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Las principales vacunas contra el Covid-19 tienen una eficacia superior al 90 por ciento. ¿Cómo es que los investigadores determinan la eficacia de una vacuna?

Para medir la eficacia de una vacuna, o qué tan bien funciona una vacuna en un ensayo clínico controlado, la FDA primero analiza la cantidad de personas que contrajeron la enfermedad. Luego, la agencia compara si recibieron la vacuna o el placebo (una sustancia inactiva e inofensiva que parece idéntica a la vacuna que se está probando). Si una mayor proporción de los enfermos recibieron el placebo en lugar de la vacuna, entonces la vacuna ha alcanzado un estándar de eficacia aceptable. Es importante saber que las personas en los ensayos no se enfermaron por la vacuna, sino por estar expuestas a la enfermedad en su vida diaria.

Para las vacunas contra el Covid-19, la FDA estableció un parámetro de al menos el 50 por ciento de eficacia en los ensayos clínicos para que una vacuna sea considerada para una Autorización de Uso de Emergencia. La FDA determinó que este era un umbral estándar, en comparación con otras vacunas (como las vacunas contra la influenza estacional), y sería lo suficientemente alto como para retrasar la propagación de la enfermedad. En sus ensayos clínicos, tanto Pfizercomo Moderna superaron este estándar con tasas de eficacia del 95 y 94.1 por ciento, respectivamente.

En el ensayo clínico de Pfizer, un total de cuarenta y cuatro mil personas participaron, de los cuales la mitad recibieron la vacuna de dos dosis y la otra mitad recibieron inyecciones de placebo, los cuales no tuvieron ningún efecto. Dos meses después de su segunda inyección, un total de 170 personas contrajeron el Covid-19. Entre los 170 infectados, solo ocho recibieron la vacuna en lugar del placebo. Mientras tanto, de los 10 casos graves, un total de nueve personas recibieron el placebo.

En el ensayo clínico de Moderna, un total de treinta mil personas participaron. De manera similar, la mitad de los participantes recibieron la vacuna de dos dosis y la mitad recibieron inyecciones de placebo. Aproximadamente dos meses después de su segunda inyección, un total de 196 personas contrajeron el Covid-19. Entre los 196 infectados, solo 11 recibieron la vacuna en lugar del placebo. Un total de 30 casos graves recibieron el placebo.

Social: ¿Cómo es que los investigadores determinan la eficacia de una vacuna? La FDA primero analiza la cantidad de personas que contrajeron la enfermedad. Luego, la agencia compara si recibieron la vacuna o el placebo. Si una mayor proporción de los enfermos recibieron el placebo en lugar de la vacuna, entonces la vacuna ha alcanzado un estándar de eficacia aceptable.

To measure vaccine efficacy, or how well a vaccine works in a controlled clinical trial, the Food and Drug Administration (FDA) first looks at the number of people who fell ill with the disease. Then the agency compares whether they received the vaccine or the placebo (a harmless, inactive substance that looks identical to the vaccine being tested). If a greater share of those who are sick received the placebo instead of the vaccine, then the vaccine has met an acceptable efficacy standard. It is important to understand that the people in the trials did not get sick from the vaccine, but rather from being exposed to the disease in their everyday life.

For Covid-19 vaccines, the FDA set a bar of at least 50 percent efficacy in the clinical trials for Emergency Use Authorization consideration. The FDA determined that this was a standard threshold, compared to other vaccines (like seasonal flu vaccines), and would be high enough to slow the spread of the disease. In their clinical trials, both Pfizer and Moderna outperformed this standard with efficacy rates of 95 and 94.1 percent, respectively.

In the large Pfizer trial, forty-four thousand individuals participated, with half receiving the two-dose vaccine and half receiving placebo shots, which have no effect. Two months after their second injection, 170 people had contracted Covid-19. Among the 170 infected, only eight had received the vaccine rather than the placebo. Meanwhile, of the 10 severe cases, nine were in the placebo group.

In the large Moderna trial, thirty thousand individuals participated. Similarly, half of participants received the two-dose vaccine and half received placebo shots. Approximately two months after their second injection, 196 people had contracted Covid-19. Among the 196 infected, only 11 had received the vaccine rather than the placebo. All of the 30 severe cases were in the placebo group.  

With the Johnson & Johnson vaccine, the FDA, "evaluated and analyzed the safety and effectiveness data from clinical trials conducted in over forty thousand study participants and manufacturing information submitted by Janssen Biotech, Inc. [...] [and] determined that the totality of the available data provides clear evidence that Janssen COVID-19 Vaccine may be effective in preventing Covid-19." 

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