For many scientists and public health experts, widescale vaccination is the only effective way to contain the ongoing coronavirus disease 2019 pandemic (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
This highly infectious RNA betacoronavirus, which belongs to the Family Coronaviridae, was first detected in December 2019 in Wuhan, China. The severity of the disease varies greatly, ranging from asymptomatic infection to the often fatal severe multi-organ system failure.
The global outbreak of SARS-CoV-2 represents the third, and the largest, betacoronavirus outbreak of the twenty-first century. Other members of this family that can infect humans and have caused epidemics in the last two decades are severe acute respiratory syndrome (SARS-CoV-1) and middle eastern respiratory syndrome (MERS-CoV).
Many countries worldwide have been badly affected by the COVID-19 pandemic, but the US, in particular, has been especially hard hit. To date, the country has reported both the highest number of confirmed cases (33.74 million) and COVID-19-related deaths (605,942).
In the US, until late December 2020, pandemic management was based exclusively on non-pharmaceutical interventions (NPIs) such as social distancing, facemasks, isolation of individuals with disease symptoms, regional lockdowns, etc.
Meanwhile, various COVID-19 vaccines were undergoing clinical trials. And, in December 2020, some candidates did finally receive emergency use authorization (EUA) from the US Food and Drugs Administration (FDA).
The vaccines developed by Pfizer-BioNtech, Moderna Inc, and Johnson & Johnson received EUA, and a rapid vaccination program commenced across the US. Except for the Pfizer vaccine, which can be given to children between the ages of 12 and 15, all other approved vaccines can only be administered to adults.
Recently, the emergence of SARS-CoV-2 variants has thrown into question the efficacy of the approved vaccines – all of which had been developed against the original SARS-CoV-2 strain that had emerged in China in late 2019.
Some of these variants have exhibited higher infectivity and virulence. For example, the B.1.1.7 variant is not only more infectious but has shown a correlation with increased hospitalizations and deaths. Various other variants have appeared in the US, which have been recognized as variants of concern (VOC).
VOCs can be more lethal than the original strain and have caused the current transmission rate to be extremely high. They can even evade diagnostic tests, cause more severe symptoms, and show resistance to vaccine-induced immunity.
The effectiveness of the vaccine-induced immunity against variants
Mathematical modeling has been used to predict the severity of the disease and gauge its impact on the community. After the commencement of vaccination, these tools have efficiently estimated the extent of cross-protective immunity from vaccines to protect the community against the newly emerged SARS-CoV-2 variants.
New research from Arizona State University, USA, uses a novel mathematical model to predict the impact of vaccination and vaccine-induced cross-protection on the B.1.1.7 variant and other SARS-CoV-2 variants circulating in the US as of June 2021. The study has been published in medRxiv* preprint server, while it undergoes peer review.
Some of the assumptions made while developing the new mathematical model include homogeneous mixing in the population, the inclusion of new susceptible individuals who are eligible for vaccination, the assumption that the participants of the study are fully vaccinated, a less severe infection of vaccinated individuals in terms of hospitalization and death, and waning off of immunity, induced via vaccine or natural infection, over time.
The main feature of this mathematical model is the estimation of the vaccine-derived herd immunity threshold for the US. Further, this model revealed that in the US, vaccine-induced herd immunity can be attained if 61% of the population is completely vaccinated with either the Pfizer or Moderna vaccines.
A parameter sensitivity analysis identified three main factors on which the COVID-19 prevalence depends in the US. These are the daily vaccination rate, the extent of cross-protection that the vaccines offer against the newly emerged variant, and the transmission rate of the dominant variant relative to the wild strain.
The current study has suggested a few conditions under which a new variant could increase the number of COVID-19 cases across the US. These conditions are as follows:
(i) the vaccine coverage against the wild strain is low (roughly < 50%)
(ii) the variant is much more transmissible (e.g., twice more transmissible) than the wild-type strain
(iii) the level of cross-protection offered by the vaccine is relatively low (e.g., less than 70%)
Numerical simulations showed that a new SARS-CoV-2 variant is unlikely to become dominant in the US if its transmission rate is moderately higher (i.e., 1.56% more infectious than the wild strain). Further, if at least 66% of the US population is fully vaccinated, with the three approved vaccines developed by Pfizer, Moderna, and Johnson and Johnson, a moderate level of cross-protection against the new variant would prevail.
One of the limitations of the current mathematical model is that it can evaluate only one SARS-CoV-2 variant. Therefore, there is a scope for improvement where it could evaluate more than one variant and predict the collective impact on the community.
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.