Influenza Vaccine Effectiveness About 48% This Season

Influenza Vaccine Effectiveness About 48% This Season

February 21, 2017 Karie Youngdahl

Influenza Virion
Influenza Virion, CDC, Frederick Murphy

Are you a glass half empty or glass half full type of person? Your orientation along the optimism-pessimism spectrum will determine whether you think this year’s seasonal influenza vaccine is a moderate success or near failure. The Centers for Disease Control and Prevention released its interim estimate of the 2016-17 influenza vaccine effectiveness (VE) last week: the vaccine appears to be 48% effective at preventing physician-attended influenza illness.

This VE estimate is similar to last year’s estimate of 47% VE. You can see a table of VE for 2004-16 here.

How does the CDC get these data? CDC used five study sites, where they enrolled patients aged ≥6 months seeking outpatient medical care for an acute respiratory illness (ARI) with cough, within 7 days of illness onset. Researchers interviewed study subjects or their parents to collect respiratory specimens, demographic data, health status, symptoms, and 2016–17 influenza vaccination status. Specimens were tested at U.S. Flu Vaccine Effectiveness Network laboratories using CDC’s real-time reverse transcription – polymerase chain reaction (rRT-PCR) protocol for detection and identification of influenza viruses.

Among the 3,144 children and adults with ARI enrolled at the study sites from November 28, 2016, through February 4, 2017, 744 (24%) tested positive for influenza virus by rRT-PCR; 656 (88%) of these viruses were influenza A, and 90 (12%) were influenza B viruses. VE was higher for influenza B (73%) than for influenza A (43%).

It’s helpful to understand what epidemiologists mean when they use the term vaccine effectiveness (VE). A simplified way to determine VE is to use the following formula:

Risk among unvaccinated group − risk among vaccinated group/Risk among unvaccinated group = VE

So, if 82% of unvaccinated people in the study became sick with influenza and 28% of the vaccinated got sick, then VE is

82%-28%/82% = 65%.

In other words, the vaccinated group had 65% fewer cases of influenza than they would have had if they had not been vaccinated.

The Philadelphia Inquirer interviewed College of Physicians of Philadelphia Fellow and Temple University School of Medicine professor Bennett Lorber, MD, about this year’s VE. While acknowledging the less than stellar performance of the influenza vaccine, Lorber noted that it is the only protection we have, besides trying to avoid those who are sick and their germs. Moreover, he noted, "Suppose you got the flu and you have a household member with life-threatening lung disease…you might as well put a gun to their head and pull the trigger." He also noted that the influenza vaccine, even when it doesn’t prevent infection, can still reduce the severity, and symptoms, of an influenza infection. He told the Inquirer’s reporter, "Influenza is a serious illness. People die from this."

Lorber is hopeful that a universal influenza vaccine will be developed in the future. Such a vaccine would protect humans from influenza infection even as influenza viruses mutate from season to season. Although this piece is several years old, it provides a useful overview of possible approaches to such a vaccine. 

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