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Influenza season likely peaked in US; illness remains widespread

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Influenza illness declined for a second straight week in the United States, a good sign that the country has passed the peak of a record-setting influenza season.

According to the latest CDC data, the percentage of outpatient visits attributed to influenza-like illness (ILI) — a key surveillance marker — dropped from 7.5% to 6.4% in the week ending Feb. 17. It peaked at 7.7% earlier this month, matching the highest level of the 2009 pandemic and setting a record for a nonpandemic season.

“It looks like we have peaked, but ILI is still high [and] higher than the peak of many seasons,” Erin Burns, a spokeswoman in the CDC’s Influenza Division, told Infectious Disease News.

Influenza activity remained widespread in 48 states, according to the CDC data, with only Hawaii and Oregon continuing to see a break. ILI activity was still at the highest level in more than half of U.S. states.

The percentage of outpatient visits attributed to influenza-like illness declined for a second consecutive week.

Source: CDC

H3N2, an elusive influenza A virus that has been the main culprit in this season’s reduced vaccine effectiveness, remained the predominant strain, but the CDC noted a continued upswing in influenza B viruses, which in not unusual for late-season activity.

Overall, 75.5% of specimens tested in clinical labs were influenza A viruses. H3N2 accounted for 86.9% of influenza A viruses tested in public labs. The percentage of specimens tested in clinical labs that were influenza B viruses rose from 36.4% to 41.8%.

Two indicators of the severity of the season remained elevated: the cumulative rate of laboratory-confirmed hospitalizations due to influenza, which rose from 67.9 to 74.5 per 100,000 people, and the proportion of all U.S. deaths attributed to influenza or pneumonia, which ticked down from 9.8% to 9.5% but remained high.

The CDC reported 13 more influenza-associated pediatric deaths, bringing the total to 97 this season. – by Gerard Gallagher

Reference:

CDC. FluView. https://www.cdc.gov/flu/weekly/index.htm. Accessed February 23, 2018.

Disclosure: Burns works for the CDC.