The following post is by Alan Cassels, a regular contributor to the blog who is also a drug policy researcher at the University of Victoria, Canada.
“Normally, the flu vaccine is between 50 to 60 percent effective”
— Dr. Tom Frieden, Director of the Centers for Disease Control and Prevention (CDC)
How effective is the flu shot?
That’s an important question that generates many headlines across North America every fall as the public health community starts ramping up its fall flu campaigns. Problem is, the media tends to generate a lot of noise around that number, but provide very little clarity.
Influenza stories this time of year swirl around similar themes, often with the CDC taking center stage (featuring the platitudinous photo op of the CDC director rolling up his sleeve for the shot), including the importance of the flu shot, the need for general hand hygiene and the expected effectiveness of the vaccine this year.
The number that arrives as predictably as the fall flu campaign is “60” as in this story that repeats the mantra: “Flu shots normally prevent 60% to 65% of infections serious enough for people to see a doctor.” Across North America, public health officers will stress that the flu vaccine “reduces the risk by 60%!” Sixty sounds good. Impressive. Powerful. But ultimately as meaningless as a used car salesman with big bright signs of “60% off” plastered on every car on the lot, without ever telling you what the regular prices are. Despite the potential for a huge bargain on that purchase, the reality is that you’re in for a much smaller deal than you think you are. MUCH smaller.
And so it goes with the flu shot. When people hear “60% reduction,” I believe this is what happens inside their thought bubble: “If my risk of getting the flu this winter is 100%, the shot will reduce that to 40%. So instead of 100 people getting the flu, only 40 would get it. Hmmm. This 60% reduction sounds like great odds.”
Even when the flu vaccine seems less effective, like in this story which said that “last year the flu shot, by the CDC’s own numbers, was 23% effective,” people might think it’s a good deal. 23% off that Chevy Impala in the back lot might be a great bargain. But what does the 23% or 60% really mean?
Since they are relative numbers, they demand us to ask “23% of what?” or “60% of what?” As we’ve made abundantly clear at HealthNewsReview.org, using relative numbers on their own are a statistical no-no because, to quote ourselves, “we think the relative risk number tends to inflate the impression of how much impact the drug [or vaccine] has.” For a quick refresher on relative numbers check out our review criteria here.
I find the 60 or the 23 percents irritating, not to mention epically misleading and I’m not alone. One commentator looking at a flu study reporting a “24% risk reduction” called it “Cockamamie propaganda”. Colorful but true.
To get some perspective I talked to Dr. Tom Jefferson in Rome who has reviewed hundreds of flu vaccine studies as part of the Cochrane Collaboration. Dr. Jefferson gives me a quick tutorial on the 60% which he calls “CDC / pharma spin of the worst kind.”
He reminded me that every flu season there are over 200 viruses which can cause influenza and influenza-like illness, all perfectly capable of making you headachy and feverish. Most people get through the flu just fine and, thankfully, the risk of death or serious illness in otherwise healthy people is rare. In a good year the vaccine might protect you against influenza A and B, which might make up about one tenth of all circulating viruses.
The best way to assess flu trials is to look at those that compared vaccinated people with unvaccinated people.
When Jefferson and his colleagues published their March 2014 review they found that under ideal conditions (when the vaccine matches the main viruses circulating that season) you need to vaccinate 33 healthy adults to avoid one set of influenza symptoms. This is what we’d call a NNV (Numbers needed to Vaccinate) of 33. When the vaccine match isn’t so good as it was last year, the NNV is about 100. That is, of 100 people vaccinated, 99 will have no benefit and one person will avoid one set of influenza symptoms. Vaccination did not seem to affect the number of people hospitalised or who lost working days.
Almost half (15 of the 36 trials they examined) were funded by vaccine companies and four had no funding declaration. His team cautioned that even these numbers may represent an “optimistic estimate” because “company-sponsored influenza vaccines trials tend to produce results favorable to their products.” You can read more details here.
As for the magical “60?” Dr. Tom Jefferson didn’t mince words: “Sorry I have no idea where the 60% comes from – it’s either pure propaganda or bandied about by people who do not understand epidemiology. In both cases they should not be making policy as they do not know what they are talking about,” he said, insisting that I quote him.
When asked to explain the 60% number via email, the CDC sent me a link to their website, which states that the 60% figure is a “point estimate” of laboratory-confirmed flu illness that results in a doctor’s visit or urgent care visit, presumably derived from studies such as this one showing a “61%” effectiveness rate for the 2013/2014 shot.
If you don’t go beyond the “60%” headlines, you probably wouldn’t question the vaccine because if A) the vaccine is so effective, B) the vaccine is ‘free’ or almost free; and C) it’s relatively safe, then how could you say no to that?
Doesn’t that equation change if the effectiveness is between 1 to 3 percent, depending on how well this season’s circulating virus has been matched with the new vaccines?
As the CDC continues to stress that “a yearly flu vaccination is still the best protection currently available against the flu,” you can imagine the confusion playing out in the thought bubbles of the general population.
What I long for—and I haven’t seen it yet—is for media coverage this season to start reporting on absolute differences related to the flu vaccine. I’d like to see how the “1-3% effectiveness of the vaccine” floats around in the public’s thought bubbles. How does that compare with something as simple as staying home and not infecting other people or washing your hands more frequently?
I think if the real numbers were out there, we might see a much broader public conversation about what other sorts of flu “protection” are worthwhile.