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Home » 2018-19 flu vaccine effectiveness – interim CDC report

2018-19 flu vaccine effectiveness – interim CDC report

Last updated on March 6th, 2019 at 11:49 am

The CDC recently issued an interim report on the 2018-19 flu vaccine effectiveness. Usually, after these reports, there are all kinds of consternation from everyone with a stake in the seasonal flu vaccine.

Last year, the flu vaccine effectiveness was a challenge. Typically, the anti-vaccine religion (or terrorists) massively understated the effectiveness of last year’s vaccine, despite good evidence that, although it wasn’t great, it was fairly good against most subtypes of flu.

Vaccine effectiveness is an important measurement to determine how many individuals who are vaccinated against the flu are actually protected from the infection by the vaccine. Despite the wild pseudoscientific claims about various flu treatments, the best preventive tool against influenza and influenza-related diseases, complications, and deaths are flu vaccines.

Preliminary statistics showing the burden of flu this year.


Well, let’s get the facts straight with what we know so far about the 2018-19 flu vaccine effectiveness.

2018-19 flu vaccine effectiveness – interim report

The interim report from the Centers for Disease Control and Prevention (CDC), which was completely funded by the CDC, used data from surveillance reports that evaluated vaccine effectiveness against laboratory-confirmed influenza between 23 November 2018 and 2 February 2019.

The CDC interim report estimates that the overall seasonal influenza vaccine effectiveness is 47% against laboratory-confirmed influenza. For children aged 6 months-17 years, overall 2018-19 flu vaccine effectiveness was 61%.

Other than vaccine effectiveness, the interim report also found the following:

  • 14% of children/adults with acute respiratory infections (ARI) tested positive for influenza (98% influenza A, 2% influenza B).
  • 74% of the influenza A viruses were the H1N1 subtype, while 26% were the H3N2 subtype.
  • Vaccine effectiveness for all ages was 46% against medically attended ARI caused by the H1N1 virus infection and 44% against the H3N2 subtype.
  • Among children aged 6 months–17 years, the vaccine effectiveness against H1N1 was 62%.
  • Among adults ≥50 years, vaccine effectiveness against all influenza types and H1N1 was 24% and 8%, respectively. However, those numbers were not considered statistically significant. 

This study has some limitations that may change significantly with larger end-of-season reports.

  • Small sample size, n=3,254 patients. The CDC’s end-of-year reports included several times that number.
  • There might be some bias in the study since all the data is self-reported.
  • Data limited to outpatient medical visits vs severe complications/hospitalizations.
2018-19 flu vaccine effectiveness
Catching the flu is more than just this. Photo by Kelly Sikkema on Unsplash

What can we say about the 2018-19 flu vaccine effectiveness?

The overall 2018-19 flu vaccine effectiveness, 47%, is much higher than last season’s 36%. However, the vaccine effectiveness for children, especially against the H1N1 and H3N2 subtypes, is about the same as last year – upper 50s to lower 60 percent.

The first words from the vaccine deniers are to claim that it’s useless. Of course, they rely upon the Nirvana fallacy, that is, if it’s not perfect, it’s useless. 

Moreover, it’s part of the bad math skills of the anti-vaccine terrorists. If vaccine effectiveness isn’t 100%, they round it down to 0%. They think the vaccines don’t work. 

And they always round up the extremely rare adverse events (and flu vaccine serious adverse events are extremely rare) to 100%. It is frustrating to engage in conversation about the vaccine when the vaccine deniers can’t even get the math right.

Another odd math failure of the anti-vaxxers is to claim that the flu vaccine is no better than random.

Uh no.

Yes, if you flip a coin, which is completely random, you would expect that heads or tails would appear 50% of the time. Any deviation from that probably means there’s some extenuating non-random factor influencing the outcome.

Vaccines and flu infections are most certainly not random, outside of some chance variables like standing in a store next to some guy hacking out his mucous for you to inhale. 

Think of it this way – a good baseball player gets a hit about 30% of the time. A really good player does it about 35%. If we thought baseball was totally random, then any player would get a hit 50% of the time based on some cosmic coin toss.

Now, there is some randomness to the game. A ball can be hit, and a fielder gets a late start and can’t catch the ball. Or a ball is hit and a breeze created by the flapping of butterfly wings in China changes the course of the ball. But all of the randomnesses of the game works both ways – it can cause a batter to get a great hit or nothing.

The difference between me, who would probably bat 0.00001 in Major League Baseball or someone who hits 0.350 isn’t randomness, it’s skill. 

So to take my analogy to vaccines, effectiveness isn’t a stochastic process, it’s based on the right choice of antigens, the timing of the vaccination, and many other factors that escape my reptilian mind. 

If you get the seasonal flu vaccine, you have a 47% chance of not contracting the flu, because of the vaccine, not because of a flip of a coin. If you don’t get the vaccine, then you will have a substantially higher risk of contracting the flu, approaching 90% if you’re in an area that has a particularly severe outbreak. 

In other words, the flu vaccine can reduce your risk of flu by 50% at least. And once again, the flu is not a trivial disease, despite the claims of the vaccine deniers. 


All that being said, 50-60% vaccine effectiveness is outstanding considering that the CDC and other country’s public health authorities must choose influenza antigens for the vaccine six months in advance. Eventually, we will have better flu vaccines cause the immune system to target antigens on the flu virus that mutates less.

The flu vaccine effectiveness myths are just a part of the larger myths against the flu vaccine. They have all been debunked dozens of times by dozens of science writers, but still, people make up these odd excuses to not get the vaccine. 

But let’s remember something – the flu is a dangerous and very deadly disease. So far this year the CDC estimates that 13-22 thousand Americans have died of the flu. And no, it’s not just the very young and very old that are at risk. It’s people with all kinds of chronic health issues who are at risk. Sometimes, it’s healthy young adults.

The vaccine works. It’s safe. And it protects us.


Michael Simpson

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