Study: COVID-19 due to the B.1.617.2 (Delta) variant compared to B.1.1.7 (Alpha) variant of SARS-CoV-2: two prospective observational cohort studies. Image Credit: Corona Borealis Studio/Shutterstock

UK study finds COVID-19 from Delta and Alpha infections are clinically similar

A recent study, published on the preprint server medRxiv*, compares the two severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants – the B.1.617.2 (Delta) variant and the B.1.1.7 (Alpha) variant and provides a meaningful clinical difference in the coronavirus disease 2019 (COVID-19) caused by the virus in the United Kingdom (UK).

The study showed that the clinical presentation of COVID-19 due to the Delta variant is similar to the illness caused by the Alpha variant. Though the Delta variant is more transmissible than the Alpha, anti-SARS-CoV-2 vaccines are effective against both variants.

Study: COVID-19 due to the B.1.617.2 (Delta) variant compared to B.1.1.7 (Alpha) variant of SARS-CoV-2: two prospective observational cohort studies. Image Credit: Corona Borealis Studio/ShutterstockStudy: COVID-19 due to the B.1.617.2 (Delta) variant compared to B.1.1.7 (Alpha) variant of SARS-CoV-2: two prospective observational cohort studies. Image Credit: Corona Borealis Studio/Shutterstock


In May 2021, the Delta variant became the predominant circulating SARS-CoV-2 strain in the UK, replacing the Alpha variant. Mutations in viruses affect their transmission ability, disease presentation, the natural or vaccine-induced protective immunity.

The dominance of a particular strain also affects other factors such as the SARS-CoV-2 prevalence, delivery of an age-stratified mass vaccination campaign, relaxation of lockdowns, and test access criteria. To date, there are little or no studies, beyond case series, that investigate symptoms and/or illness duration due to Delta variant infection otherwise in comparison with the Alpha. Therefore, it is crucial to prospectively assess the epidemic.

In a previous study, the researchers described the COVID-19 profile, the transmissibility, re-infection risk, and the vaccine effectiveness when the Alpha variant was predominant. In the present study, they look at the data after the Delta variant became the dominant circulating form of SARS-CoV-2, (from 0·09% at the beginning of April 2021 to >98% at the end of June 2021). They present a comparative study on illness from Delta vs. Alpha infection, in a large UK community cohort.

Study findings

The researchers collected the prospective longitudinal observational data as part of the King’s College London [KCL]/ZOE COVID Symptom Study. The study cohort included symptomatic adults participating in the app-based COVID Symptom Study (using the ZOE COVID Study App). The data was obtained from approximately 1 million UK app users. This study uses data from one of the largest UK citizen science epidemiological initiatives.

The participants tested positive for SARS-CoV-2 from May 26th to July 1st 2021 – the period when the Delta variant was the overwhelmingly predominant circulating UK variant. The researchers compared (1:1, age- and sex-matched) with individuals presenting from December 28, 2020, to May 6, 2021 – when Alpha was the predominant variant.

The number of individuals in the study cohort included: 19,118 individuals when Alpha was predominant and 3,581 when Delta was predominant. The researchers presented a table listing the detailed characteristics of the UK individuals presenting with COVID-19, during periods of Alpha and Delta SARS-CoV-2 variant predominance.

Because the participants with a positive test did not have the variant confirmation by sequencing, illness within these two timeframes was attributed in the study to the predominant circulating variant.

The researchers described the illness, which includes the symptoms, duration, burden, profile, risk of a long illness, and hospital attendance, in symptomatic adults with either the Alpha or Delta variant during Alpha-predominant and Delta-predominant timeframes in the UK.

Assessing evidence of transmission, reinfection, and vaccine effectiveness, the researchers in the present study showed that the seven most common symptoms of Delta infection were the same as with Alpha infection (though varied in prevalence):  headache, fatigue, rhinorrhea (nasal discharge), anosmia/dysosmia (smell and taste disorders), sneezing, sore throat, and persistent cough.

The burden of symptoms was higher with Delta infection in the initial week. However, long COVID-19 illness with the duration of symptoms ≥28 days trended towards lowered risk with Delta infection than the Alpha infection, though unchanged in unvaccinated individuals. Researchers also observed that the risk of hospital presentation was moderately lower for Delta versus Alpha infection.

Here, the researchers repeat that the observations need to be interpreted in the context of vaccination rollouts among the UK population, along with many other environmental and societal changes.

For the percentage of daily positive SARS-CoV-2 testing information, the researchers extracted the time-series data from COVID-19 Genomics UK Consortium (COG-UK) – where a random sample of positive PCR tests from the general community is sequenced. The researchers found that the transmissibility of the Delta variant was more than 1·47 than the transmissibility by Alpha – despite pertinent differences, e.g., viral prevalence, lockdown restrictions (between the current and previous studies).

On the effect of Delta variant on re-infection, the researchers observed that for most regions of the UK, the rise in COVID-19 correlated more with increased prevalence of SARS-CoV-2 overall than rather than the increased proportion of circulating SARS-CoV-2 due to the Delta variant specifically.

Post-vaccination with BNT162b2 (BioNTech-Pfizer) or ChAdOx1 nCoV-19 (Oxford-Astra Zeneca), infection during the Delta period was enormously reduced. Thus, this supports the effectiveness of both the vaccines against the Delta variant.


Despite higher transmissibility of the Delta variant than the Alpha, the study observed that the COVID-19 from Delta or Alpha infections is clinically similar.  Although symptom burden in the first week is modestly higher, the risk of LC28 was lower. The Delta variant was more transmissible than the preceding predominant Alpha variant. However, it did not increase the risk of reinfection per se.

Importantly, the Delta variant is more responsive to the current vaccines, which show good efficacy against disease. This supports the prosecution of the anti-SARS-CoV-2 vaccination campaign internationally.

*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.