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Flu Vaccine Effectiveness 101

Posted on January 15, 2020

One of the common reasons people cite for not getting an annual flu vaccine is that they think it doesn’t work. Flu vaccine effectiveness (VE) indicates how well flu vaccination helps protect individuals against flu-related illness, hospitalization, and even death. The important thing to remember is that regardless of how high (or low) VE may be every year, so long as it’s above ZERO, that means flu vaccination can be beneficial in some way. Even if flu vaccination doesn’t prevent you from getting sick, it can still be effective at preventing severe flu-related outcomes such as hospitalization and death.    


Even if flu vaccination doesn’t prevent you from getting sick, it can still be effective at preventing severe flu-related outcomes such as hospitalization and death.  


What exactly does flu vaccine effectiveness mean?

VE compares flu-related illnesses, hospitalizations, or deaths in vaccinated individuals to unvaccinated individuals, and VE estimates vary from season to season. VE is not just represented by a single number, but actually represents many different numbers based on the type of study, flu strain(s), age groups, and outcome (i.e., prevention of flu-related illness, hospitalization, or death). 

The VE number most of us are familiar with is the one overall number that represents average effectiveness against medically-attended flu illness (meaning that someone was so sick they had to go to a doctor) for ALL flu strains across ALL age groups. This estimate comes from the U.S. Flu VE Network (described below).  

For the 2018-2019 flu season, the overall flu VE number was 29%, meaning that flu vaccination, on average, reduced a vaccinated individual’s risk of medically-attended, flu-related illness by 29% – again, for all flu strains and across all age groups. This observational study included over 10,000 patients. If we dive deeper into the available data for the 2018-2019 season, we can see that VE varied according to flu strain (i.e., H1N1, H3N2, and influenza B) and age groups (i.e., 6 months to 8 years, 9 – 17 years, 18 – 49 years, 50 – 64 years, and 65+ years). Of note, VE against H1N1 for children age 6 months to 8 years was 59% (see table), which is considerably higher than the overall VE of 29%. So it’s important to remember that VE represents more than just a single data point.  

How does the CDC estimate flu vaccine effectiveness every year? 

The Centers for Disease Control and Prevention (CDC) conduct studies every year to determine how well flu vaccines are working. The CDC works with universities and hospitals and uses three different networks consisting of several study sites to estimate flu vaccine effectiveness using observational studies: 

  • The U.S. Flu VE Network collects and tests respiratory specimens from outpatient visits to measure the number of patients with laboratory-confirmed flu infections. Comparisons are made between vaccinated and unvaccinated individuals. 
  • The Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) measures flu vaccine effectiveness at preventing flu-related hospitalizations in adults greater than 17 years of age. Hospitalized patients with respiratory illness are tested for influenza. The frequency of laboratory-confirmed flu illness in hospitalized patients is compared between vaccinated and unvaccinated individuals. 
  • The New Vaccine Surveillance Network (NVSN) measures flu vaccine effectiveness at preventing flu-related hospitalizations in children ages 6 months to less than 17 years of age. The frequency of laboratory-confirmed flu illness in hospitalized patients is compared between vaccinated and unvaccinated individuals. 

Besides these three CDC networks, are there other studies on flu vaccine effectiveness?

Yes. CDC, as well as other organizations and institutions, have conducted flu vaccine effectiveness studies throughout the years, focusing on different target populations, flu strains, age groups, and outcomes. As examples, one study found that flu vaccination in pregnant women can reduce their chances of flu-related hospitalization by 40%, and another study found that flu vaccination in children can reduce pediatric deaths in otherwise healthy children by 65%.  

Can flu vaccination still be beneficial even if it’s not well-matched with circulating strains?

Yes. Although flu strains may change from year to year (known as “antigenic drift”), if those changes are only mild to moderate, it’s still possible that people may receive some protection through vaccination. And remember the flu vaccine provides protection against three or four different flu strains, so even if one strain changes slightly, the vaccine still offers protection against the other flu strains included in the vaccine.    

Sources:

https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htm 

https://www.cdc.gov/flu/vaccines-work/effectivenessqa.htm 

https://pediatrics.aappublications.org/content/early/2017/03/30/peds.2016-4244 

https://academic.oup.com/cid/article/68/9/1444/5126390 

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