On February 16, 2018, the Centers for Disease Control and Prevention (CDC) published a report on the effectiveness of this year’s flu vaccine. The news was not good. At mid-season, the effectiveness of the vaccine against the most common type of flu virus circulating in the community was a disappointing 25%. The overall effectiveness, including against the less-common types of flu, was 36%.
Seasonal influenza virus changes every year. It appears that the virus is very good at changing its protein coat so as to evade immune detection. Every year, epidemiologists at the CDC gather in meeting rooms with reams of data in front of them, all of which represent data regarding the patterns of influenza infection in the U.S. They try to recommend the composition of next year’s vaccine. What they are doing is not unlike predicting the next hurricane.The epidemiologists scrutinize all the relevant data. They run the data through sophisticated machine-learning algorithms.
Then they take a guess.
More often than not, the CDC guesses wrong as to how effective the vaccine will be. In a review of the influenza vaccine effectiveness over the last 14 flu seasons (including this one), the average effectiveness of the vaccine overall was 40.5%. The range was a low of 10% in the 2004-2005 season to a high of 60% in 2010-2011. The effectiveness of the flu vaccine was less than 50% in all but three of the last 14 seasons. Even when the CDC correctly predicts the strains of the virus that will appear, as was the case this year, the vaccine that is produced still doesn’t work very well. When it guesses wrong about the strain, the situation is even worse. A vaccine against strains that are not in circulation is no good to anyone.
What does effectiveness mean when you’re talking about flu vaccine? It’s really quite simple. The CDC picks several clinic sites to monitor influenza activity. These are called “sentinel sites.” When patients appear with flu-like symptoms, the patients are tested for the flu virus. They are also asked if they received the vaccine. Based on the number of vaccinated patients who nevertheless contract the flu, a calculation is done that returns the effectiveness of the vaccine. Roughly speaking, 25% effectiveness means that if you got your flu shot this year, and you are exposed to the flu, there is a 25% chance that the vaccine will protect you. Another way of looking at is that if four vaccinated people are exposed to the same flu virus, three of them will get sick.
With this kind of track record for the vaccine, it’s reasonable for people to ask whether it’s worth it to get the vaccine. According to the CDC, the answer is yes. Based on its calculations from the 2014-2015 season, when the vaccine was only 20% effective, it estimates that a minimum of 11,000 hospitalizations and 300 deaths were avoided.
What do you think?