specimens testing positive for influenza

31.6%

32.8%

29.4%

26.1%

25.5%

States reporting widespread influenza activity

41

47

48

47

42

Deaths attributed to influenza and pneumonia in 122 benchmark cities

7%

7.3%

8.3%

9.8%

9.4%

Hospitalizations for influenza per 100,000 population (cumulative for influenza season)

8.1

13.3

18.8

22.2

25.9

Outpatient visits for ILI nationwide

5.6%

4.8%

4.6%

4.3%

4.2%

States reporting high levels of ILI

29

24

30

26

24

Source: Centers for Disease Control and Prevention

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