Let’s say when crossing a busy highway on foot without looking you have a 50% chance of getting hit by a car. But if you look before crossing, then you have only a 20% chance of getting hit. By looking, you’ve reduced your risk of getting hit by 60% [(50-20)/(50)=60%]. It’s not perfect, and there’s room to improve, but it’s clearly wiser to look before crossing.
How is Flu Vaccine Effectiveness measured?
The CDC recently announced this past season’s flu vaccine effectiveness was 47% – a marked improvement over last year’s abysmal 23%, and slightly better than the 10-year average of about 40%.
But what exactly does 47% mean? A 47% less chance of getting the flu this past winter if you got the flu shot in the fall? A 47% less chance of dying? Of going to hospital?
And does it measure ‘flu’ in terms of lab-confirmed blood tests? Doctor visits for flu-like illnesses? Or self-reporting (oh man I feel awful…must be the flu!)
Is that 47% effectiveness the same for all people? Healthy, elderly, children?
In fact, the way vaccine effectiveness is measured varies. But for the past 13 years, the CDC has used a “very accurate and sensitive laboratory test” to confirm that a person who has gone to the doctor because of a flu-like illness actually has the flu (and not just something else like a bad cold or similar infection). They then compare between those who have the flu and those who don’t, those who got the flu shot and those who didn’t.
What does Flu Vaccine Effectiveness mean?
There are a few ways of thinking about what vaccine effectiveness (VE) actually means. Let’s take VE=60% as an example. The CDC indicates that a VE of 60% “means that the flu vaccine reduces a person’s risk of developing flu illness that results in a visit to the doctor’s office or urgent care provider by 60%.”
Put another way, a VE=60% indicates a 60% reduction in disease occurrence among a vaccinated group versus an unvaccinated group, or a 60% reduction from the number of cases you would expect if they have not been vaccinated. [Source: CDC]
Flu vs. Car
Returning to our laissez-faire pedestrian, even when looking before crossing, you’re still quite susceptible (20%) to getting hit. What if we added novel technologies, like cross walks, traffic lights, and pedestrian bridges? Along with looking before crossing, the new measures would likely approach 100% effectiveness!
With an average ~40% flu vaccine effectiveness, the NIAID/NIH, the WHO, and CDC are calling for R&D into improved vaccines. There are a number of groups pursuing the goal of “universal” flu vaccines that provide enhanced and broadened protection from the flu. Each group is exploring different methods (to return to the analogy, some are developing crosswalks, others traffic lights, and others pedestrian bridges). BiondVax, having completed 5 human clinical trials and currently conducting two additional Phase 2 trials, is likely the closest to the finish line. The promising results give hope that within a few years your relatively ineffective annual flu shots will be replaced by an improved flu shot effective for multiple years against all seasonal and pandemic flu viruses.
> The numbers I’ve used are just for example. In reality, seasonal flu infects around 5-15% of the unvaccinated population.
> Big thanks to Matan Animation Studios for permission to share their video.
> Further reading:(1) http://www.who.int/influenza/vaccines/virus/recommendations/201502_qanda_vaccineeffectiveness.pdf (2) http://www.cdc.gov/flu/about/qa/vaccineeffect.htm