Flu Shot a Mismatch for Main Strain, Season Far From Over: CDC

Troy Brown, RN

January 15, 2020

Influenza is still going strong in the United States and isn't expected to slow down for at least several more weeks, according to a report from the Centers for Disease Control and Prevention (CDC).

What's more, this season's vaccine is only a 58% match for B/Victoria, the strain that is hitting children especially hard.

"It's not a very good match for B/Victoria," Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, told CNN. "It's not an awful match, but it's not a very good match."

Nationally, the predominant virus is B/Victoria, followed by A(H1N1)pdm09 viruses, with the predominant virus varying by region and age group. A(H3N2) and B/Yamagata viruses are circulating in low numbers.

"Key indicators that track flu activity remain high, but indicators that track severity are not high at this point in the season," the CDC explains in the report.

Although levels of outpatient visits for influenza-like illness (ILI) remain elevated, hospitalization rates and the percentage of deaths resulting from pneumonia and influenza remain low.

"This is likely due to the predominance of influenza B/Victoria and influenza A(H1N1)pdm09 viruses which are more likely to affect children and younger adults than the elderly. Because the majority of hospitalizations and deaths occur among people age 65 and older, with fewer illnesses among that group, we expect, on a population level, to see less impact in flu-related hospitalizations and deaths," the CDC explains.

Outpatient visits for ILI activity fell from 7% the previous week to 5.8%. "The decrease in the percentage of patient visits for ILI may be influenced in part by changes in healthcare seeking behavior and influenza virus transmission that can occur during the holidays," the CDC notes in its report.

Regionally, the percentage of outpatient visits for influenza ranged from 3.6% to 8.6%, with all regions reporting a percentage of outpatient visits for influenza that were equal to or higher than their region-specific baselines.

The percentage of respiratory specimens that tested positive for influenza in clinical laboratories fell to 23.6% from 26.4% during the last week of 2019.

ILI activity was high in the District of Columbia, New York City, Puerto Rico, and 33 states; moderate in six states (Alaska, Indiana, Michigan, Minnesota, Rhode Island, and South Dakota); and low in eight states (Florida, Hawaii, Missouri, Montana, Nevada, Ohio, Vermont, and Wyoming).

Activity was minimal in New Hampshire and there were insufficient data to calculate ILI activity for Delaware, Idaho, and the US Virgin Islands.

Geographically, influenza activity was widespread in Puerto Rico and 46 states, regional in three states (Mississippi, North Dakota, and Vermont), local in the District of Columbia and Hawaii, and sporadic in the US Virgin Islands. Guam did not report.

Hospitalizations and Deaths

According to the CDC's estimates, there have been at least 9.7 million illnesses, 87,000 hospitalizations, and 4800 deaths from influenza this season.

Between October 1, 2019, and January 4, 2020, 4228 laboratory-confirmed hospitalizations were reported. Of those, 2299 (54.4%) were linked to influenza A virus, 1906 (45.1%) to influenza B virus, 13 (0.3%) to influenza A virus and influenza B virus coinfection, and 10 (0.2%) to influenza virus, which had not been typed.

Among patients for whom influenza A subtype information was available, 461 (86.0%) were A(H1N1)pdm09 virus and 75 (14.0%) were A(H3N2) virus.

The overall cumulative hospitalization rate was 14.6 per 100,000 population. The rate was highest for those aged 65 years or older (33.3%), followed by children younger than 5 years (26.8) and those aged 50 to 64 years (17.0).

The percentage of deaths from pneumonia and influenza edged up to 5.8% (epidemic threshold 6.9%), from 5.5% during week 52 of 2019.

During week 1 of this year, the CDC received reports of five influenza-associated pediatric deaths that occurred during the weeks ending December 28, 2019, and January 4, 2020. Three of those were linked to influenza B viruses for which lineage was not determined, and two were related to influenza A(H1N1)pdm09 viruses.

So far, the CDC has received reports of 32 influenza-associated pediatric deaths that occurred this season, compared with 16 at this point last season. Of those, 21 deaths were related to influenza B viruses, five of which had lineage determined and were all B/Victoria viruses. Eleven deaths were linked to influenza A viruses, six of which underwent subtyping and were all A(H1N1)pdm09 viruses.

Influenza B viruses can cause severe illness in adults and children; however, adults often have built-up immunity from previous infections, whereas children may not have.

Both influenza A viruses and influenza B viruses can cause serious illness in adults and children; therefore, the CDC recommends vaccination for everyone aged 6 months or older and antiviral medications as soon as possible after illness onset.

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