There's been a lot about vaccines in the news recently, especially the flu vaccine, so we're going to concentrate on that in today's Healthcare Triage News.

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Unless you've been living under a rock, you've seen some headlines about how the flu shot this year is only 23% effective. Most of the headlines are panicky and angry. I don't know why.

Let's start with a simple explanation of what it means that this year's flu shot is "23% effective". What happens is that the CDC monitors people coming to doctors' offices with acute respiratory infections and checks them for influenza by laboratory testing. It then checks the proportions: who were vaccinated or unvaccinated. Then, they calculate effectiveness as 100% x (1-(the odds of being vaccinated among those with influenza/ the odds of being vaccinated among those without influenza)).

This is what they found: Of those with influenza, about 49% were vaccinated. Of those without influenza, about 56% were vaccinated. Converting those to odds gets you 0.957 for influenza positive people and 1.283 for influenza negative people.

So, effectiveness=100x(1-(.957/1.283))=about 25%, but that's unadjusted, so the CDC did some adjusting and it's 23%.

Let's remember the reality here: First of all, the differences in the percentages were 49% of those with flu were vaccinated vs. 56% of those without flu were vaccinated. That's a bad year. In a good year, say, 2013, the numbers were 32% and 56%, and that yields an effectiveness of 62%, but even in a good year, a third of people who were influenza positive were vaccinated.  My concern here is that people, and a lot of people in the media, have a rather poor understanding in general of what 'effective' means when it comes to healthcare.  For instance, people assume that a good flu shot is a lock against not getting the flu.  The CDC report isn't a randomized control trial, and I'm not gonna try to calculate the NNT, and you better have watched that video.  It's also not a perfect methodology to establish how the influenza vaccine reduces the absolute probability of your getting the flu.  But the differences between this year and others just really aren't as huge as many are saying they are, and it's just a shot, it's hard to see how the benefits don't still massively outweigh the harms, especially since we DO have randomized controlled trials.

To the research!  A meta-analysis of 17 flu shot studies showed that when the vaccine is well-matched, 1.2% of vaccinated people were infected with influenza, versus 3.9% of people not vaccinated.  That makes the number needed to treat, or NNT, for a well-matched flu shot to prevent one influenza infection to be 37.  When the match is poor or uncertain, however, the NNT is 77.  A couple of points here.  First of all, NNTs of 37 and 77 are pretty good, especially for something like a shot.  The NNT for many, many, many other things we consider no-brainers are much higher.  Also, 37 and 77, while different, aren't so different that we'd tell you to do one but not the other.  They're both in a class that would be recommended.  It's important to understand that even if you're vaccinated, you can still get the flu.  It's not like your chance went from 90% to 1%.  No matter what, you still need to practice good hygiene.  Wash your hands, stay home if you're sick, don't sneeze or cough on others, common sense stuff.  My issue here is that many are treating the news of this year's flu shot as if there's some monstrous difference from other years, and that we should behave differently this year than other years, and that's just not the case.  The difference between 60% effective and 23% effective is subtle.  It's real, but not that huge.  Yes, you should get a flu shot, because the NNT is still pretty great.  Herd immunity is important.  But you should always, always practice good infection control as well.

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