General Information about Vaccine Efficacy


Last updated: Assessing the efficacy of a vaccine usually starts with testing its immunogenicity, which is the same as saying we measure the body’s antibody response to the vaccines. If there are enough antibodies, then this is usually equated with immunity, even though it has long been proved that a high level of antibodies does not mean a person cannot get ill. [1,2,3,4,5,6,8] In fact, antibody levels have been shown to tell us very little about the likelihood of a person becoming ill. The immune system is only partly understood and blood antibodies are only a small part of it. In addition to this, vaccines sometimes also undergo a clinical trial where a large number of people are vaccinated and the researchers look at whether these people get ill less often than a “placebo” group. If they do, the vaccine is also considered efficacious. Here we have a more realistic view of whether a vaccine works but it is still important to remember that these studies are carried out by the manufacturers of the vaccines and that they control how the data is handled. In addition, the placebo is, in many cases, another vaccine. Sometimes, so-called non-inferiority studies are used to show that a vaccine is at least as good as one which is already in use, once again based on immunogenicity. Effectiveness, as opposed to efficacy, of a vaccine is usually assessed retrospectively by looking at whether a vaccine has made a positive impact in real life. Sometimes this is easy. If almost every student in a school was fully vaccinated and the school still suffered an outbreak, then the vaccine was not effective. But most of the time it is highly complicated, as data needs to be adjusted for so-called confounding factors. For example care home residents may…

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