Thursday, February 15, 2018

MMWR: Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness (VE) — US Feb. 2018


Two weeks ago in Eurosurveillance: Early Season Flu Surveillance & Vaccine Effectiveness (VE) - Canada, we saw estimates of Canada's VE with their H3N2 component providing only 17% protection, while their influenza B  component produced  a respectable 55%, even though the trivalent shot often used in Canada contained a Victoria strain this year, suggesting some cross protection.
While similar to what we've seen this year, Canada's flu season has featured a higher percentage of influenza B infections, and a less diverse set of H3N2 viruses than reported by the CDC.
Today, the CDC's MMWR has a preliminary analysis of the effectiveness of U.S. vaccines halfway through the flu season, and while similar to the Canadian results, nevertheless shows some interesting differences. According to today's report:
VE was estimated to be 25% (CI = 13% to 36%) against illness caused by influenza A(H3N2) virus, 67% (CI = 54%–76%) against A(H1N1)pdm09 viruses, and 42% (CI = 25%–56%) against influenza B viruses.
The overall U.S. VE (Vaccine Effectiveness) against both A & B subtypes was 36% (compared to Canada's 42%). Some age groups however - notably adolescents (9-17), and adults 50 and older - showed no statistically significant protection from the vaccine.
One bright spot is that kids aged 6 months through 8 years saw as much as 59% protection from this year's vaccine. 
The VE against influenza B (42%) - given the 55% VE from Canada and our greater use of quadrivalent vaccines - came in lower than I think most of us were expecting. Protection ranged from a respectable 50% for those aged 18-64, to a modest 25% for those 65+.
The best performance was against H1N1, but since that has been a minor player in this year's flu season, it has done little to bolster the overall VE numbers.
These are preliminary numbers, and are subject to revision as more data comes in. We'll get a more complete analyses after the flu season has ended.
While this year's vaccine may have produced lackluster results there is some evidence that even when it doesn't prevent the flu, it may reduce the severity of one's illness (see CID Journal: Flu Vaccine Reduces Severe Outcomes in Hospitalized Patients).
That said, we desperately need better flu vaccines.  While some progress has been made (quadrivalent vaccines, high-dose & adjuvanted vaccines) much more needs to be done (see CIDRAP: The Need For `Game Changing’ Flu Vaccines).

I've only included some excerpts from a much longer, far more detailed report.  Follow the link to read it in its entirety.
Interim Estimates of 2017–18 Seasonal Influenza Vaccine Effectiveness — United States, February 2018

Weekly / February 16, 2018 / 67(6);180–185

Brendan Flannery, PhD1; Jessie R. Chung, MPH1; Edward A. Belongia, MD2; Huong Q. McLean, PhD2; Manjusha Gaglani, MBBS3; Kempapura Murthy, MPH3; Richard K. Zimmerman, MD4; Mary Patricia Nowalk, PhD4; Michael L. Jackson, PhD5; Lisa A. Jackson, MD5; Arnold S. Monto, MD6; Emily T. Martin, PhD6; Angie Foust, MS1; Wendy Sessions, MPH1; LaShondra Berman, MS1; John R. Barnes, PhD1; Sarah Spencer, PhD1; Alicia M. Fry, MD1

In the United States, annual vaccination against seasonal influenza is recommended for all persons aged ≥6 months (1). During each influenza season since 2004–05, CDC has estimated the effectiveness of seasonal influenza vaccine to prevent laboratory-confirmed influenza associated with medically attended acute respiratory illness (ARI). 

This report uses data from 4,562 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network (U.S. Flu VE Network) during November 2, 2017–February 3, 2018. During this period, overall adjusted vaccine effectiveness (VE) against influenza A and influenza B virus infection associated with medically attended ARI was 36% (95% confidence interval [CI] = 27%–44%).
Most (69%) influenza infections were caused by A(H3N2) viruses. VE was estimated to be 25% (CI = 13% to 36%) against illness caused by influenza A(H3N2) virus, 67% (CI = 54%–76%) against A(H1N1)pdm09 viruses, and 42% (CI = 25%–56%) against influenza B viruses. 

These early VE estimates underscore the need for ongoing influenza prevention and treatment measures. CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks. Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths. Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated.


Early and widespread influenza activity during the 2017–18 influenza season provided the opportunity to estimate interim VE against several circulating influenza viruses, including the predominant A(H3N2) virus. These interim estimates reflect ongoing challenges with the A(H3N2) vaccine component since the 2011–12 season. 

The interim estimate of 25% VE against A(H3N2) viruses this season indicates that vaccination provided some protection, in contrast to recently reported, nonsignificant interim estimates of 17% from Canada and 10% from Australia (4,5) and is similar to final (32%) VE estimates in the United States against A(H3N2) viruses during 2016–17§ (6).

However, among children aged 6 months through 8 years, the interim estimates against any influenza and A(H3N2) virus infection were higher; the risk for A(H3N2) associated medically-attended influenza illness was reduced by more than half (59%) among vaccinated children. 

Also, with interim VE estimates of 67% and 42% against influenza A(H1N1)pdm09 and B viruses, respectively, vaccination provided substantial protection against circulating A(H1N1)pdm09 viruses, as well as moderate protection against influenza B viruses predominantly belonging to the B/Yamagata lineage, the second influenza type B component included in quadrivalent vaccines. 

What is added by this report?
So far this season, influenza A(H3N2) viruses have predominated, but other influenza viruses are also circulating. Based on data from 4,562 children and adults with acute respiratory illness enrolled during November 2, 2017–February 3, 2018, at five study sites with outpatient medical facilities in the United States, the overall estimated effectiveness of the 2017–18 seasonal influenza vaccine for preventing medically attended, laboratory-confirmed influenza virus infection was 36%.

What are the implications for public health practice?

CDC continues to monitor influenza vaccine effectiveness. Influenza vaccination is still recommended; vaccination reduces the risk for influenza illnesses and serious complications. Treatment with influenza antiviral medications, where appropriate, is especially important this season.

(Continue . . . )

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