The mass vaccination programs primarily include the BNT162b2 (Pfizer/BioNTech) vaccine that has shown more than 90% efficacy in preventing symptomatic COVID-19. In a letter to The New England Journal of Medicine, publishing online yesterday September 1, an interdisciplinary team of physicians and public health experts at University of California San Diego measured the effectiveness of COVID-19 mRNA vaccines among health workers at UC San Diego Health, most notably during the emergence of the highly transmissible delta virus variant and coincident with the end of the state's mask mandate, allowing fully vaccinated persons to forgo face coverings in most places. The letter's authors report that the effectiveness of both the Pfizer and Moderna vaccines significantly waned over time. Both vaccines were granted emergency use authorization by the FDA in December 2020, with vaccinations of the UC San Diego Health work force beginning the same month for health care workers with direct, patient-facing duties.
In the letter, the authors note that from March through June 2021 vaccine effectiveness against symptomatic infection was estimated to exceed 905; by July, however, it had fallen to approximately 65%. Despite experiencing a drop in infection rate soon after initiation of vaccination, Israel has faced a sharp rise in new infections, including vaccine breakthrough infections during June-July 2021, because of the predominantly circulating Delta variant. The emergence of vaccine breakthrough cases could be due to waning vaccine efficacy or immune escape ability of the dDelta variant. Given the waning vaccine efficacy, the Israeli government has initiated a third-booster dose vaccination program in August 2021. Initially, individuals aged 30 years or above and high-risk populations have been prioritized for the third booster dose, which is administered at least 5 months after the second dose immunization. Alike Israel, the USA, and UK governments have decided to immunize their residents with a third booster dose.
In a latest study, the scientists have examined the efficacy of a third booster dose of the BNT162b2 vaccine in comparison to the original two-dose regimen. Among individuals who had received two vaccine doses and were tested for SARS-CoV2, about 5.6% exhibited a positive test result. In contrast, only 3.6% of individuals who had received the third booster dose tested positive for SARS-CoV2. The frequency of a positive test result was highest among individuals who had not received the third dose or those who had received the third dose within 7 days of testing positive. In contrast, the frequency was lowest among individuals who had received the third dose more than two weeks before testing positive. Although no marginal effectiveness of the third dose compared to the two-dose regimen was observed within 7 days of the administration, about 48% and 79% increase in marginal effectiveness were observed 7-13 days and 14-20 days after administration of the third booster dose, respectively.
Since the booster vaccination program has been introduced only recently, the study could not provide the long-term effectiveness of the third vaccine dose. Moreover, the efficacy of the third dose against severe COVID-19 has not been addressed in the study. Overall, the study highlights the significance of a third vaccine dose in counteracting waning vaccine immunity. The authors conclude on the importance of rapidly reinstating key interventions, such as indoor masking and intensive testing strategies, plus continuing efforts to boost vaccination rates.
Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Scientifc references
Keehner J et al. New Engl J Med 2021 Sept 1.
Patalon T et al. medRxiv 2021 Aug 29:21262792.
Keehner J et al. New Engl J Med 2021; 385(2):e8.
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