COVID-19 Vaccine Boosters – 8.10.21 Update

August 10, 2021 Todd Raehtz

Will We Need COVID-19 Vaccine Boosters?

(August 10, 2021) According to the CDC, the need for COVID-19 booster doses has not been established and no additional doses are recommended at this time (1). However, research is underway regarding the necessity and efficacy of a booster dose. Ongoing analysis of participants in Pfizer’s original Phase I/II/III trial has shown that the vaccine’s effectiveness in preventing symptomatic infection remains high four to six months after vaccination, but drops from a peak of 95% to 84% (2).

Pfizer and BioNTech are currently conducting a study looking at the efficacy of a third dose of their BNT162b2 vaccine given six months after the second dose. While no data has been formally published from this trial, they released a statement on July 8, 2021, with their interim results (3). According to Pfizer, the booster dose of their COVID-19 vaccine exhibits a similar safety profile to initial dosing and elicits neutralization titers against the wild type and Beta variant that are 5 to 10 times higher than after two primary doses (3).

Pfizer has not released information regarding the Delta variant (currently the predominant strain of COVID-19 circulating in the United States). One study published in Nature in early July examined the immune sera from patients who received two doses of the Pfizer or AstraZeneca vaccines (the AztraZeneca vaccine is not currently approved for use in the United States). They found that vaccine-elicited antibodies generated a neutralizing response in 95% of participants, but that the two-dose effectiveness against the Delta variant was estimated to be 60% and 88% for AstraZeneca and Pfizer respectively (4). Pfizer has stated that they expect the third dose of their vaccine to boost antibody titers against the Delta variant and are currently conducting preclinical and clinical studies (3). They are also in the process of developing a new version of the COVID-19 vaccine that specifically targets the Delta variant spike protein, with clinical trials set to begin in August 2021 (3).

Data released from the Israel Ministry of Health on July 7, 2021, following their vaccination program suggests a decline in the effectiveness of the vaccine since June 6, 2021, corresponding to the spread of the Delta variant (5). Currently, their real-world data indicates 64% effectiveness in preventing infection and symptomatic illness and 93% effectiveness in preventing serious illness and hospitalization (5).

On Thursday, July 22, the CDC Advisory Committee on Immunization Practice (ACIP) discussed the potential benefit of providing additional doses of the COVID-19 vaccine to individuals with impaired immune systems (e.g persons living with HIV or certain types of cancer, or those taking immunosuppressive medications) (6). They have yet to make an official recommendation, but this suggests an initial prioritization of booster vaccine efforts to patients at risk of experiencing a reduced antibody response to the initial vaccine, leaving them vulnerable to COVID-19 infection.

While discussion of a COVID-19 vaccine booster dose will undoubtedly continue in the coming months, much remains unknown. Topics still requiring investigation include the optimal timing for additional dosing, the possibility of administering a mixed-dose series with vaccines from different manufacturers, the populations most in need of booster dosing, and the role of new versions of the vaccines targeted toward new variants. As we learn more, recommendations will likely change to respond to the most up-to-date medical evidence. For now, booster doses of the COVID-19 vaccine are not recommended for any Americans, but the situation continues to rapidly evolve, and continuing to follow any forthcoming CDC recommendations will be vital to the effort to combat COVID-19.

Author: Kirsten Smith | PharmD Candidate 2022 | University of Michigan

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