Vccines and Communication

 

A brief note on how mass communication is often approaching very delicate issues, such as vaccines, in a very rough way.

Two essential definitions:

Incidence: the ratio between the number of new cases observed over a fixed time-window and the number of people in the reference group (or population).

Prevalence: the ratio between the number of active cases in a given instant of time and the number of people in the reference group (or population)

How is the effectiveness of a vaccine measured? It is measured by the attributable risk. The attributable risk is estimated as (incidence in the placebo group - incidence in the vaccinated group) / (incidence in placebo).

Corriere della Sera (https://www.corriere.it/dataroom-milena-gabanelli/covid-vaccino-quante-probabilita-ci-sono-chi-guarito-ricontagiare-immune-immuni/f1fd386a-42c5-11eb-a388-78033ff67873-va.shtml) offers us an interesting analysis, many numbers, many comments, little statistics. At first glance,  it is to be trusted, especially since it is promoted by Milena Gabanelli. Still, there is something wrong. What's the point of comparing the vaccine's estimated efficacy (the attributable risk) with the number of reinfections (the incidence)? The answer is simple, it doesn't make any sense. It's like comparing pears with fried potatoes.

Even if you want to derive the attributable risk of reinfection, you have to be very careful.

 Indeed, incidence always refers to a specific interval of time. Remark that the 1.8% reinfections reported by the Corriere are not referred to the same interval of time of vaccine evaluation. The attributable risk calculation only makes sense as long as the incidences are calculated over the same time interval, with similar circulation and transmissibility of the virus (therefore in the same geographical area as a minimum). The incidence is not intrinsic to the virus but results from the interaction between it and the population.

The very question posed by the Corriere makes little sense.

A vaccine is evaluated in terms of risks (of adverse events), costs, and benefits. The Pfizer vaccine, which is approved, and the Moderna, which is being approved, are safe and effective vaccines. The benefits in health, social and economic terms are so much greater than the risks that it is useless to even discuss them. Only those who have prejudices, or delusions of conspiracy, can think the opposite. EMA and AIFA are independent bodies that evaluate benefits and risks and then make informed decisions based on scientific evidence.

The Sweden model has largely failed (https://doi.org/10.1093/ectj/utaa025). The way to return to normal is the vaccine. We have just undertaken it, it will take time, but we are sure that we will get out of it.