The vaccination program has commenced in many countries worldwide, which is marked as a significant milestone in curbing the spread of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causal agent of the coronavirus disease 2019 (COVID-19) pandemic.
These vaccines, which have received emergency authorization from different regulatory bodies around the world, such as the Food and Drug Administration in the USA, have different mechanisms of delivery of antigens. Researchers have revealed that different vaccines have varied levels of efficacy, i.e., between 60% and 95%.
-6.jpg "Study: The importance of time post-vaccination in determining the decrease in vaccine efficacy against SARS-CoV-2 variants of concern. Image Credit: Rido / Shutterstock")
*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
The production of neutralizing antibodies is the main determining factor of the host’s immune response against viral infections. Previous studies have reported that during the early phase following vaccination, there was a correlation between the neutralizing antibodies produced by different COVID-19 vaccines and their efficacy.
Scientists around the world are concerned because of the emergence of SARS-CoV-2 variants such as the United Kingdom (B.1.1.7), South Africa (B.1.351), and Brazil (P.1) variants, which have a higher rate of transmission and can evade the vaccine-induced immunity. A reduction in the neutralization capacity was observed in the laboratory studies associated with vaccinated individuals or people who were naturally infected with a non-variant of concern (VOC). Researchers have reported a significant reduction in the neutralization capacity of the antibodies stimulated by the AstraZeneca ChAdOx vaccine against the B.1.351 variant in South Africa. Such an occurrence is associated with a loss of vaccine efficacy in a phase 1b/2 clinical trial. Similar to this report, the adenovirus vaccine by Johnson and Johnson and the Novavax vaccine has also shown a decreased efficacy in clinical trials which had been carried out in South Africa. During this time, the B.1.351 had been the dominant circulating variant over the area.
Individuals who have been vaccinated are less inclined to be severely infected than those who have not received vaccines. However, the degree of protection may vary between the VOC and the non-VOC. This is because certain VOCs have the capacity to evade the host’s immune system. One of the strategies to reckon vaccine efficacy against VOC is by comparing the efficacies of vaccines between locales with different circulating variants, e.g., the Pfizer BNT162b2 vaccine was found to be highly effective against B.1.1.7 variant in Israel. Another method is the use of case-control studies. In this method, the vaccinated individuals are compared to unvaccinated individuals with similar demographics and statistical tools are used to evaluate the VOC infection in a vaccinated population.
In several studies, quantification of the degree of immune escape by VOC was evaluated for a specific period of time after vaccination. In these studies, “fully immune” individuals are those who have passed enough time after receiving both the vaccine doses. A new study has been published on the medRxiv* preprint server, which deals with the importance of time post-vaccination based on the quantification of VOC immune evasion. The current study developed a toy model to estimate the effectiveness of the vaccines against VOC over time. Researchers have shown that the estimated value is highly dependent on time post-vaccination. Additionally, this model can describe the implications of changes in vaccine efficacy against VOC.
The long-term effectiveness of the available COVID-19 vaccines has not been assessed properly. This model does not depict the mechanism by which VOC escapes the immune system. However, this model aims to predict the dynamics of antibody neutralization as well as the efficacy of vaccines in the simplest manner. This would help understand how the duration after the vaccination is correlated with the efficacy of vaccines against VOC.
While constructing the model, researchers of the current study made a couple of assumptions. One of the assumptions is that the functional relationship perceived between the highest level of neutralization for each vaccine and the efficiency of the vaccines is linked with each other post-vaccination. Additionally, the same functional relationship is associated with non-VOC as well as VOC. However, a reduced neutralization is observed only against VOC.
There are certain limitations in calculating the efficacy of vaccines against VOC as a function of time from empirical data. For instance, the number of studies supporting the immune escape of the B.1.351 variant was small, thereby, creating an error in the prediction model. Also, maximum cases related to infection by the B.1.351 variant were restricted to the first week after the second dose of the vaccine. This data creates an uncertainty of the efficiency of the vaccine against B.1.351 in later weeks.
The current research suggested that the period of time after vaccination is an important factor that should be considered while estimating vaccine efficacy. Researchers of this study pointed out that real-world measurements are required to make solid predictions by the model. This study showed how the effect of the emerging variants in a vaccinated population changes as a function of time. This could help understand the transmission of the variants and efficacy of the vaccines against them.
*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
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