The Ministry of Health presents below what the effectiveness of a vaccine actually means, and shows how this type of immunization serum helps people, and can be extremely important in the entire context of the Coronavirus pandemic around the world.
"The effectiveness of a vaccine shows how much the cases of disease decrease in the vaccinated population, compared to the non-vaccinated population.
"Effectiveness 95%" does not mean, as it might be easy to believe, that in 5% of cases the vaccine fails and is useless. In fact, "95% effectiveness" means that, compared to an unvaccinated person, a vaccinated person has a 20 times lower risk of developing symptomatic COVID-19*.
How is this estimate of effectiveness arrived at? Through clinical trials. For example, the clinical trial that led to the approval of the Pfizer BioNTech vaccine followed 43.448 people, divided into two groups: those who received the vaccine and those who received a placebo (a saline solution).
In both groups, the injections were given 21 days apart. One of the main objectives of the study was to determine the effectiveness of the vaccine against the development of symptomatic COVID-19 at least 7 days after the administration of the second dose. It was observed that:
Of the 21.720 people who received the vaccine, only 8 developed specific symptoms of COVID-19, being confirmed by RT-PCR testing
Of the other 21.728 people who received placebo (saline solution), 162 developed specific symptoms of COVID-19, being confirmed by RT-PCR testing
Based on the above data, vaccine efficacy was calculated at 95%. Efficacy in clinical trials is defined mathematically by the formula:
(the disease risk of the unvaccinated population – the disease risk of the vaccinated population)*100/the disease risk of the unvaccinated population,
where the risk of disease in the two groups is calculated as the ratio of the number of people who became ill to the total number of people in that group.
* A case of symptomatic COVID-19 is medically defined as infection with SARS-CoV-2 manifested by signs and symptoms that can range from mild to very severe. A case of asymptomatic COVID-19 is defined as infection with SARS-CoV-2 that does not manifest clinically in any way."
