An appropriate question for this time of year, to be sure. As with many healthcare topics, the various terms and concepts surrounding vaccines can make
it difficult to find trustworthy, understandable information on the topic. Fortunately,
that’s exactly what we do here.
At the time of writing, the only nasally administered influenza (flu) vaccine FDA approved for sale in the United States is called FluMist, manufactured by MedImmune. This product is unique in that it uses four different strains of “live,” attenuated influenza virus (or LAIV), as opposed to the inactivated influenza virus found in the flu shot.
Some quick definitions are in order. You may hear the vaccine referred to as quadrivalent. This just means it has four strains of the virus that are expected to be most prevalent that year. Live refers to a whole virus capable of replicating and infecting. Attenuated, used here, means that the live viruses are made to be lousy at said replicating/infecting.
The idea is that a live virus can be administered directly where it naturally initiates infection, which is exactly what the vaccine does. This allows the immune system to deal with it as a kind of dress rehearsal, producing stronger immunity that would also cover a broader range of strains. The immune system can handle a lot, but as George Carlin said, “…it needs practice, it needs germs to practice on.”
So why don’t you get the flu from it? Simply put, it can’t handle the heat. Viruses are phenomenal at mutating and rearranging the genetic material they carry, which is why the flu vaccine is reformulated every year to keep up. Taking advantage of this property, the manufacturer replicates the selected four strains of flu virus in a culture using chicken eggs as a growth medium.
The viral strains are run through 30-40 cycles of replicating, each time lowering the temperature until it reaches around 30°F. Along the way, each new generation of viruses adapts to replicate at the lower temperatures. In doing that, they lose the ability to replicate at “nose-temperature,” if you will. The result is a vaccine that can be stored in the refrigerator safely, but when administered loses the ability to cause serious infection.
Now, the Center for Disease Control (CDC) studies the effectiveness of all approved flu vaccine products in the United States every year, using data gathered through the U.S. Influenza Vaccine Effectiveness Network. After reviewing the data from the 2015-16 flu season, they estimated the vaccine effectiveness for LAIV in the studied age group (children 2 years to 17 years) was around 3%. In comparison, the inactivated flu vaccine (the “flu shot”) had an estimated effectiveness of 63%. Similarly poor or lower-than-expected effectiveness was also reported for LAIV in 2013-14 and 2014-15 seasons.
Nasal Flu Vaccine Recommendations
The Advisory Committee on Immunization Practices (ACIP) is panel of immunization experts that advises the CDC on immunization recommendations. Because of these poor effectiveness results, ACIP recommended that the nasal flu vaccine should not be used for the 2016-17 flu season. This recommendation against use was renewed for the 2017-18 flu season.
While the vaccine (sold as FluMist in the US) is still FDA-approved and being manufactured, such a strong recommendation against use from a major regulatory body makes it very unlikely that any medical practitioner would administer it. However, legally, they still can.
It is not yet known what exactly caused the decreased effectiveness. This uncertainty is made more confounding because this decline was not seen from this product in Canadian and European patient populations. The manufacturer is reportedly working on it, but the feeling is that ACIP is unlikely to change their recommendation for several years, and the effectiveness data will have to be consistent and at the previous level for it to do so.
Usually “shots for everyone” is heard in a more celebratory atmosphere, but it appears that’s the way it is for the time being. Thanks for your question!
Lisa A. Grohskopf, MD, Leslie Z. Sokolow, MSc, MPH, Karen R. Broder, MD, et al. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2017–18 Influenza Season. Recommendations and Reports; 66(2);1–20. Published August 25, 2017. Accessed December 2, 2017.
CDC Newsroom. ACIP votes down use of LAIV for 2016-2017 flu season. CDC Newsroom Media Statement. https://www.cdc.gov/media/releases/2016/s0622-laiv-flu.html (archived). Published June 22, 2016. Accessed December, 2017.
Wen He, Wei Wang, Huamin Han, et al. Molecular Basis of Live-Attenuated Influenza Virus. PLoS. Published: March 26, 2013. https://doi.org/10.1371/journal.pone.0060413