Booster Dose Reduced COVID-19 Across Adult Age Groups in Israel

A booster dose of the BNT162b2 (Pfizer-BioNTech) m-RNA vaccine reduced incidence of COVID-19 and rates of severe illness in all adult age groups 16 years or older, in a nationwide assessment in Israel that accounted for participants' environmental risk and interval after receiving the booster.

After initial vaccine effectiveness appeared to wane with emergence of the delta variant, Israel authorized a booster dose for those 60 years of age and older in July, 2021, The availability expanded stepwise by 10-year age groups, starting with those 50-59 on August 13, 2021 to those 12 years or older on August 29. With the robust nationwide program, investigators were able to access the Ministry of Health database of over 4.6 million person 16 years or older who had received the 2-dose initial regimen at least 5 months prior between July 30 to October 10, 2021.

Yinon Bar-On, MSc, Department of Plant and Environmental Sciences, Weizmann Institute of Science, Rehovot, Israel, and colleagues compared the rates of confirmed COVID-19, severe illness, and death among those who had received a booster at least 12 days earlier to those vaccinated but without booster dose, and to those who had received the booster 3 to 7 days prior to contracting the illness (early booster). In addition, they adjusted for environmental exposure risk for each participant's area of residence, based on the number of confirmed infections in the past 7 days per 1000 residents.

"Although observational studies suggest that the booster dose is effective against both confirmed infection and severe disease in the elderly population, the extent of protection of an additional dose in younger age groups requires further clarification," the investigators explained.

Bar-On and colleagues reported that the rate of confirmed infection was lower in the booster group after at least 12 days than in those with just the 2-dose vaccination by a factor of approximately 10 (ranging from 9.0 to 17.2; an adjusted difference in rates ranging from 57.0 to 89.5 infections per 100,00 person-days across the 5 age groups). The adjusted difference in rate of infections among those contracting COVID-19 within 3 to 7 days of the booster dose was also lower, but closer tothose without booster (ranging from 34.4 to 38.3 across the age groups).

The rates of severe illness in both the booster and early booster group were lower than in the vaccinated group without booster, by a factor of 17.9 (95% CI 15.1-21.2) and 6.5 (5.1 to 8.2), respectively.The mortality among those 60 years of age or older was lower among the booster group by a factor of 14.7 (10.0 to 21.4), and by a factor of 4.9 (3.1 to 7.9) in the early booster elderly.

The investigators concluded that the 3rd dose booster reduced the incidence of COVID-19 and of severe illness relative to the 2-dose initial vaccination across all adult age groups, and was particularly effective at least 12 days after administration.

"Understanding the protective effect of the booster dose in young age groups is key for forming public health policy," Bar-On and colleagues noted.

In an accompanying editorial, Minal Patel, MD. COVID-19 Response International Task Force, Centers for Disease Control and Prevention (CDC), Atlanta, GA welcomed the evidence of benefit from the booster dose, but pointed out that many countries are yet to provide initial vaccination.

"It is important that vaccination with the primary series, especially in high-risk populations, remain a top priority everywhere, because this will ultimately lead to a greater reduction in severe disease and death," Patel stated.