In this catatrophic situation, several vaccines, including Pfizer-BioNTech, Moderna, AstraZeneca, protect against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These vaccines teach our cells how to make a protein (or even just a piece of a protein)—that triggers an immune response inside our bodies. Theoretical benefits of these vaccines are that those vaccinated gain protection without ever having to risk the serious consequences of getting sick and or die of COVID-19. Several studies found that these vaccines trigger an immune response against the virus 27,38,39. These vaccines have been approved for clinical use by the World Health Organization based on comprehensive assessments of non-clinical, laboratory confirmation, clinical utility, and manufacturing data provided by manufacturers to respective regulatory bodies (i.e., U.S. Food and Drug Administration (FDA) and the Department of Health and Human Services or related institutes). Though these vaccines' efficacy and side effects have not yet been widely discussed, numerous claims have been made by popular media and politicians 40,41. During this process, respective government bodies examine the quality and consistency of the vaccine so that the potential benefits offset the potential adverse effects of the vaccine. Despite unsubstantiated claims on the COVID-19 vaccines efficacy and associated side effects, there is a lack of accumulated evidence on the effectiveness of this vaccine 27. However, among different variants of SARS-CoV-2, the delta variant has significantly increased virulence and transmission capacity 42. In addition, recent evidence indicates that the vaccine's effectiveness declined with the emergence and promotion of the delta variant 6. Therefore, this meta-analysis presents a piece of systematic and evidence-based information regarding the effectiveness of different COVID-19 vaccines against specific delta variant (B.1.617.2).
The B.1.617.2 has seen a sharp rise in infections and mortality in 186 countries 2, including those with relatively high vaccine coverage and increased hospitalization and mortality in the U.K. 20,43. The high transmissibility of SARS-CoV-2 mainly of delta variant is a considerable socioeconomic global burden. Previous evidence on vaccine effectiveness reported reduced protection against the delta variant compared to the Alpha variant 1,31. Even the efficacy of the 2-dose ChAdOX1 vaccine decreased by 59.8% after exposure to B.1.617.2 31. Recent reports determined a higher vaccine breakthrough infection in terms of reduced prophylactic vaccine effectiveness against the SARS-CoV-2 delta variant (B.1.617.2) 1,31. While more contributing factors may reduce immunity, a recent Israeli study has found higher infection rates even among the earliest vaccinated individuals 44. Another study among nursing home residents found the effectiveness of the mRNA vaccine as 74.7% against infection, but the efficiency decreased significantly by 53.1% when the B.1.617.2 circulation predominated 28. The interpretation of this report is complex due to the bias of the methodology, research population, care settings, immunisation timeline and the potential selection and case diagnosis 6. Our study found that the effectiveness of COVID-19 vaccine was 80% more protective among those vaccinated with the delta variant (RR = 0.20, 95% CI: 0.07-0.54), compared to unvaccinated populations. A previous study also found a higher rate of vaccine infection breakthrough, indicating lower effectiveness of vaccines in protecting infection with the delta variant 34. Another study also argued that infections with delta variant that are transmitted despite the vaccine carry a similar viral burden for people who have not been vaccinated 14. Though the effectiveness of vaccines is lower among delta cases, completing the second dose of vaccines reduces the complication among those vaccinated 6. In addition, a growing number of cases detected after one or two doses of vaccination are more likely to be infected with B.1.617.2 than other variants 12. Since the efficacy of the vaccines against the B.1.617.2 is lower, several studies recommended an additional dose to curve the infection of the variant and optimise the immune response 12,28. Single-dose vaccine recipients with high infection rates of B.1.617.2 or who have been infected before with another variant can risk vaccine-adapted forms of evolution 12. We argues that the population should be vaccinated as much as possible since those who have not been vaccinated cannot be protected by a significant reduction in the number of vaccinated populations as seen by other infections, making herd immunity could be unachievable due emerging variants 14.
Our study is unique with several strengths. Firstly, this meta-analysis study that provides evidence-based data on vaccines' effectiveness against the B.1.617.2 variant. Secondly, we only considered the studies that provide symmetrical information of individuals vaccinated with two doses. Finally, a large population was considered in this study that yields a piece of adequate evidence on this topic. However, we acknowledge some limitations with our study design and selection of published studies. Effectiveness of vaccines (e.g., BNT162b2 or ChAdOx1 nCoV-19) after one dose was significantly lower among population with the B.1.617.2 variant (30.7%) than among those with the alpha variant (48.7%) 31. Studies on single-dose vaccination and its efficacy were excluded in this meta-analysis due to lack of reported data, lower effectiveness with the delta variant, insufficient number of studies, and we did not employ any analysis considering the sub-groups of different types of vaccines due to insufficient number of studies and reported combined results with more than one vaccine.
Nonetheless, our study provides valuable insights into the effectiveness of the vaccines against the delta variant since there is a dearth of information on this topic. Depending on the predominance of the delta variant, the findings of this study may help policy-makers take action or control measures, and coordinate vaccination effects. Recently, caution has been elevated that vaccine performance may diminish in the future, considering the possibility of a series of potential viral genetic drift 6. These claims highlight the importance of both pharmaceutical and non-pharmaceutical interventions intandem to reduce SARS-CoV-2 infection. Future research can examine the combined efficacy of mass vaccination, contact tracing and social distancing measures in minimising the socioeconomic burden arising from COVID-19. Our study's practical implication is that taking the mounting evidence on the efficacy of pharmaceutical intervention into account, mass vaccination against should be continued in a planned manner to lessen the socioeconomic burden of COVID-19. To do so, COVID-19 vaccines should be categorised as a global humanitarian good emphasising the optimisation of production and equal distribution.
On the other hand, the demand side should target reducing vaccine hesitancy so that more vulnerable populations are adequately covered. The effectiveness of COVID-19 vaccines was noted with the delta variant among populations after receiving two vaccine doses as compared with unvaccinated populations. This finding would help efforts to maximise vaccine coverage with two doses among vulnerable populations.