(The Hill) — A new study in Israel has found that the fourth dose of the Pfizer-BioNTech COVID-19 vaccine improves protection against infection and severe COVID-19; however, protection against confirmed infection appears to be short-lived.

The study, published in the New England Journal of Medicine on Tuesday, found that the effectiveness against COVID-19 infection in the fourth week after receiving the fourth dose was lower when compared to protection after the third dose of the vaccine.

It added that protection against severe illness did not wane during the six weeks after the fourth dose was administered and found that the rate of confirmed infection in the fourth week after was lower than that in the group with three vaccine doses.

However, the study added that protection against infection waned in later weeks.

Israel was the first country to begin administering a fourth dose. The study, conducted by the Sheba Medical Center, included more than 1.25 million vaccinated people in Israel from Jan. 10 to March 2.

It included those who were 60 years of age or older and had received three doses of Pfizer-BioNTech COVID-19 vaccine at least four months before. It took place as the omicron variant led to a winter surge of coronavirus cases around the world.

The study also noted that the omicron variant is genetically divergent from the ancestral SARS-CoV-2 strain for which the Pfizer-BioNTech vaccine was tailored.

It added that the findings suggest that the protection against confirmed infection with the omicron variant is at its highest level in the fourth week after vaccination, after which the efficacy of the booster decreases by the eighth week.