Clearing Up Misunderstandings on Recent Flu Efficacy Reports

Clearing Up Misunderstandings on Recent Flu Efficacy Reports

December 08, 2017

By Dr. Bob Lutz
Spokane Regional Health District, Health Officer

My counterpart at Public Health Seattle-King County, Dr. Jeff Duchin, who is a highly regarded expert in the realm of infectious diseases and epidemiology, wrote this week about the effectiveness of this year's flu vaccine and I wanted to take a moment to echo his sentiments.

Early reports out of the Southern Hemisphere are indicating that one component of the flu shot, for influenza A H3N2, may have only been 10% effective in Australia. Dr. Duchin points to several reasons, that I agree with, why Australia's situation is not necessarily predictive of what we should expect here. And most importantly, both of us, and numerous experts are still strongly recommending getting the flu shot.

Here is those reasons that Australia's situation may not be predictive of ours:

  • It is not unusual for vaccine effectiveness study results to vary from country to country (live attenuated influenza vaccine is a recent example where we saw low vaccine effectiveness in the U.S., while Canada and UK saw good effectiveness over multiple seasons).
  • Several factors can all lead to different vaccine effectiveness results:

    • Differences in underlying population immunity
    • Extent of previous vaccine coverage in the community
    • Relative proportion of infections in the community caused by the two types of influenza A (H3N2 versus H1N1)
    • Two types of influenza B that may circulate each season, as well as other factors, can all lead to different vaccine effectiveness results. Spokane County's first flu death this season was due to influenza B.
  • The current predominant strain in the U.S. is influenza A H3N2, which is the same strain that circulated last season and that circulated in the recent Southern Hemisphere flu season, and there have been no significant mutations detected in the influenza H3N2 strain. Last year in the U.S., vaccine effectiveness for the same influenza A H3N2 virus was 34%. Influenza B is also circulating, though at lower levels.
  • We can't be sure of how much illness each of the circulating types influenza viruses (A H3N2, A H1N1, two types of influenza B) will contribute until the season is over. Vaccine effectiveness against influenza A H1N1 and influenza B strains is usually much better than against H3N2, about 50-60% overall.

Bottom line: Even though we cannot predict which strains will predominate in the US this season, it is likely that influenza vaccination will provide meaningful protection against severe illness, hospitalization and death.

Influenza vaccine is especially important for pregnant women to protect both mother and her newborn, and for other people at increased risk for serious influenza infections, including complications, hospitalization and death. See the link below for who is at increased risk and should get vaccinated:

About Dr. Lutz
In addition to being the health district's health officer, Bob Lutz, MD, MPH, is also clinical director of Urgent Care for MultiCare Rockwood.

The health officer provides leadership for the protection of public health; enforcement of public health regulations; and serves as an advisor on matters affecting community health and safety to elected and appointed officials, members of the medical profession, and the public.

Dr. Lutz obtained his medical degree from Temple University; completed a residency in family medicine at Bayfront Medical Center in St. Petersburg, Florida, and fellowships in sports medicine and integrative medicine at Bayfront and University of Arizona respectively; and holds a master’s of public health degree. He is licensed to practice in Washington state and board certified in family medicine. Dr. Lutz is a member of the American Academy of Family Physicians, American Public Health Association and Wilderness Medical Society.