The coronavirus disease 2019 (COVID-19) pandemic spread worldwide in 2019, causing a global health and economic crisis. The rapid transmission of the disease combined with the high death rates in at-risk groups led to millions of deaths, and many countries introducing social distancing measures, lockdowns, and other restrictions.

Study: The impact of SARS-CoV-2 vaccination on Alpha and Delta variant transmission. Image Credit: BaLL LunLa/ Shutterstock

Recently with the help of mass vaccination schemes, drug repurposing programs, and monoclonal antibodies, the pandemic has begun to be brought under control. However, worries are rising over variants of concern, such as the Delta variant, which is known to avoid both vaccine-induced and natural immunity.

Due to the increased threat posed by the variant, researchers from the University of Oxford have been examining the impact of vaccination on the spread of the Alpha and Delta variants through the population.

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

The study

The researchers used national contact testing data from England to examine the rate of transmission for the variants. They performed a retrospective observational cohort study of contacts of both symptomatic and asymptomatic convalescent individuals. The scientists only included results from those who had contact with an infected individual – proved with a PCR test – who then took a PCR test themselves within 1-10 days. They also only included cases from three laboratories, as they all used the same standardized procedure looking for the S gene encoding for the spike protein, the N gene encoding for the nucleocapsid, or the ORF1ab target.

Vaccination details for individuals meeting these criteria were then obtained from the national immunization database. The results of these tests estimated the specific strain – the Alpha strain will show a negative result for the S gene but positive results for the rest. Any full set of results before the 10th of May 2021 was considered to be for the original strain. Still, given the prevalence of the Alpha and Delta strains following this date, any non-Alpha positive test was assumed to be Delta.

Multivariable logistic regression was used to examine the transmission from the original positive test – the index case – to other contacts. This technique is often used to examine a series of predictor variables, which in this case were vaccine status, type of vaccine, contact event type, age, sex, symptoms, local prevalence of COVID-19, and local deprivation, to return a dichotomous result. Natural cubic splines and log transformation helped to account for non-linearity.

In total, nearly 100,000 index cases with a positive PCR test were identified, and over 150,000 positive contacts. Twelve thousand six hundred fifty-seven of these contacts had to be excluded due to incomplete information. Fifty-one thousand seven hundred ninety-eight of the remaining contacts tested positive. Ages ranged from 26-50 and were roughly split between men (44%) and women (56%). 70% of contact events were within the same household, with the rest evenly split between visitors, events, and work/education.

The vast majority of positive contacts were from unvaccinated individuals (49%). Around 27% were partially vaccinated, and the remaining 23% were partially vaccinated. These figures are for the ChADOx1 vaccination individuals; BNT162b2 cases showed small differences but followed roughly the same pattern.

Multivariable models showed that double vaccination for either vaccine significantly reduced the odds of infection for the Alpha strain, with BNT162b2 potentially providing a small amount of extra protection following either one or two doses.

The Delta variant showed increased transmission from the index case compared to Alpha for both asymptomatic and symptomatic individuals. Outside of vaccine status, other factors that were positively associated with a positive test were contact event type, age, the index case and the contact being different sexes, higher deprivation index, and higher local severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) incidence.

These findings are supported by previous studies showing higher risk for more deprived individuals, as well as up to a 12-fold lower antibody response from vaccinated individuals to the Delta strain as compared to the original strain found in Wuhan.

The authors highlight the importance of their results in showcasing the increased danger of the variants of concern, particularly considering the relatively high cases of double-vaccinated individuals who still tested positive for COVID-19. These results could be very important in informing public health policy.

*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.

Written by

Sam Hancock

Sam completed his MSci in Genetics at the University of Nottingham in 2019, fuelled initially by an interest in genetic ageing. As part of his degree, he also investigated the role of rnh genes in originless replication in archaea.