US Influenza Activity Widespread and Intense, May Be Peaking

Megan Brooks

January 12, 2018

The United States is in the midst of a very active influenza season, with much of the continental United States experiencing widespread and intense flu activity, and although activity may be peaking, hospitalizations have almost doubled in just the past week, federal health officials with the Centers for Disease Control and Prevention (CDC) said today.

So far this season, influenza A(H3N2) has been the most common form of influenza. These viruses are often linked to more severe illness, especially in children and people age 65 years and older. "When H3 viruses are predominant, we tend to have a worse flu season, with more hospitalizations and more deaths," CDC Director Brenda Fitzgerald, MD, said during a press briefing.

The latest data from the national influenza surveillance system show that the influenza season may be peaking now, but "we know from past experience that it will take many more weeks for flu activity to truly slow down," Dr Fitzgerald said.

Hospitalizations Double in 1 Week

The preliminary vaccine effectiveness data from studies ongoing this season will not be available until at least mid-February. However, the information so far suggests that vaccine effectiveness against the predominant A(H3N2) viruses will probably be somewhere around the 30% range, the CDC said.

"While our flu vaccines are far from perfect, they are the best way to prevent getting sick from flu, and it is not too late to get one," Dr Fitzgerald said. As of this last month, manufacturers have shipped more than 151 million doses of flu vaccine, "so it should be readily available."

In the outpatient setting, there has been a "very rapid increase" in the numbers of people with influenza coming in to see their doctor, reported Dan Jernigan, MD, MPH, who directs the Influenza Division in the CDC's National Center for Immunization and Respiratory Diseases.

"We are also seeing a rapid rise in the numbers of people being hospitalized with laboratory-confirmed influenza," Dr Jernigan reported, with 22.7 hospitalizations per 100,000 this week, up from 13.7 per 100,000 last week. "That's almost a doubling of the numbers in just the last week," he noted.

The highest hospitalization rates are among those over age 65 years, but rates are also increasing for adults aged 50 to 64 years. Hospitalization rates for children younger than 5 years has "almost doubled" in the last week, Dr Jernigan said.  This week, an additional seven influenza-associated deaths have occurred among children, bringing the total number of flu-related pediatric deaths reported so far to CDC to 20, he noted.

Right now, this influenza season seems to be tracking "somewhat worse than 2012-13, but perhaps not as bad as 2014-15," Dr Jernigan said. In both seasons, H3 viruses were the predominant strains. In the 2014-15 season, vaccine effectiveness was very low and the United States had upward of 700,000 influenza hospitalizations. The current flu season is "bad," but we don't know yet whether it will be a "high-severity season or not," he said.

Also of note, although the influenza season is likely peaking, at least 11 to 13 more weeks remain in the influenza season and strains other than A(H3N2) will undoubtedly show up and be "a major cause of disease." The B viruses will show up later in the season, which is another reason to get vaccinated now. Influenza A(H1N1) is also starting to show up in some states, Dr Jernigan reported.

Start Antiviral Drugs Early

Given the characteristics of this season's flu, the CDC in late December 2017 issued a health advisory recommending, in addition to the flu vaccine for prevention, increased use of neuraminidase inhibitor  antivirals for treatment.

Antivirals can lessen the symptoms and shorten the duration of illness and prevent serious influenza complications, Dr Jernigan noted.  "CDC recommends that people who are very sick or people with flu symptoms who are at high risk for serious flu complications should be treated as early as possible with flu antiviral drugs.  This includes people 65 and older, young children, people with chronic conditions like diabetes, heart disease and asthma, and pregnant women.  Clinicians shouldn't wait for confirmatory testing but should begin treatment if they suspect flu in a severely ill or high risk patient," he said. Currently, there is no evidence of resistance to influenza antiviral drugs, he added.

While the national antiviral drug supply should be "sufficient to meet this high seasonal demand," some manufacturers are reporting delays in filling orders and spot shortages in some areas. CDC is working closely with manufacturers to address any gaps in the market. 

"In summary, we are seeing a very active flu season and early signs that the season may be severe," Dr Jernigan said. "Right now our most important tool for treatment is influenza antiviral medication. Our main tool for ongoing prevention should be flu vaccine. And of course we cannot forget the everyday common sense behaviors — stay away from people who are sick, stay home from work or school when sick, and cover your cough and wash your hands often."

More information on the influenza is available on the CDC's website.

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