Covid-19: vaccine boosters are useful but their effectiveness does not last long

in collaboration with

Dr Gérald Kierzek (Medical Director of Doctissimo)

What is the protection provided by a vaccine against Covid-19 several months after a booster injection? To answer this question, scientists from the EPI-PHARE group – the scientific interest group associating the National Health and Medicines Agency (ANSM) and the National Health Insurance Fund – carried out a pharmaco- epidemiology to assess the effectiveness of booster doses of monovalent mRNA vaccines, Comirnaty (Pfizer-BioNtech) and Spikevax (Moderna). The work was supervised by epidemiologist and director of EPI-PHARE, Dr. Mahmoud Zureik.

A study based on data from different information systems

For this study, the scientists worked from data taken from different information systems. The objective is to trace the vaccination and the status of the tests (positive or not) carried out in 38,839 hospitalized patients between 1^er June and October 15, 2022. Conclusion: “The results of this new study show that booster doses are effective against the risk of hospitalization for Covid-19 in times of predominance of the Omicron BA.4 and BA.5 subvariants, but that this protection decreases over time. The level of efficacy is all the more important as the number of doses received is high or the time since the last dose is short.“.

Vaccination “effective against severe forms”

More simply, vaccination against Covid-19 has therefore demonstrated its effectiveness against hospitalizations for severe forms, even against the latest sub-variants of the virus, in particular BA.4 and BA.5 from Omicron. On the other hand, it does not reduce the risk of being contaminated in the medium or long term.

Nevertheless, even after two years, people who have received two doses of vaccine are still better protected than the non-vaccinated against severe forms of Covid-19, with an effectiveness of 45% against the risk of hospitalization.

Reminders, an extra dose of protection

Finally, the scientists point out that “booster doses are 56% effective for the first booster dose (3rd dose) and 75% effective for the 4th and 5th doses” and “compared to the primary vaccination, the additional efficacy of a booster dose (3rd, 4th or 5th dose) is estimated at 69% in the first two months following the injection”.

But the effect of these reminders quickly diminishes over time. After six months, there is little difference compared to people who have only had one vaccination: “The effectiveness of booster doses decreases over time, dropping from 55% between two and four months, to 30% between four and six months and then to 22% six months after the primary vaccination. The additional efficacy of the second booster vaccine (4th dose) compared to the first booster (3rd dose) is estimated at 44%”.

Asked about this work, Dr Gérald Kierzek believes that “the targeted vaccination strategy must continue, by offering the vaccine to subjects at risk”. The doctor stresses, however, that the spacing of doses and boosters is made difficult by the more unpredictable nature of the Covid epidemic.