Influenza Season Still Going Strong, CDC Says

Seasonal influenza activity continued to rise in the United States during the week ending January 26 (week 4) and is not expected to peak for several weeks, according to the Centers for Disease Control and Prevention (CDC).

Key influenza indicators are still lower than they were last season, the CDC says.

National, Regional Influenza Activity Increased

During week 4, seasonal influenza activity was widespread in 45 states, up from 36 during week 3. Puerto Rico, Colorado, Indiana, and West Virginia reported regional activity, and local activity was reported by Alaska and Hawaii. The District of Columbia and US Virgin Islands experienced sporadic influenza activity (Guam did not report), according to the CDC's weekly FluView and an accompanying summary.

Two more children died during week 4.

Also during week 4, the proportion of outpatient visits for influenza-like illness rose from 3.3% to 3.8%, which is higher than the national baseline of 2.2%. "For comparison purposes, over the past five flu seasons, the peak percent of visits due to [influenza-like illness] has ranged between 3.6% (2015-2016) and 7.5% (2017-2018)," the CDC writes in its report summary.

High influenza-like illness activity increased to New York City and 23 states — up from New York City and 18 states the previous week. Activity was high in New York City and Alabama, Alaska, Arkansas, Colorado, Connecticut, Georgia, Indiana, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, Nebraska, New Jersey, New Mexico, North Carolina, Oklahoma, Rhode Island, South Carolina, Texas, Utah, Vermont, and Virginia.

Influenza-like illness activity was moderate in Puerto Rico and 10 states; low in the District of Columbia and 13 states; and minimal in four states (Delaware, Montana, North Dakota, and Ohio).

The highest rate of influenza-like illness this season occurred during the week ending December 29, 2018. All 10 regions reported a proportion of outpatient visits that was at or above their region-specific baseline level.

Influenza-Associated Hospitalizations

A total of 4423 laboratory-confirmed influenza-associated hospitalizations have been reported since October 1, 2018 through the Influenza Hospitalization Surveillance Network (FluSurv-NET). The Network covers approximately 9% of the United States, translating to a cumulative overall US hospitalization rate of 15.3 per 100,000 population.

The hospitalization rate was highest among adults aged 65 years and older (39.8 per 100,000), followed by children younger than 5 years (27.3 per 100,000) and adults aged 50-64 years (20.5 per 100,000).

Last season, the final cumulative overall hospitalization rate for week 4 was 57.7 per 100,000. During the last five influenza seasons, cumulative end-of-season hospitalization rates have ranged from 31.4 per 100,000 (2015-2016) to 102.8 per 100,000 (2017-2018).

Mortality

Deaths caused by pneumonia and influenza (P&I) have hovered at or above the epidemic threshold for 3 weeks in a row; however, "hospitalization rates remain relatively low compared with last season," the CDC explains.

Five more children died during week 3 (three children) and week 4 (two children), bringing the total this year to 24. During week 4 of 2018, 17 children died, bringing the total to 53 by that point in the season.

Both pediatric deaths reported during week 4 were related to an influenza A virus for which no subtyping was performed.

Of the three pediatric deaths reported during week 3, two were linked to an influenza A(H1N1)pdm09 virus and occurred during the weeks ending December 22, 2018 and January 12, 2019. One death was linked to an influenza A virus for which no subtyping was performed and occurred during the week ending January 5, 2019.

During week 3 (reported during week 4), the proportion of deaths caused by P&I was 7.2% — the epidemic threshold for that week in the National Center for Health Statistics Mortality Surveillance System. "P&I data for week 1 backfilled to 7.0%, which is at the epidemic threshold for that week. P&I has now been at or above the epidemic threshold for 3 weeks so far this season," the CDC writes.

"For comparison purposes, over the last five seasons, P&I has been at or above epidemic threshold for a range of 4 weeks (2015-2016) to 16 weeks (2017-2018)."

Influenza Type, Antiviral Susceptibility

Nationally, the percentage of respiratory specimens that tested positive for influenza viruses during week 4 was 19.2%, up from 15.7% during week 3 — the highest so far this season.

"For comparison purposes, since laboratory data from clinical and public health laboratories were disaggregated three seasons ago, the peak percent of respiratory specimens testing positive for flu at clinical laboratories has ranged from 23.6% to 27.4%," the CDC explains.

At the regional level, the 3-week average ranged from 11.3% to 21.9%.

Most (98.7%) influenza-positive laboratory specimens were influenza A, 75.2% of which were influenza A(H1N1)pdm09 viruses and 24.8% of which were influenza A(H3N2) viruses.

More than 99% of influenza viruses tested during week 4 were susceptible to oseltamivir, zanamivir, and peramivir; no new viruses with resistance to antiviral drugs were reported.

To date this season, two (0.5%) influenza A(H1N1)pdm09 viruses showed highly reduced antiviral susceptibility to oseltamivir and peramivir. Two other (0.5%) influenza A(H1N1)pdm09 viruses showed reduced antiviral susceptibility to oseltamivir only. All influenza viruses tested were susceptible to zanamivir.

The CDC cautions that reporting delays can result in data changing over time. The most current data for all weeks is available on the current FluView and FluView Interactive.

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