Why Are Flu Vaccines Sometimes Ineffective?

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Why Are Flu Vaccines Sometimes Ineffective?

The Centers for Disease Control and Prevention (CDC) are reporting that 36 states are in the midst of widespread influenza outbreaks. It appears that this flu season might be a bad one, but why? And will the usual vaccines be effective in preventing you for getting the flu this winter?

You might think that, okay, I got the vaccine, so I can’t get the flu this winter. You might be surprised to know, however, that the CDC shoots for about 40-60% vaccine effectiveness in preventing the illness, not the 100% you expected. In fact, last year’s vaccine was 42% successful (less against H3N2), on the low end of the CDC’s target, but in 2014-2015, the prevention rate from the flu vaccine was about 19%.

(Note: This is neither an anti-vaccine nor pro-vaccine article. It is simply a discussion as to why influenza vaccines seem to be less effective in preventing disease in recent years.)

Influenza vaccines produced in a particular year are derived from certain proteins found in last year’s virus. The CDC makes a determination about what virus they expect to be dominant in the coming year, and companies base their production on those predictions. If you have the material from last-year’s flu, why are vaccines sometimes so ineffective in achieving their purpose? Mainly because of two concepts we’ll discuss today: Antigenic Drift and Antigenic Shift.

Antigenic Drift: Viruses are notorious for their ability to mutate, but fortunately their mutations rarely make a major change in their genetic makeup. When the changes are so small that the current virus going around is essentially the same as the previous one, it is called antigenic drift. In this circumstance, vaccines are more effective; that is, they reach the CDC goal of 40-60% prevention rate because they are fighting essentially the same virus.

Antigenic Shift: Sometimes, a virus undergoes a major mutation or two viruses combine to make a significant change in the nature of a virus. For example: If the Ebola virus primarily lives in fruit bats in Africa, some mutation along the way made it able to live in human beings. Perhaps another mutation made it more easily passed along to other humans in bodily secretions. These significant changes in the genetic makeup are called antigenic shifts. With antigenic shifts, all bets are off with regards to predicting the success of a vaccine in preventing disease. To take an extreme example, if the Ebola virus mutated to make it easily transmissible via airborne droplets, we’d be in a lot of trouble.

Classifying Influenza Viruses

The dominant virus causing this year’s influenza cases in the H3N2 Type A virus. Influenza viruses are classified according to the proteins that exist on their surface. These are called Hemagglutinins (HA) and Neuraminidases (NA). There are more and more different HA and NA subtypes discovered every year. The Swine flu, for example, is H1 N1.

Why is flu vaccine less effective against influenza A(H3N2) viruses?

The journal Proceedings of the National Academy of Sciences suggests that last year’s vaccine’s ineffectiveness occurred as a result of an antigenic shift in the H3N2 virus. Indeed, the flu vaccine seems to be generally less effective against influenza A(H3N2) that other viruses. Why? This from the CDC:

“While all influenza viruses undergo frequent genetic changes, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic change) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when the composition of the flu vaccine is recommended and the flu vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the flu vaccine works.”

In other words, H3N2 tends to develop mutations more often and faster than other common influenza viruses.

The CDC goes on to say: “Growth in eggs is part of the production process for most seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses.”

Therefore, the flu vaccine as currently produced is less effective in its purpose than we would like. More research is necessary to produce a vaccine that we can depend on to prevent the disease, and to make it safer for those opting to take it.


In the meantime, ask your physician for an anti-viral med (not an antibiotic!) that may decrease the duration and severity of the disease, such as oseltamivir (Tamiflu) or zanamivir (Relenza). As these drugs are more effective in the first 48 hours of symptoms, you should have these on hand, just in case your doctor isn’t able to see you quickly. I recently received an official CDC health advisory emphasizing the increased importance this year of these medications due to the issues related to the H3N2 virus. It also stated that early treatment may decrease the risk of hospitalization.

Joe Alton MD

Dr. Alton
Dr. Alton

Find out more about influenza and 150 other medical topics in the award-winning Third Edition of “The Survival Medicine Handbook: The Essential Guide for When Medical Help is Not on the Way”

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