Over 55 in France and Belgium, over 60 in Germany, Spain or Italy and over 30 in the United Kingdom: the ages at which the AstraZeneca vaccine is reserved vary by country. A decision based on what scientists call the benefit / risk balance.

Like any drug, this vaccine indeed has benefits – effective protection against covid – and risks – side effects, including the very rare occurrence of blood clots.

If the benefits outweigh the risks, the balance is “favorable” or “positive”. Otherwise, it is “unfavorable” or “negative”.

One death in a million vaccinated

What do we know about this scale for the AstraZeneca vaccine? Globally, health authorities count less than one death related to thrombosis in a million vaccinated.

And if this million covid catches covid for lack of vaccination, it could lead to between 2,000 and 50,000 deaths, depending on the age group.

And it is these age groups that, precisely, make all the difference, as the British health authorities explained on April 7 during a press conference based on Cambridge University data.

Benefits at any age when the incidence is high

First situation: the virus is circulating strongly, with an incidence of at least 200 cases of covid per 100,000 inhabitants. This is the current case in France, where the average incidence exceeds 400 per 100,000.

© Winton Center for Risk and Evidence Communication, University of Cambridge.

In blue, on the left of the graph: the potential benefits, namely ICU admissions for covid averted by the vaccine, over 16 weeks of exposure to the virus per 100,000 people.
In orange, to the right of the graph: the potential serious risks caused by the vaccine, mainly cases of severe thrombosis.

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Here, the older we get, the greater the benefit compared to the risk. For example, the benefit ratio for 60-69 year olds is 127.7 when the risk ratio at the same age is 0.2. Among 20-29 year olds, the profit ratio is 6.9 versus a risk ratio of 1.1.

This age-related change can be explained simply because the risk of severe forms of covid increases with age. In 100,000 people, when the virus circulates strongly, it is therefore significantly more interesting to vaccinate than not to vaccinate at all ages. But the benefit of vaccinating remains significant at all ages.

Less benefits for young people when incidence decreases

Second situation: the viral circulation is intermediate, with an incidence of 60 in 100,000. Vaccination with AstraZeneca remains interesting for all subjects, in particular for those over 30 years of age.

© Winton Center for Risk and Evidence Communication, University of Cambridge.

Third situation: the circulation of the virus is low, as for example in the United Kingdom today, at an incidence of 20 cases per 100,000 inhabitants. Here again, the advantage of the vaccine remains very significant, but can be discussed for the youngest: the difference between benefit and risk for 19-29 year olds is indeed quite low.

© Winton Center for Risk and Evidence Communication, University of Cambridge.

Also count long covid and transmissions

But to assess the benefits of the vaccine, other factors than intensive care admissions must be taken into account, the scientists note: hospitalizations outside of intensive care units, long covid conditions, which exist even in minor forms of the disease and even in young patients, and the risk of infection of vulnerable people by unvaccinated young people.

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Over time, profits increase

Another clarification: these scales are only valid at the time of vaccination. Over time, the benefits increase, but the risks do not. The balance therefore becomes more and more favorable, because “a vaccinated person will continue to accumulate these benefits for the duration of the protection offered by the vaccine” while “the risk (of thrombosis, editor’s note) linked to vaccination only occurs at the time of vaccination“explains the University of Cambridge.

At each age its vaccine?

What to retain from these analyzes? Overall, the vaccine is interesting at all ages in terms of benefit / risk balance when viral circulation is strong.

This balance is discussed for those under 30 when the virus is less circulating, in particular “if other vaccines which do not carry a risk of a blood clot are available immediately “, points to the University of Cambridge.

This is the case in Europe, where Moderna and Pfizer vaccines are therefore preferred for the youngest age groups. It remains to define an age limit, ideally common to all countries, or even a condition of incidence to apply or not this limit.

Last precision: this is an individual benefit / risk balance. But beyond the individual benefit, the overall benefit of vaccination should be taken into account, since massive vaccination against covid will make it possible to control or even stop the epidemic.