There has been a lot of buzz and confusion about the efficacy of influenza vaccines in the last couple of weeks. The Center for Disease Control and Prevention’s (CDC) recent press release has led to many articles claiming that the influenza vaccine will not work and questioning the need for the vaccine. The CDC’s press release stated that early data suggests the 2014-2015 flu season could be more severe and that the vaccine might be less effective. To understand why this year’s influenza vaccine will be less effective, one must understand the influenza virus.
There is no one influenza virus. There are three many type of influenza, type A, B, and C. All influenza viruses belong to the Orthomyxoviridae family. Influenza C rarely causes serious infections in human beings. Influenza A and B more commonly cause infections, with A being the most virulent.
Influenza A is further broken down into different subtypes. The subtypes are based off viral components called hemagglutinin and neuraminidase. Hemagglutinin is a glycoprotein that allows the attachment of the virus to target cells. Neuraminidase is a glycoprotein that allows the viral RNA to enter the target cell. Sixteen hemagglutinin subtypes are known (H1-H16), but only H1, H2, and H3 are known to cause disease in humans. Nine neuraminidase subtypes (N1-N9) are known, but only N1 and N2 are known to cause infections. The two Influenza A viruses in this year’s vaccine are H1N1 and H3N2. Influenza A can be broken down one more time to the specific strain. This involves the site of origin, the isolate number, and year of isolation. So the full names of the two influenza A viruses in this year’s vaccine are A/California/7/2009 (H1N1) and A/Texas/50/2012 (H3N2).
Now that the specific type of Influenza is understood, there are a couple of terms that one must understand: epidemic, pandemic, antigenic drift and antigenic shift. Epidemic is a widespread occurrence of an infectious disease in a community at a particular time. Pandemic is prevalent over a whole country or the world. An example of an epidemic would be a large amount of cases of influenza in a certain city. A pandemic would be a large amount of cases over the entire United States. The CDC is warning that there might be more severe epidemics, but not pandemics.
The other two terms are not as easy to define. Antigenic drift and antigenic shift both involve the influenza virus changing, but to different extents. Antigenic drift are slight mutation of the virus, but the H and N categories stay the same. Antigenic shift is when there is a major change in the virus, either the H or N or both categories change. An antigenic shift is what leads to pandemics. Examples of antigenic shift are the Spanish Influenza outbreak of 1918, the Asian Influenza outbreak of 1957, and the Hong Kong Influenza outbreak of 1968.
With this information, we can now understand what is happening this year. There has been an antigenic drift of the influenza A H3N2 virus. The World Health Organization and CDC release their recommendations for the influenza vaccines usually in February of the year the vaccine is used. This allows pharmaceutical companies time to manufacture the vaccine. Most years this time delay doesn’t affect the vaccine’s effectiveness. But this year the influenza A H2N3 has mutated since the recommendation. This antigenic drift has led to the influenza vaccine becoming less effective. The last time an antigenic drift like this occurred was in the 2007- 2008 flu season. In that flu season the vaccine had an overall efficacy of 37% and an efficacy of 42% against H3N2.
So should a patient still receive the influenza vaccine this year? Yes. Even though the H3N2 virus has mutated, the other viruses appear to have stayed the same. Many patients have a hard time understanding why they need to get a new vaccine every year. As demonstrated above, the influenza virus is constantly changing. Most other diseases with vaccines, such as rubella, measles, mumps, and chickenpox, only require a one or two time vaccination. Those viruses do not mutate or mutate very rarely compared to influenza.
While the vaccine has shown to be less effective this year, it might help prevent future influenza viruses. In 1977 an antigenic shift H1N1 virus emerged, causing a pandemic. The pandemic was more severe for younger patients, those born after 1957. The proposed reason for this was the prevalence of H1N1 from 1918 to 1956. This allowed those born before 1957 to have immunity to the new H1N1 that came about in 1977. The ability for immunization to work years later on a similar but mutated virus is called original antigenic sin.
Understanding the influenza virus is the first step in fighting its spread. Over the course of the flu season approximately 25 to 50 million people will become infected with influenza. Around 200,000 will be hospitalized and around 30,000 will die in the United States. The total cost to the United States healthcare system is more than $40 billion. Hopefully with this information, pharmacists will be able to convince their patients to get the influenza vaccine. While it may not be as effective this year, it is still the best tool we have to fight influenza.
Pharm.D. Candidate 2015
St. Louis College of Pharmacy
Early Data Suggests Potentially Severe Flu Season. Center for Disease Control and Prevention. http://www.cdc.gov/media/releases/2014/p1204-flu-season.html. Published: December 4, 2014. Accessed: December 10, 2014.
Dolin R. Chapter 187. Influenza. In: Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson J, Loscalzo J. eds. Harrison’s Principles of Internal Medicine, 18e. New York, NY: McGraw-Hill; 2012. http://accesspharmacy.mhmedical.com/content.aspx?bookid=331&Sectionid=40726944. Accessed December 10, 2014.
Brooks GF, Carroll KC, Butel JS, Morse SA, Mietzner TA. Chapter 39. Orthomyxoviruses (Influenza Viruses). In: Brooks GF, Carroll KC, Butel JS, Morse SA, Mietzner TA. eds. Jawetz, Melnick, & Adelberg’s Medical Microbiology, 26e. New York, NY: McGraw-Hill; 2013. http://accesspharmacy.mhmedical.com/content.aspx?bookid=504&Sectionid=40999961. Accessed December 10, 2014.