A nurse with a sample of the vaccine produced by the Chinese company Sinovac Biotech Photo: AFP.
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The percentage of effectiveness of a vaccine is measured in the famous phase 3, which is where various vaccine projects are found. In addition to that of Pfizer-Biontech, the prototypes of Sinovac, the Wuhan Institute, Beijing-Sinopharm Institute, AstraZeneca-Oxford, CanSino, Gamaleya Institute (Russian Sputnik), Janssen-Johnson and Johnson have reached this final phase of clinical trials. , Novavax and Moderna.
In previous phases, it had been verified that these nine vaccines worked by measuring that an immune response was indeed aroused to a greater or lesser degree in the subjects who were inoculated with the vaccine. Thus, news emerged, such as those recently published, that the AstraZeneca-Oxford prototype stimulated antibodies and T cells in the elderly.
This type of news was easy to understand, a vaccinated person was pricked both doses and, some time later, the pertinent serological analyzes were carried out where the presence of antibodies was measured, etc.
The difference between groups
Measuring effectiveness, however, is more complicated since what you are trying to know is to what extent those antibodies that we know are going to be there are capable of stopping the virus in real life. During phase two, the company researchers have been experimenting with the quantities until they get the answer that they have considered sufficient. Phase three is the one that determines whether or not they got it right.
To find out how effective different vaccines are, phase 3 of each includes tens of thousands of participants. Among those people, there are those who receive a placebo and those who have been inoculated with the real vaccine. Typically, there are as many people who receive the placebo as those who do, but it doesn’t have to be that way. In the case of Russian Sputnik, only a quarter of the study received the placebo.
As these samples are large enough so that coronavirus cases appear among the participants, the researchers are monitoring them. Basically they count how many of these cases occur among people who have received the placebo and how many among those who are actually vaccinated. The percentage difference between one and the other will be the effectiveness.
Obviously, the larger the size of the studio and the more time passes, the better. However, this is not decisive, the key is how the cases are distributed between vaccinated and unvaccinated.
Imagine a study with 10,000 people with the same number of vaccinated as people who have received the placebo. In this trial, 100 infected with coronavirus have appeared. Of those 100 cases, 80 have been among the placebos and 20, among those vaccinated. In this case, the number of infected vaccinated has been 25% with respect to those who have received the placebo (since 20 is 25% of 80). We would say then that the effectiveness of the supposed vaccine would be 75%.
In other words, since we started from the idea that both groups (placebos and vaccinated) should have been infected in the same way and it has been proven that in fact the vaccinated have been infected to a lesser extent, the result of this imaginary study would conclude that the vaccine has been effective three out of four times.
Be careful with this formula to calculate, because if in that same study of 10,000 people 800 cases had appeared in the group of placebos and 200 in vaccinated, the efficacy of the vaccine would continue to be 75%, despite the fact that infections are multiplied by ten.
The Pfizer ad
Now that we have clarified all of the above, we are going to analyze exactly what the pharmaceutical company Pfizer-Biontech has announced about its RNA vaccine (similar to that of Moderna). What it has announced is that 43,538 people participate in its phase 3. Seven days after receiving the second dose (of placebo or vaccine), 94 coronavirus infections had been recorded in all participants. And more than 90% of these cases were among people who had received the placebo.
If this is confirmed, the news would be good, very good. But is this the end? Do we already have the vaccine that will kill the virus? Unfortunately not. These are very preliminary results. According to the experts consulted to prepare this article, it would be even more relevant to know how effective the vaccine is in specific risk groups. And even if that 90% effectiveness is confirmed (the pharmacist will review when they reach 164 cases within their own sample), their safety in the short, medium and long term will also have to be verified.
(With information from I am)
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