CDC Says Influenza Activity as Widespread as 2009 Pandemic

Alicia Ault

January 26, 2018

US influenza activity is now the most widespread since the 2009 influenza A(H1/N1) pandemic, said the Centers for Disease Control and Prevention (CDC) today in its latest weekly update on flu activity.

The rate of hospitalizations and deaths is now approaching — and may exceed, especially in pediatric deaths — that recorded in the severe 2014-2015 season, said CDC officials.

The entire country is being simultaneously walloped. Most of the severe illness is due to influenza A(H3N2), but the H1N1 subtype is also contributing to higher hospitalization rates among 50- to 64-year-olds, said Dan Jernigan, MD, MPH, director of the Influenza Division in the CDC's National Center for Immunization and Respiratory Diseases, in a briefing with reporters.

"For the past 3 weeks, the entire country has been experiencing lots of flu all at the same time," said Dr Jernigan. A rapid rise in cases after the winter holidays is likely being driven by children who returned to school, he said.

Forty-nine states have reported widespread activity for the third week in a row. Only Hawaii; Guam; Washington, DC; and the US Virgin Islands are reporting less illness. The hospitalization rate in the week that ended January 20 reached 41.9 per 100,000, equivalent to what was seen in 2014-2015, when some 710,000 Americans had been hospitalized by the end of the season. It's a substantial rise from the January 12 weekly report, when the rate was 22.7 per 100,000.

Nine percent of all deaths recorded in the United States for the week ended January 20 were due to the flu, and seven children died, bringing the season's total to 37 pediatric deaths. But this is likely a huge underestimate, said Dr Jernigan, noting delays in reporting. "There may even be twice as much as the number we have," he said, adding that the agency anticipates more reports next week.

In the 2014-2015 season, 148 children died.  "We anticipate that there will be more pediatric deaths this year," Dr Jernigan said. 

Almost 7% of all individuals being seen as outpatients or in the emergency department had influenza. "This is the highest level of activity recorded since the 2009 H1N1 pandemic, which peaked at 7.7%," said Dr Jernigan.

And, he said, "we have several more weeks of flu to go," noting that the flu has been active for 9 weeks currently and the average season lasts 16 weeks.

Baby Boomers Hard Hit

Although hospitalizations were highest among individuals over age 65 years (183.1 per 100,000), the rate continues to rise for 50- to 64-year-olds, said Dr Jernigan. That age group now has the second highest hospitalization rate—44.2 per 100,000—overtaking the youngest, the 0- to 4-year-old group (at 27 per 100,000), that usually occupies that slot most flu seasons.

"Baby boomers have higher rates than their grandchildren right now," said Dr Jernigan.

H3N2 is the most common strain identified in patients over 65 hospitalized, and in the baby boomer group. But H1N1 is contributing to higher hospitalization rates in 50- to 64-year-olds, Dr Jernigan said. For individuals over age 65, H3N2 is responsible for about 90% of hospitalizations, but in the younger group, it's a factor in 80%, he said. H1N1 causes about twice as much disease proportionately in 50- to 64-year-olds.

Different explanations exist for this difference, said Dr Jernigan. One is the concept of imprinting. "The first influenza virus that someone is exposed to as a child has a way of determining how you respond to influenzas the rest of your life," he said.

The over-age-65 cohort was likely first exposed to an H1N1 strain, which was the predominant circulating virus from 1918 to 1947. That could explain a strikingly low hospitalization rate for Americans older than age 65 after the 2009 H1N1 pandemic, he said. People born after 1947 were likely first exposed to an H3N2 virus, which could mean they are more susceptible to H1N1 strains. But it is a complicated mix of environmental, host, and viral changes, he said.

Vaccination Still Urged

The CDC is continuing to urge Americans to get a flu vaccine if they haven't already. "There is still a lot of the season to go, and vaccination now could still have some benefit," said Dr Jernigan.

The agency is also encouraging hand washing, covering a cough, and staying home from work or school if an illness is at all suspected. The very young, very old, and people with underlying heart or lung conditions should seek treatment immediately. Healthy individuals who have chest pain, ear pain, difficulty breathing, or persistent high temperatures should also visit their doctor or an emergency department, where they might be prescribed neuraminidase inhibitor (NAI) antiviral treatment, said Dr Jernigan.

Some spot shortages of oseltamivir suspension and generic capsules have been reported, but Dr Jernigan said the CDC is working with manufacturers to address the gaps. Three NAIs are approved by the US Food and Drug Administration and are recommended for the 2017-2018 season: oseltamivir, zanamivir, and peramivir.

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