SARS-CoV-2 Delta variant's effect on COVID-19 vaccine efficacy

By Shanet Susan AlexReviewed by Emily Henderson, B.Sc.Feb 7 2022

In a recent research paper published in the journal Infection, the scientists estimated the transmission and disease severity associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant and the effectiveness of the current coronavirus disease 2019 (COVID-19) vaccines against the same by conducting a literature review. Further, a meta-analysis was conducted to obtain a numeric estimate of the vaccine effectiveness against the Delta variant.

Study. Implication of the emergence of the delta (B.1.617.2) variants on vaccine effectiveness. Image Credit: Carl DMaster/Shutterstock

Background

Several vaccines have been developed and distributed worldwide to prevent SARS-CoV-2 infection and mitigate the ongoing COVID-19 pandemic. These vaccines have been effective in lowering SARS-CoV-2-related hospitalizations, severe forms of the disease, and mortality. 

SARS-CoV-2 variants associated with high virulence have been designated as a variant of concern (VOC), and those with high potential to cause the infection are termed as a variant of interest (VOI) by the World Health Organization (WHO). The emergence of mutated SARS-CoV-2 VOCs like the Delta variant, including B.1.617.2, has been a cause of concern for healthcare systems across the world since it resulted in additional SARS-CoV-2 waves. 

About the study

In the current literature review, the researchers estimated the effect of SARS-CoV-2 convalescent sera, sera from vaccinated subjects, and monoclonal antibodies (mAbs) on Delta from in vitro studies. Then, the impact of vaccination on Delta was estimated from in vivo studies. The databases such as Embase, Scopus, and PubMed were searched till August 4, 2021, using keywords such as vaccinations, neutralizing antibody, and Delta variant to conduct the literature review.

Additionally, a meta-analysis was conducted to evaluate the COVID-19 vaccine effectiveness against Delta variants and obtain a numeric estimate for vaccine efficacy by screening 21 articles based on the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) protocol. Among the five articles identified to be eligible, four were analyzed in the present meta-analysis.

Findings

In vitro studies included in this review indicated that the sera samples from COVID-19 convalescent individuals demonstrated four- to six-fold lower neutralization rates against the SARS-CoV-2 Delta variant than the D614G strain. Hence, SARS-CoV-2-recovered individuals were associated with a higher risk of reinfection with the Delta variant.

In vitro analysis of clinically approved monoclonal antibodies (mAb), such as etesevimab, casirivimab, imdevimab, and bamlanivimab, for SARS-CoV-2 treatment, indicated a lower sensitivity to the Delta variant, and bamlanivimab did not show any neutralizing activity against Delta.

The in vitro and in vivo studies showed that sera samples from individuals vaccinated with a single dose of COVID-19 vaccines, such as Oxford-AstraZeneca and Pfizer-BioNTech, had reductions in the neutralizing capacity against Delta. However, individuals who received two doses of Moderna, Pfizer-BioNTech, and Oxford-AstraZeneca vaccines, were associated with a lower SARS-CoV-2 Delta incidence.

In one of the studies, two trekkers fully vaccinated with AstraZeneca and Moderna tested positive for the SARS-CoV-2 Delta variant. Severe pneumonia was observed in the AstraZeneca-vaccinated individual, whereas the Moderna-vaccinated individual developed only mild sore throat and congestion. Both individuals recovered from the Delta infection eventually. Similarly, in another study, an outbreak of Delta was reported in a gymnastics facility with 47 infected cases. However, Delta variant-associated hospitalizations were reported only in two unvaccinated individuals.

Further, an outbreak of the Delta variant was reported in six COVID-19 two-dose vaccinated attendees of a wedding event. One attendee who received COVAXIN died, while some recipients of the Pfizer vaccine required an infusion of mAbs. 

In another instance, the SARS-CoV-2 Delta outbreak was reported at a hospital in Finland among 45 healthcare workers (HCWs) and 58 patients. The mean age of the patients was 80 years, and SARS-CoV-2-related death was reported in one fully vaccinated, 11 single-dose vaccinated, and six unvaccinated patients. The mean age of the HCWs was 38 years, and no deaths were reported among them. Secondary infections from the HCWs were reported in the community.

The initial willingness of HCWs in the Kingdom of Saudi Arabia to receive Pfizer and AstraZeneca vaccines were 20.9% and 24.4%, respectively. Vaccination rates reported in the United States (US) ranged from 44.8% and 75.4% in Mississippi and Vermont, respectively, given that the seven-day moving average of SARS-CoV-2 Delta cases in the US was 350%. 

Furthermore, few studies indicated that heterologous vaccinations with Pfizer and AstraZeneca vaccines were associated with strong immune responses.

Conclusions

The study findings highlighted the importance of vaccination to mitigate the transmission of the SARS-CoV-2 Delta variant since the symptomatic Delta infection was significantly lower in fully vaccinated individuals. Therefore, complete vaccination together with adherence to the currently implemented health protocols to control COVID-19, such as masking, social distancing, and sanitizing, are required to harness the transmission of Delta. 

Additionally, the study emphasized the requirement of policies to identify the population at risk and inadequacies of vaccination, early SARS-CoV-2 detection, timely quarantining of COVID-19 patients, preparing healthcare systems, and broadening vaccination coverage at the local levels.

Furthermore, continued investigations on the SARS-CoV-2 variants and the development of vaccines with the potential to neutralize current and future variants are needed to tackle the ongoing COVID-19 pandemic.