Egg-Based Flu Vaccines May be Less Effective: The Case of the H3N2 Vaccine

How are egg-based vaccines made?
Egg-based vaccines are made by injecting a virus into a fertilized, hen egg. The virus then continues to replicate in the egg as it is incubated, and then the virus is either killed ("inactivated") or weakened (attenuated). Inactivated viruses are more commonly used for flu shots, while nasal sprays commonly contain attenuated viruses (1).

Why are egg-based vaccines making headlines?
According to a study published on October 23rd, by researchers Wu, Zost, Thompson, Oyen, Nycholat, McBride, Paulson, Hensley,  and Wilson, egg-based vaccines appear to be getting less effective over time (2), and these observations may be possibly explained by the mutations that occur in the virus during the process of making egg-based vaccines (2). These mutations affect the structure of the virus, specifically receptor binding sites which are usually recognized by the immune system  and are critical for the immune system to recognize in order to build immunity(2). 

Wu et. al are recommending that manufacturers change their vaccine production method to a method in which the virus is less prone to mutations in these critical structures. An alternative method of vaccine production that has been proposed is cell-based flu vaccine production. The advantage to this method is that the virus can replicate more quickly, cells can be "frozen" and stored for later use, and the virus may be less likely to develop mutations which decrease its effectiveness (and which may be a product of the virus adapting to its egg host (4).

-EM

References:
1. https://www.cdc.gov/flu/protect/vaccine/how-fluvaccine-made.htm
2. http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1006682
3. http://www.contagionlive.com/news/egg-based-flu-vaccines-offer-lower-levels-of-protection-against-certain-virus-subtypes
4. https://www.cdc.gov/flu/protect/vaccine/cell-based.htm