February 20, 2019, 02:02 pm News Staff – On Feb. 15, the CDC released a pair of Morbidity and Mortality Weekly Reports (MMWRs) offering interim information on the 2018-2019 influenza season and addressing this season's flu vaccine effectiveness (VE).
First, for the period of Sept. 30 through Feb. 2, the CDC said that although influenza A(H1N1)pdm09 viruses have predominated in most areas of the country, influenza A(H3N2) viruses have predominated in the southeastern United States.
The agency also said the overall number of influenza B viruses reported has been low; influenza B/Yamagata viruses were more commonly reported from September through late December, and influenza B/Victoria viruses have been reported more frequently since late December.
Influenza-like illness (ILI) activity and the percentage of respiratory specimens that have tested positive for influenza in clinical laboratories have been increasing since mid-January, agency officials said.
In good news, VE data collected from children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network from Nov. 23 through Feb. 2 indicate that the flu vaccine has reduced the risk of medically attended acute respiratory virus infection (ARI) by 47 percent.
Even better, children ages 6 months to 17 years who were vaccinated against flu this season reduced their risk of contracting influenza A(H1N1)pdm09 illness by more than 62 percent and their risk for illness from all influenza types by 61 percent.
For all ages, the CDC said VE against medically attended ARI caused by A(H1N1)pdm09 virus infection was 46 percent, and VE against illness due to influenza A(H3N2) was 44 percent.
This season, the percentage of outpatient visits for ILI reached 4.3 percent as of Feb. 2. In comparison, peak ILI activity the past two A(H1N1)pdm09-predominant seasons was 3.6 percent during the 2015-2016 season and 4.6 percent during the 2013-2014 season.
Influenza-associated hospitalization rates and pneumonia-/influenza-attributed mortality also have been relatively low this season and are consistent with previous seasons when influenza A(H1N1)pdm09 viruses predominated, the agency noted.
"However, preliminary cumulative in-season prevalence estimates indicate that influenza has caused 155,000-186,000 hospitalizations and 9,600-15,900 deaths," the CDC said.
This is the first season that the CDC has reported preliminary estimates of influenza prevalence, the agency noted, and prevalence estimates will be updated each week for the remainder of the season.
Current CDC forecasts predict that influenza activity will remain elevated in parts of the United States for several more weeks.
From Oct. 1 to Feb. 2, a total of 5,791 laboratory-confirmed influenza-related hospitalizations were reported to the CDC, for a cumulative incidence of 20.1 per 100,000.
As is seen during most seasons, adults 65 and older had the highest hospitalization rates, followed by young children. This season, the hospitalization rate was 53 per 100,000 among adults 65 and older and 33.5 per 100,000 among children younger than 5.
Of the total number of hospitalizations, 93.8 percent were for influenza A virus infection, and 5.2 percent were for illness caused by influenza B virus. Digging deeper, among hospitalizations associated with influenza A infection for which subtype information was known, the vast majority were for A(H1N1)pdm09 virus illness; less than one-quarter were for illness caused by A(H3N2).
According to one MMWR report, as of Feb. 2, a total of 28 laboratory-confirmed influenza-associated pediatric deaths among children and adolescents younger than 18 had been reported to the CDC from New York City and 21 states; one death occurred in a non-U.S. resident.
Of these pediatric deaths, 54 percent were associated with an influenza A(H1N1)pdm09 virus infection, 7 percent with an influenza A(H3N2) virus, 36 percent with an influenza A virus for which no subtyping was performed, and 4 percent with an influenza B virus.
The mean age of pediatric deaths reported to the CDC was 6.5 years, with a range from 8 months to 15 years.
The CDC continues to recommend that health care professionals, including family physicians, offer and encourage influenza vaccination for all unvaccinated patients ages 6 months and older as long as influenza viruses are circulating.
"Annual influenza vaccination is the first and best defense against influenza infection," the agency said in its MMWR. "Depending on the vaccine formulation (trivalent or quadrivalent), influenza vaccines can protect against three or four different influenza viruses.
"With vaccine effectiveness in the range of 30 percent to 60 percent, influenza vaccination prevents millions of infections and medical visits and tens of thousands of influenza-associated hospitalizations each year in the United States."
For example, during the 2017-2018 season, the CDC said vaccination averted an estimated 7.1 million illnesses, 3.7 million medical visits, 109,000 influenza-associated hospitalizations and 8,000 influenza-associated deaths.
Finally, the agency noted that regardless of vaccination status, it's important that patients with confirmed or suspected influenza who have severe, complicated or progressive illness; who require hospitalization; or who are at high risk for influenza complications be treated with antiviral medications.
The FDA has approved four antiviral drugs for treatment of acute uncomplicated influenza within two days of illness onset that the CDC recommends for use during the 2018-2019 flu season: oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) and baloxavir marboxil (Xofluza).
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