A study done by the Clalit HMO’s research institute in collaboration with Harvard University, just published in the New England Journal of Medicine, has studied the efficacy of a first and second dose of the Pfizer vaccine against the delta variant on adolescents aged 12 to 18. The study included people receiving the vaccine between June 8 and September 14, 2021, spanning the fourth wave of the epidemic. The study methods used were the same as in other large studies conducted by Clalit and published in the same journal.
Researchers reviewed data obtained from 94,354 vaccinated adolescents who were carefully matched to a control group of exactly the same size but which had not been vaccinated. The control group matched the vaccinated one in terms of demographics, geographic location, clinical background and health-linked behaviors that are linked to risks of infection. The subjects were divided into groups dynamically, based on their changing vaccine status (during the study period, 13,423 of the adolescents changed from unvaccinated to vaccinated). The study carefully adjusted personal and clinical characteristics among the groups, allowing researchers to overcome potential biases. The large sample size allowed for a more precise assessment of the efficacy of the vaccine over different periods after the vaccine was administered.
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The results:
Among youths who were fully vaccinated (one week or longer after the second dose) there was a 93 percent reduction in symptomatic COVID-19, compared to the unvaccinated group. There was also a 90 percent reduction in documented cases of infection.
The study did not provide a precise estimate of the effects of the vaccine on the reduced intensity of the disease in cases of serious illness, since there is not enough information so far in this age group, due to the rarity of serious illness among adolescents.
A researcher manipulates proteins in a laboratory as part of COVID vaccine researchStephane Mahe/Reuters
According to Prof. Ran Balicer, the head of innovation at Clalit, “with the outbreak of the fourth wave of the delta variant, the question arose whether the vaccine used in Israel provides sufficient protection against it, as it did against the alpha variant in January. Since most of the adolescents were first vaccinated during the later wave, estimating the efficacy of the vaccine could provide the answer.”
The results show unequivocally that the vaccine is very effective in reducing infections with the delta variant and in preventing symptomatic illness one week after receiving two doses. The results show that the degree of protection against the delta variant is similar to that seen among young adults with the alpha variant, six months ago. These results provide important and confirmed information for parents who are deliberating whether to vaccinate their adolescent children.
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Prof. Ben Rice from Harvard University, also the head of the Predictive Medicine Group at the Boston Children’s Hospital, says that the results of this observational study, which complement the clinical study done by Pfizer, provide well-grounded scientific proof regarding the efficacy of the vaccine in preventing infection and serious illness among youths, as well as regarding the efficacy against the delta variant, which is currently the dominant one in Israel and around the world.
The study was enabled through a donation by the Ivan and Francesca Berkowitz Family Living for Laboratory Collaboration at Harvard Medical School and the Clalit Research Institute.
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