Dec 07, 2021

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As the US government prepares to offer COVID-19 vaccine to children aged five to 11 in November, researcher at Israel’s Clalit Research Institute – in collaboration with colleagues from Harvard University –  have proven the effectiveness and safety of the Pfizer vaccine against the Delta variant in teens aged 12 to 18. 


The peer-reviewed study has just been published in one the world’s leading medical journal, The New England Journal of Medicine, under the title “Effectiveness of the BNT162b2 COVID-19 Vaccine against the B.1.617.2 (Delta) Variant in Adolescents.” 


The team analyzed one of the world’s largest integrated health record databases to examine the effectiveness in teens of the Pfizer vaccine against the Delta variant of SARS-CoV-2, which is the most common form of the Coronavirus currently challenging the world. It is the first such study in which the Delta variant was dominant. The study was conducted in Israel, an early global leader in COVID-19 vaccination rates.         


The study took place from June 8, 2021 through September 14, 2021. It coincided with Israel’s fourth wave of coronavirus infection and illness. 


The research was conducted by Dr. Noam Barda, Dr. Noa Dagan, Michael Leshchinsky, Dr. Eldad Kepten, and Prof. Ran Balicer from the Clalit Research Institute of Israel’s largest health maintenance organization Clalit Health Services, along with Prof. Miguel Hernán and Prof. Marc Lipsitch of the Harvard’s School of Public Health and Prof. Ben Reis of Boston Children’s Hospital and Harvard Medical School.

They reviewed data from 94,354 vaccinated adolescents aged 12 to 18, who were carefully matched with 94,354 unvaccinated adolescents based on an extensive set of demographic, geographic and health-related attributes associated with risk of infection, risk of severe disease, health status and health seeking behavior. 


The results show that in fully vaccinated adolescents (seven to 21 days after the second dose), the risk of symptomatic COVID-19 decreased by 93% compared with the unvaccinated, while the risk of documented infection decreased by 90%. In the period immediately preceding the second dose 14 to 20 days after the first dose, vaccine effectiveness was lower, but still substantial – the risk of symptomatic COVID-19 decreased by 57% in vaccinated individuals, and the risk of documented infection by 59%. There was insufficient data to provide an estimate on the reduction in the incidence of severe disease, hospitalization and mortality, as these outcomes are rare among adolescents.


“The extensive nationwide rollout of Israel’s COVID-19 vaccination campaign to adolescents at the very beginning of the delta variant wave, provided the Clalit Research Institute with a unique opportunity to assess, through its rich and comprehensive digital datasets, the effectiveness of the vaccine for adolescents against delta in a real-world setting,” said Balicer, senior author of the study, director of the Clalit Research Institute and the health fund’s chief innovation. 


“These results show convincingly that one week after the second dose, this vaccine is highly effective in adolescents against symptomatic COVID-19 and against all documented infections. These data should facilitate informed individual risk-benefit decision-making, and, in our view, make a strong argument in favor of opting-in to get vaccinated, especially in countries where the virus is currently widespread,” said  Balicer, who also serves as chairman of israel’s national expert advisory team on COVID-19 response.


Reis, director of the predictive medicine group at the Boston Children’s Hospital computational health informatics program and Harvard Medical School, said, “To date, one of the main drivers of vaccine hesitancy has been a lack of information regarding the effectiveness of the vaccine. This careful epidemiological study provides reliable information on vaccine effectiveness, which we hope will be helpful to those who have not yet decided about vaccination.”


Hernán, director of the CAUSALab and professor at the Harvard School of Public Health, said, “This research is a perfect example of how randomized trials and observational healthcare databases complement each other. The adolescent-focused trial of the Pfizer/BioNTech vaccine provided compelling evidence of its effectiveness to prevent symptomatic infection, but the estimates for severe disease and specific age groups were too imprecise. This analysis of Clalit’s high-quality database emulates the design of the original trial, uses its findings as a benchmark, and expands upon them to confirm the vaccine’s effectiveness in adolescents. This combination of evidence from randomized trials and observational studies is a model for efficient medical research, something which is especially important in COVID times.”


Lipsitch, director of the center for communicable disease dynamics and professor at the Harvard School of Public Health, concluded that “in all studies of vaccine effectiveness, a major challenge is to ensure that those we are comparing to identify the vaccine’s effect are similar in the other characteristics that may predict whether they get infected or ill. This is especially hard in the context of a rapidly growing, age-targeted vaccine campaign. Clalit’s extraordinary database made it possible to design a study that addressed these challenges in a way that provides tremendous confidence in the inferences that come out of the study.”