I have previously provided examples of how the illusion of efficacy over a period of time can be created for a vaccine intended to avoid getting infected with a virus. For example, this article shows that if the vaccine is a placebo (i.e. has no effect at all) it will appear to be effective if there is a delay in reporting infections of those vaccinated. But it is just an inevitable statistical illusion. In this article I showed the same illusion is created if those infected shortly after vaccination are classified as unvaccinated. I also produced a detailed video about it and its implications in assessing Covid vaccine efficacy and safety.
In assessing the efficacy of Covid vaccines in observational studies (such as in the large Israel study which claimed 95% efficacy of the Pfizer vaccine) it is now standard to assume that the vaccine takes 14 days to 'work' and hence to classify a person as 'unvaccinated' within 14 days of vaccination. But, as the previous example shows, such an approach inevitably exaggerates efficacy.
Because it is such a critical issue and lots of people still don't 'get it' I have tried to explain in the simplest way possible in this short video why assuming a person is 'unvaccinated' until 14 days after vaccination is such a problem:
The video also shows how vaccine effectiveness in observational trials is further exaggerated if the unvaccinated are less likely to get tested for the virus than the vaccinated (as happened in the Israel Pfizer study).