COVID-19 boosters to prevent SARS-CoV-2 infections in adults

COVID-19 booster vaccine

Since it was initially announced on March 11, 2020, the unusual coronavirus disease 2019 (COVID-19) pandemic produced by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has posed a threat to world health. Since then, the BNT162b2/Pfizer and mRNA-1273/Moderna messenger ribonucleic acid (mRNA) vaccines against SARS-CoV-2 have been rolled out in two-dose regimens in the United States, with a December 2020 deadline. Both of these vaccines provide at least six months of protection against COVID-19-related hospitalization and mortality.

COVID-19 booster vaccine

Because of the weakened immunity, the transmission of the SARS-CoV-2 B.1.617.2 (Delta) variant increased, resulting in a larger number of COVID-19 cases in the summer of 2021. Booster vaccines for higher-risk persons were approved by the US Food and Drug Administration in response to the abrupt surge in illnesses.

Serological studies have since showed that the antibody response increases significantly from the first to the second dosage of the mRNA vaccines. However, after six months of full vaccination, the duration and amount of antibody response to booster doses are uncertain.

Understanding COVID-19 booster antibody responses

Researchers measured anti-receptor-binding domain (RBD) immunoglobulin G (IgG) and surrogate virus neutralization of the interaction between the SARS-CoV-2 spike protein and the human angiotensin-converting enzyme (ACE2) receptor before and after vaccination with boosters in 33 healthy adults in a recent study published on the medRxiv* preprint server. Participants were requested to fill out an e-consent form and complete online surveys about their COVID-19 vaccination status and history.

Before receiving the booster dose, the subjects supplied finger stick dried blood spot samples, which were collected 6-10 days later. The study’s findings were compared to data from a previous community-based study that followed the identical protocols.

The antibody responses were assessed in a prior community-based trial after SARS-CoV-2 infection or after receiving the second dose of the mRNA vaccine. The presence of anti-RBD IgG before vaccination was used to classify the participants as seropositive or seronegative.

Findings of the research

The study’s findings revealed that antibody responses after 6-10 days of receiving the booster dose are higher than natural SARS-CoV-2 infection, after two doses of the mRNA vaccine, and after both natural infection and immunization. Notably, females’ post-booster IgG levels were greater than men’ and were adversely associated to age.

In addition, the SARS-CoV-2 Delta variant displayed strong surrogate neutralization, but this response was still lower than that shown after exposure to the wild-type SARS-CoV-2 strain. There were no differences in SARS-CoV-2 Delta variant neutralization between males and females, although the inhibitory concentration of 50% (IC50) was adversely related to age.

COVID-19 boosters to prevent SARS-CoV-2 infections in adults
A) Response to COVID-19 mRNA vaccine and booster was measured as anti-RBD IgG antibodies from dried blood spots. Median IgG concentration (black dashed line) increased from 4.4µg/ml pre-booster to 101.6µg/ml post-booster (*p<0.001). Grey dotted lines represents paired samples. n=33. B) There was a median 25-fold change post-booster. C) Median anti-RBD IgG concentration (black dashed line) are shown. Individuals with outpatient COVID-19 had a median of 1.92 µg/ml (n=76) 14-42 days after infection, while individuals with a history of COVID-19 followed by vaccination were higher (60.61µg/ml, n=73, 5-42 days after 2nd dose). Individuals without a known history of COVID-19 who were either seropositive or seronegative and then 2-dose vaccinated had median IgG of 34.15µg/ml (n=181) and 33.09µg/ml (n=687), respectively. Pre-booster levels mean 237.9 days after 2-doses of vaccine were 4.4 µg/ml (n=33) compared to post-booster vaccination level of 101.6 µg/ml (n=33). Dotted lines represent the 25th and 75th percentiles. (*p<0.001).


The findings suggested that giving BNT162b2/Pfizer or mRNA-1273/Moderna boosters to healthy adults could prevent infections from progressing due to the production of significant antibody responses. Furthermore, when compared to antibody-mediated immunity created after the second vaccine dosage, antibody-mediated immunity may be sustained for a longer period of time.

The study has several drawbacks, including a short timeframe, a small sample size, and the lack of cellular immunity tests. Future research can look into the impact of boosters on cell-mediated immunity.

“These data support the use of boosters to prevent breakthrough infections and suggest that antibody-mediated immunity may last longer than after the second vaccine dose.”

*Important notice

Preliminary scientific papers published on medRxiv are not peer-reviewed and should not be regarded as conclusive, should not be used to influence clinical practice or health-related behavior, and should not be recognized as established information.

Journal reference:

  • Demonbreun, A. R., Sancilio, A., Vaught, L. A., et al. (2021). Antibody titers before and after booster doses of SARS-CoV-2 mRNA vaccines in healthy adults. medRxiv.