Why is the delta variant of the coronavirus so contagious?

The delta variant of the coronavirus was discovered last October in India. Since then, it has spread to around 100 countries, including Spain, where it became the dominant strain in four regions in July. By mid-July, it already accounted for 68% of new coronavirus cases in the country, according to Health Ministry data. The massive spread of the virus is attributed to the fact that it is 60% more contagious than other strains. Last Friday, the US Centers for Disease Control (CDC) released a document that stated that the delta variant is more contagious than the MERS virus, ebola, the common cold, the seasonal flu and smallpox, and as transmissible as chickenpox. Another study indicates that the viral load of those infected with the delta strain is up to 1,260 times greater than the original coronavirus variant, a factor which could be key to its high transmissibility – but not the only one.

This investigation was carried out by the Guangdong Provincial Center for Disease Control and Prevention in China, and has yet to be reviewed by other scientists, which is key to validating the work. According to the study, the first local infection of the delta variant in the city of Guangzhou in Guangdong province was detected on May 21, 2021. A total of 167 cases were detected between that date and June 18. Researchers carried out daily PCR tests on the close contacts of these cases, and noted the viral load of each infected person on the first day that they tested positive for the virus. Afterwards, they compared these figures with those from 63 coronavirus cases in 2020, who contracted the original strain of the virus. The researchers found that the viral load of those who had contracted the delta strain was 1,260 higher than the group with the original variant.

According to the CDC, the delta variant is more contagious than the MERS virus, ebola, the common cold, the seasonal flu and smallpox

A greater viral load may be linked to a greater number of superspreader events, says Juan Bárcena, a virologist and investigator at Spain’s National Institution of Agricultural Research (INIA). “With this strain, which has higher loads at the beginning, it is easier for superspreader events to happen,” he explains. In this situation, a single person is able to infect 10 or more of their contacts. At the beginning of this year, a residential apartment building in Bilbao in Spain’s Basque Country had to be quarantined after a superspreader sparked a coronavirus outbreak in the block. More than 30 people contracted the virus from the building’s common elevator and its old ventilation system, five of whom died.

In the Guangdong study of the close contacts of delta cases, researchers were also able to determine the time between exposure and detection of the virus. In the case of the delta virus, this was an average of four days, compared to six days for the original strain of the virus. According to the report, on June 6, the provincial government of Guangdong, required people leaving Guangzhou city from airports, train stations and shuttle bus stations to show proof of a negative Covid-19 test within 72 hours. One day later, this was reduced to 48 hours. The comparable time window in the 2020 epidemic was seven days. Because the delta variant has a shorter incubation period, tracking systems have to work faster – which is not always possible due to the workload.

The study also indicated that the delta variant could be more infectious in the first days of contagion, even before symptoms develop. However, the authors of the investigation say it is difficult to measure infectiousness from clinical investigations since “more than 50% of transmission occurs during the pre-symptomatic phase.” The researchers explain that pinpointing when an infected person can transmit the virus “is essential for designing intervention strategies that break chains of transmission.”

Juan Bárcena believes that while the Guangdong study may have relied on a small sample of cases, its findings are valid. “We are seeing a virological study in real-time. To be able to speak about this it would be better to have five years of work, with people in many countries, but we are doing things this way,” he says. “The strength of the work is that they have followed cases from the time before they were positive. I believe it is fast and precise work, fairly well-framed, with clear conclusions.”

Nuria Izquierdo-Useros is the leader of IrsiCaixa, a research group on emerging pathogens, and although she is wary of the figures in the study, she trusts its overall conclusions. “There is something that is undeniable and that is that the delta variant is dominating the world. This is because it has to have some advantage,” she explains. “This data has to be taken with great caution, because these quantifications have been made comparing populations in China, but they don’t say very well what the clinical characteristics of these people are. And the viral load can change depending on these clinical characteristics.” Izquierdo-Useros says the experiment should be repeated with a larger sample, from different countries in order to confirm the results and find out definitively why the delta variant is so contagious.

Delta mutations

A second study, published in the scientific journal Nature on July 28, indicated that the delta variant also has many different mutations, although there are three specific to the strain that improve receptors’ ability to bind to cells. This allows contagions to happen more quickly and for the virus to partially evade the immune system. Another study published in the same journal on July 8, found that while both doses of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines against Covid-19 were able to inhibit the delta variant, they offered less protection against the strain than other mutations. Despite this, Pfizer and BioNTech announced on July 8 that they were developing an updated version of their Covid-19 vaccine to target the delta variant. Clinical studies of this version are set to begin in August.

English version by Melissa Kitson.