Vaccine definition change?

So… I know a fellow who says they’re ‘not to be against vaccines’, but claims they changed the definition of vaccines to include things that don’t make you 100% immune. I keep getting stuck on this point with him. Is any of this true?

IANAD but vaccines have never been 100% effective. They will boost your resistance, so that if you cache the illness, you won’t get so ill.

That’s all vaccines?

I did find this, from almost a year ago. The changes were made because previous definitions could be interpreted to mean that vaccines are 100% effective, and as Mikkel says above, that was never true.


“100% immune” is such a nonsensical criteria that no immunologist would even try to answer that question without first qualifying what vaccines do, which is to prime the adaptive immune system to make genetic modifications to respond to pathogens and toxoids. Immune system response is as unique to the individual as their genome so in every vaccine there is a wide variation in efficacy; the ability of SARS-CoV-2 to adapt through rapid changes to the genes coding for the receptor binding domain (RBD) and other identifiable proteins such as the envelope (although mRNA vaccines have specifically focused on RBD sequences in the S-protein) have made what initially appeared to be highly efficacious vaccines to be less effective against emerging variants even in the population cohorts they were originally tested against.

For some historical context, the Salk vaccine against Poliomyelitis—arguably more one of the first preventative vaccination campaigns to fight an ongoing epidemic nationwide instead of attacking outbreaks piecemeal—was only about 70% effective, and was only delivered to about 70% of the affected population for a composite of ~50% protection, and it still managed to curtail outbreaks to below an epidemic threshold within a few months. Very few vaccines offer so-called “sterilising immunity” and there is active debate in the immunology community about whether that terminology should be completely retired.

None of which is going to make a whit of difference to people who want to question the science of vaccines and public health in general as a philosophical issue of government overreach or for political opportunism. But the reality is more complicated than a binary of protection or vulnerability as the public insight into the sausage-making process of vaccine development and public health during the ongoing COVID-19 pandemic has shown.


I just spent 10 min trying to find a summary table listing at least the common ones. This seems like something that should exist, but I’m coming up dry.

One of the more effective ones, measles, is still only 97% effective. And that takes two shots.

Thank you for searching.

To understand this, you have to know how the immune system works. The main players are B-cells, which can produce each a single specific type of antibody, and T-cells, some of which help to modulate the activity of the B-cells, and others are able to attack and kill cells that are infected by a pathogen (e.g. a virus or bacterium). If you get infected by a pathogen your immune system has not encountered before, you are unlikely to have any B- and T-cells that strongly recognised that pathogen. However, amongst the billion or so different immune cells in your system, a few will recognise the pathogen, although weakly, and these get stimulated to propagate and, in the case of B-cells, to mutate the antibody they produce. If they thereby evolve better-binding antibodies, they propagate even faster, thereby evolving a strong antibody response. To produce a strong antibody response from scratch takes several weeks. Once the pathogen is vanquished, antibody production diminishes, and some of the immune cells become long-lived memory cells. These are able to, upon renewed contact with the pathogen, to very quickly (within a week or so) mount a strong immune response, since they already have the mutated antibody genes enabling them produce strong antibodies from the get go. Therefore, being vaccinated gives your immune system a two- to three weeks head start in fighting the pathogen compared to someone not vaccinated. In many cases, this enables your immune system to vanquish the infection before you even experience symptoms, or at least to greatly diminish the severity of symptoms and to speed up recovery. However, there are other factor which influence how fit your immune system is, and therefore, sometimes even a good head start is not enough to allow it to win the race. Therefore, it is not in all cases able to stop the infection before the first symptoms appear.

Nothing makes you “100% immune” so, in that regard, nothing has changed.