A third (booster) dose of the Pfizer-BioNTech vaccine against Covid-19 is 93 percent effective in protecting individuals against severe Covid-19-related outcomes, compared with two doses received at least five months earlier.

This conclusion is based on data from Clalit Health Services, the largest of Israel’s four nationalized healthcare maintenance organizations. The study was published in The Lancet on October 29.

“To our knowledge, the present study is the first to estimate the effectiveness of a third dose of an mRNA COVID-19 vaccine … against severe outcomes with adjustment for various possible confounders … and within subgroups,” state the study authors from Clalit, Bar-Ilan University and Harvard University.

These subgroups include males, females, Arabs, Jews, ultra-Orthodox Jews, and people with various preexisting conditions and body-mass indexes. Risks were found to be higher in males and in people 70 and older.

The authors explain that despite good results of nationwide vaccination campaigns, many countries – including Israel – experienced a resurgence of Covid-19, dominated by the Delta variant of SARS-CoV-2. This is because the protective effect of Pfizer’s mRNA vaccine wanes over time and especially after six months.

The Israeli Ministry of Health therefore rolled out a vaccine booster campaign, beginning with immunocompromised citizens, last July, and by the end of August was offering the third shot to anyone over the age of 12 years who had received the second dose at least five months previously.

Researchers examined Clalit’s records of individuals receiving a third vaccine dose between July 30, 2020, and Sept 23, 2021, comparing them with demographically and clinically similar controls who did not receive the third dose. Each group contained 728,321 individuals.

Crunching the data revealed that at least seven days after receipt of the third dose, individuals who did become infected were 95% protected against hospitalization, 92% against severe disease and 81% against Covid-19-related death. Overall, the booster was deemed 93% effective.

The study was funded by The Ivan and Francesca Berkowitz Family Living Laboratory Collaboration at Harvard Medical School and Clalit Research Institute.