The protection against Covid-19 infection offered by the vaccines wears off rapidly, especially against symptomatic infection with the Omicron variant, a study conducted by the Sciensano public health institute shows. Protection against hospitalisation remains stable.
The researchers analysed test results, vaccination status and hospital admissions of 1.5 million Belgian residents between July 2021 and April 2022 – when Belgium dealt with the Delta and Omicron variants.
"The Covid-19 vaccines used in the Belgian vaccination campaign provide good protection against hospitalisation for Covid-19 infection with the Omicron variant, and lower protection against symptomatic infection," the Sciensano study concluded. "Although protection decreases several months after vaccination, protection against hospitalisation increases to 87% after booster vaccination."
Data from 1,433,135 people were analysed to calculate the protection rate against symptomatic infection (falling slightly ill as a result of a Covid infection), and data from 662,220 people to calculate the rate against hospitalisation.
Omicron escapes the immune system more easily
Those who received the first two doses – the so-called primary or basic vaccination – were initially 81% more protected against symptomatic Delta infection than unvaccinated people. After 100 to 150 days (three to five months), this percentage dropped to 56%. After a booster dose, it climbed back to 84%.
It was not possible to ascertain how long the protection from the booster shot lasted against Delta because the extra doses were only administered when the variant had already almost disappeared in Belgium.
For protection against the Omicron variant, the results are even more striking: the initial protection against symptomatic infection after the first two shots was just 37%. This was halved after three to five months.
The booster shot surpasses the initial protection (52%) but appears to wear off after a few months (25%). "This has to do with the characteristics of Omicron. It escapes the immune system of the human body more easily than, for example, a Delta or Alpha variant," Sciensano researcher Joris van Loenhout told VRT.
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As the vaccine was designed for the first variant of the virus, it is "not surprising" that its efficacy against Omicron is fairly low and declines rapidly. "Still, the booster does increase it, especially if we look at protection against hospitalisation. We see that in other studies as well."
The Sciensano study, which has not yet been peer-reviewed, indeed shows that people who had both basic vaccination and the booster shot continue to be between 90% and 94% more protected against hospitalisation due to Delta than those who have not been vaccinated.
Against Omicron, the booster provides 87% protection, although it declines to 66% after three to five months. In April, pharmaceutical company Pfizer also announced that its booster shot was still 55% effective against hospitalisation after three months.
In practice, this means that the vaccines still protect against severe illness "reasonably well," but no longer help contain the pandemic, clarified Van Loenhout. "The protection against infection goes down quite rapidly against Omicron and even after the booster, we see a decrease. This ensures that the virus continues to circulate more than with the Delta variant."
People who have been infected, either recently or a year ago, and have also been vaccinated appear to be best protected, said Van Loenhout.
"Depending on exactly when you were infected, the protection lies between 70% and 80%," he explained. "People with hybrid immunity are better protected than people who have only been vaccinated or only have immunity through an infection."