COVID-19 booster doses could significantly reduce the incidence of SARS-CoV-2 infection among young adults

A recent study published in the journal PLOS Medicine examined the effectiveness of booster doses of coronavirus disease 2019 (COVID-19) vaccines in protecting vaccinated young adults against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections during the dominant period of the Omicron BA.1 variant.

Study: Booster vaccination protection against SARS-CoV-2 infections in young adults during an Omicron BA.1-predominant period: A retrospective cohort study. Image Credit: Jo Panuwat D / Shutterstock

Background

The emergence of the immune evasive Omicron subvariants and the waning of vaccine-induced immunity have resulted in an increase in breakthrough infections. With the efficacy of the messenger ribonucleic acid (mRNA) vaccines mRNA-1273 and BNT162b2 and the adenoviral vector vaccine Ad26.COV2.S decreasing against the emergent variants, the United States (U.S.) Centers for Disease Control and Prevention (CDC) has recommended booster doses of any of the three vaccines to be administered six months after primary vaccinations with either of the two mRNA vaccines or two months after primary Ad26.COV2.S vaccinations.

While studies have found that the booster doses are effective in preventing severe outcomes such as hospitalization and death during SARS-CoV-2 infections involving the Omicron subvariants, the efficacy of booster vaccines in preventing mild or asymptomatic SARS-CoV-2 infections is not well understood. Given that the Omicron subvariants’ increased transmissibility is linked to the number of mild or asymptomatic cases, it is vital to understand how effective the booster doses are in the overall prevention of SARS-CoV-2 infections.

About the study

In the present study, the researchers conducted a retrospective, cohort-based analysis of deidentified records of Cornell University students obtained from their COVID-19 surveillance database. Participants were recruited from Cornell University’s mandatory surveillance testing implemented for faculty, students, and staff as part of their reopening strategy.

The eligible participants had to be students enrolled at the Ithaca campus of the university, with primary vaccinations with any of the World Health Organization (WHO)-approved COVID-19 vaccines. The inclusion criteria also stipulated that the participants must have no positive SARS-CoV-2 polymerase chain reaction (PCR) test in the 90 days leading up to the study. Additionally, the vaccination records, including the booster status, also had to be provided.

Since sex and the coding of the primary vaccination completion date were considered confounders in the analysis, students of unspecified sex or with incomplete, preliminary vaccinations or invalid vaccination records were excluded from the study. The study was conducted between December 5^th and 31^st, 2021, considered the dominant period of the Omicron BA.1 variant.

The measured primary outcome was SARS-CoV-2 infection confirmed by a positive PCR test. The person-days for each student were calculated, which was the number of days contributed by each participant to the boosted or non-boosted cohort during the study. The person-days were classified as boosted or control based on whether the booster had been administered at least seven days ago, as other studies had reported that booster vaccines become effective seven days after administration.

The probability of a positive PCR-based SARS-CoV-2 diagnosis after receiving a booster dose was calculated using multivariable Poisson regression analysis. The analysis was controlled for covariates such as sex, participation in fraternities or sororities, varsity athletics participation, and primary vaccination type and completion date. Participation in varsity athletics, fraternities, or sororities was included to account for the heterogeneous risk of infection across the study group. The completion date of the primary vaccination accounted for heterogeneity in social behavior, vaccine uptake, and waning immune response.

Results

The results reported that a booster dose decreased the probability of a PCR-positive SARS-CoV-2 diagnosis during the dominance of the Omicron BA.1 variant by 56%, accounting for covariates such as sex, date of completion of primary vaccination, heterogeneous behavior across the study group, and temporal effect.

The observed effectiveness of the booster dose was slightly lower than that reported by other studies against symptomatic SARS-CoV-2 infections among adults during the same time. Students who had received the Ad26.COV2.S vaccine had a higher risk of SARS-CoV-2 infection than students who had received other COVID-19 vaccines, although the difference in risk was not significant, possibly due to the small sample set of students who received the Ad26.COV2.S vaccine.

Students who had completed their primary vaccinations more recently had lower SARS-CoV-2 infection incidence rates, suggesting the waning of vaccine-induced immunity. Additionally, students with increased social contact through participation in varsity athletics and sorority or fraternity events were at increased risk of SARS-CoV-2 infections.

Conclusions

Overall, the results indicated that booster doses of COVID-19 vaccinations effectively reduced the incidence of SARS-CoV-2 infections by half among the vaccinated young adult population during the dominance of the Omicron BA.1 variant. However, since mild and asymptomatic cases of COVID-19 can still transmit SARS-CoV-2, booster doses may be essential in reducing the incidence of COVID-19.