Why is the Delta variant spreading so quickly?

The Delta variant replicates rapidly in cells, infected people have a short incubation period, the virus is concentrated in the upper respiratory tract so it is easily spread into the environment.

The number of Covid-19 cases worldwide is increasing alarmingly. Experts say the cause lies in the more contagious Delta variant. According to the World Health Organization (WHO), the variant spreads about 55% faster than the Alpha strain, which first appeared in the UK late last year.

The Delta variant was discovered in India in December 2020. It quickly became the dominant strain, overwhelming the country's health system and soon spreading to the UK. "This is the most contagious version of the virus since the beginning of the pandemic," said Ashish Jha, dean of the Brown University School of Public Health.

The first case of the Delta variant in the US was reported in March. By early July, the variant accounted for more than 50% of positive cases in the country. Today, that number has risen to 83%, according to the Centers for Disease Control and Prevention (CDC).

New daily cases are up 55% this week compared to last week, according to data from Johns Hopkins University. Hospitalizations in the past 14 days are up 52%.

Since March of this year, scientists have been trying to figure out why Delta spreads so rapidly.

A study published in July on the Virological website found that the variant produced more copies of the virus in a shorter period of time when it was present in cells. Chinese scientists compared dozens of Delta cases with the early strain of nCoV, and found that patients had 1,260 times more virus.

"A lot of the virus is in the body. The amount of virus multiplies rapidly over a shorter period of time, five minutes, seven minutes. You can get sick even if you're 1.8 meters away from the source of infection," Dr. Jha explained. "Unvaccinated people get sick over a much shorter period of exposure," she said.

A Covid-19 patient at a hospital in St. Petersburg, Russia, July 14. (Photo: AFP)

The incubation period for people infected with the Delta variant is shorter. Symptoms will appear after 4 days, instead of 6 days like the original virus strain.

Researchers also measured the viral shedding of 62 people during the outbreak in Guangzhou, China, from May 21 to June 18, and compared it to 63 patients in 2020. They found that people infected with the Delta variant spread the virus earlier. Experts stressed the importance of immediately isolating positive cases.

In addition, according to Professor David Wohl, an infectious disease specialist at the University of North Carolina School of Medicine, the Delta variant is concentrated in the upper respiratory tract such as the nose and throat . When coughing or sneezing, patients release large amounts of virus particles into the air.

Young people are also at higher risk of infection with the variant in the new global outbreak . This is a population that travels a lot, meets a lot of people, but most of them have not been vaccinated.

Professor Wohl said that young people have a different pattern of symptoms than older people. He said that the spread of the variant depends on the community group in which it is circulating, not just the virus itself.

Research published in the journal Nature on July 8 showed that Delta avoids some antibodies that target specific parts, and is much less sensitive to natural antibodies than Alpha.

In people who received one dose of the vaccine, only 10% had neutralizing antibodies against Delta and Beta (first seen in South Africa). After the second dose, the number increased to 95%.

The finding is consistent with data from trials in Israel and the UK. The new study also found that the Delta variant did not respond to bamlanivimab, a monoclonal antibody developed by Eli Lilly. But three other monoclonal antibodies from the company were effective.

Current vaccines are known to be effective against the Delta variant. People who have received two doses rarely become severely ill. But the vast majority of the world's population has not been fully vaccinated. Just over 25% have received two doses of vaccine, and 13% are fully vaccinated, according to Our World in Data.

"If you are not vaccinated, you are at high risk of getting sick. You need to take precautions like wearing a mask, avoiding indoor gatherings and keeping a safe distance," said Vivek Murthy, the US Surgeon General.