Preliminary research tries to explain why the delta variant is so contagious

The study conducted in China investigates the characteristics that have made the delta variant of Sars-Cov-2 the one prevalent in most of the world in a short time

(photo: Ayelt Van Veen / Unsplash) It is now established: the delta variant of Sars-Cov-2 has become predominant in many countries of the world, including Italy. But if the numbers immediately indicated how easily it spread, the mechanisms behind its contagiousness had not yet been investigated. Now a pre-print study by the Center for Disease Prevention and Control of Guangdong Province, China, has analyzed the characteristics of delta infection compared to that of the original strain of the coronavirus. The results seem to explain why the variant is so contagious: those affected by the former Indian variant would have a viral load about a thousand times higher than the original, favoring super-diffusion phenomena and also hindering an effective treatment of infections. >

The situation in Italy

Appearing for the first time in India in late 2020, the delta variant of Sars-Cov-2 (also called B.1.617.2) has become the predominant strain largely of the world. In Italy, according to the international database Gisaid, which collects the deposited sequences of the coronavirus, the prevalence of delta in the last four weeks is equal to 72.3%.

As also underlined by the latest report on the weekly monitoring of Sars-Cov-2 by the Ministry of Health, the circulation of delta is constantly increasing in Italy, making it the prevailing variant. These last few weeks, in fact, have marked, in Italy, the overtaking by the delta of the alpha variant (B.1.1.7, the former English variant). This is not surprising, considering that even in Great Britain this variant is now present in almost all cases (again from Gisaid data, its prevalence amounts to 99.5%).

According to data from the Public Health England delta would have, compared to alpha, a 40-60% greater ability to be transmitted from person to person (and in turn the alpha variant was also more contagious than the original Sars-Cov-2 strain). For the World Health Organization, delta is the fastest and most able to transmit variant. The Chinese study, led by epidemiologist Jing Lu, tried to explain why.

Measuring the delta variant

The research (which, remember, is in pre-print and therefore not still undergoing peer review) monitored 62 people in quarantine for being exposed to the coronavirus, among the first to be infected with the delta strain in mainland China. The researchers quantified their viral load (the numerical estimate of viral particles in the body) every day, for the duration of the infection. All this was then compared with the trend of the infection of 63 people who contracted Sars-Cov-2 in 2020, before the diffusion of the variants.

According to the data deriving from the study, the delta variant is it reproduces within the organism much more than the original strain and even faster. If, in fact, in people in contact with the variant, the virus was detected after four days of exposure, normally those who contracted the infection in 2020 had an average of six days of incubation. Furthermore, some individuals infected with delta also possessed viral loads that were a thousand times higher (up to 1,260 times, to be precise) than those of people infected with the original strain.

New analyzes

Although the study is still preliminary, according to several scientists, such as Emma Hodcroft of the University of Bern in Switzerland and Benjamin Cowling of the University of Hong Kong, the data relating to viral load and reduced incubation times could explain the transmissibility of delta, so high compared to the original Sars-Cov-2 strain, but also compared to the other variants.

In particular, the presence of many more virus particles in the respiratory tract would greatly facilitate the phenomena of "superdiffusion" ( that is, when a single person infected with the coronavirus infects many others), while a faster incubation would hinder an effective tracing of the infections.

The proposed mechanism, according to experts , it would seem sensible, but further studies conducted on large populations of infected people are needed to confirm the differences between the delta variant and the original Sars-Cov-2 strain. Differences that do not stop only at the viral load: it is not yet clear, for example, how much contracting delta causes a more serious disease or how much the variant manages to escape our immune system. “The virus surprised us”, concludes Hodcroft, underlining the importance of continuing this type of research.


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Coronavirus Sars-Cov-2 variants globalData.fldTopic = "Coronavirus, Sars-Cov-2 variants "

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