CDC influenza surveillance data confirms severity of flu season

Analytics help agency forecast that country will likely have several more weeks of elevated disease activity.

As the Centers for Disease Control and Prevention continues to track this year’s severe flu season, initial data from the CDC’s national surveillance system indicates that the severity will be as high or higher than what was seen in the 2014-2015 season, agency officials predict.

In fact, hospitalizations for the 2017-2018 flu season have already reached the highest levels since the CDC began tracking such data closely in 2010. Overall, a cumulative rate of 51.4 laboratory-confirmed, influenza-associated hospitalizations per 100,000 population was reported by the agency, with the highest hospitalization rate for people who are 65 years and older.

Alicia Budd, an epidemiologist in the Influenza Division of the CDC’s National Center for Immunization and Respiratory Diseases, provided the latest update on flu activity for clinicians last week, using data based on the most recent report for week 4, ending on January 27.

If results continue to track with the agency’s analysis of this past data, Budd believes there could be about 35 million illnesses this season, approximately 700,000 hospitalizations and at least 56,000 deaths associated with the flu.

According to Budd, the CDC works with public health partners to track influenza-like illnesses (ILI) nationally, using a surveillance system that leverages eight different sources of data, including information from physicians’ reports about patients with ILI who are seeking medical attention.

She said data from the CDC’s Influenza-like Illness Surveillance Network (ILInet), which includes about 2,500 outpatient providers, indicate that nationally for the week ending January 27 the proportion of outpatient visits for ILI was 7.1 percent—which is the tenth consecutive week that it has been above the national baseline of 2.2 percent.

“This season is currently following within expected range in terms of the number of weeks of elevated activity, but the actual level of activity is pretty remarkable,” Budd observed.

Through the end of December, ILI was tracking very closely with the 2014-2015 and 2012-2013 seasons, she added. However, since the end of December, ILI activity this season has continued to increase beyond those two prior seasons and is “now at the highest level that we’ve seen since the pandemic in 2009.”

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“The data are reported to CDC at least weekly, and each week, we analyze and summarize the data and feed the information back to both public health providers and to the general public,” said Budd, who added that the weekly data is reported to the agency from a network of about 250 clinical laboratories across the country that test respiratory specimens for diagnostic purposes.

So far this flu season, these labs have reported information on almost 600,000 tested specimens. The CDC uses this data to determine the timing and intensity of flu activity.

“The overall percent of specimens testing positive for influenza was rising sharply through late December and has been holding roughly steady for the past three weeks at about 26 percent positive,” noted Budd. “In recent weeks, the percent of specimens testing positive for influenza A has been decreasing, while the percent positive for influenza B viruses has continued to increase. This isn’t really an unexpected finding as we often do see increases in influenza B activity later in the flu season.”

There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses.

When it comes to pneumonia and influenza mortality, as of the week ending January 13, 9.1 percent of deaths were attributed to pneumonia and influenza. It’s the fourth week that the system has been above the epidemic threshold this flu season.

Budd said influenza A H3N2 is the predominant strain of the virus this season, which is often linked with higher mortality and hospitalization rates among older adults and young children, though influenza A(H1N1)pdm09 and influenza B viruses are circulating as well.

“During this most recent week, 80 percent of the viruses reported by public health labs were influenza A, and 20 percent were influenza B,” she observed. “Of the influenza A viruses with subtype information, 84 percent were H3 viruses. For the B viruses that we had lineage information on, 91 percent belonged to the Yamagata lineage.”

Influenza B viruses are not divided into subtypes but are further broken down into lineages and strains. Currently circulating influenza B viruses belong to one of two lineages: Yamagata and Victoria.

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