The CDC recently shared interim statistics on the effectiveness of the influenza vaccine for the 2019-2020 flu season. I wanted to link to their main article, and provide some summary highlights that you can discuss with your colleagues and your patients and their families.

  • The data comes from the US vaccine effectiveness network and 4,112 children and adults with the flu between October 23, 2019 and January 25,2020. The network data is from 5 states; Michigan, Pennsylvania, Texas, Washington, and Wisconsin.
  • The predominant lineage for influenza B this year is B/Victoria lineage. Recall that B flu viruses are generally a little more virulent to younger children, and that they predominated earlier this season.
  • The predominant influenza A lineage this year has been our good friend H1N1 (97%), with a much smaller proportion being H3N2 (3%).
  • Vaccine rates were low overall, with 51% of 6 month to 8 year olds (470/915) and 34% of 9-17 year olds (164/481).
  • 37% of influenza-positive participants received the 2019–20 seasonal influenza vaccine, versus 55% of influenza-negative participants
  • The overall adjusted vaccine effectiveness was 45% against influenza A and B virus types combined, 50% against influenza B/Victoria, and 37% against A(H1N1)
  • Vaccine effectiveness was higher in the pediatric population and lower among adults aged 18–49 years, especially versus A(H1N1) (Vaccine effectiveness = 5%; 95% CI = -45% to 37%).
Overall interval influenza vaccines effectiveness for the 2019-20 season. Data abstracted from the CDC.

What does this data mean?

I could totally see a parent seeing the 45% number, not reading into the data and saying – “Well, confirms my suspicions. The flu vaccine doesn’t help so I’m not getting it.” It is important to note that the CDC defines this 45% effectiveness as “reduction in influenza illness associated with a medical visit.” Given that the estimate is as many as 21 million flu related doctor’s vistas in the last decade 45% ain’t so bad after all.

The data also has some limitations. Notably, it is only from 5 states, 4 of which utilized vaccine status self report as the marker of whether or not subjects were vaccinated. Second, there are a lot of patients, but ~4,100 pales in comparison to the total number of people with the flu. This data also is insufficient to truly estimate the overall effectiveness against B – H3N2, which again, was the early predominant lineage. Finally, and this is the big one, the effectiveness estimate is limited to the prevention of outpatient medical visits rather than more severe illness outcomes. The data on more severe outcomes (hospitalization and death) is promised for later this year.

Should we still be giving flu vaccines at this point in the season?

I’ll let the CDC handle this one…

The CDC recommends that health care providers continue to administer influenza vaccine to persons aged ≥6 months because influenza activity is ongoing, and the vaccine can still prevent illness, hospitalization, and death associated with currently circulating influenza viruses as well as other influenza viruses that might circulate later in the season.

Dawood et al, 2019


Dawood et al, Interim Estimates of 2019–20 Seasonal Influenza Vaccine Effectiveness — United States, February 2020. CDC Morbidity and Mortality Weekly Report, 2020.