Description: Why Delta Variants Spread More Rapidly Than Other Covid-19 Strains

Recent epidemiological studies have shown that the delta variant (B.1.617.2) is at least 40% more infectious than the alpha variant first identified in the United Kingdom in late 2020. Multiple studies Shows reduced vaccine efficiency against delta mutants, and fully vaccinated individuals remain vulnerable Breakthrough infectious disease..

WHO Chief Tedros Adhanom Ghebreyesus previously stated at a news conference that Delta is the most contagious subspecies identified so far and is becoming more rapid. Dominant Covid-19 shares In many countries.According to an internal presentation distributed within the US Centers for Disease Control and Prevention (CDC), the Delta Variant More contagious It is at least as infectious as chickenpox than the viruses that cause MERS, SARS, Ebola, colds, seasonal flu, and smallpox.

Source: JHUCSSE and covariants.org

Why are delta variants more infectious?

Recent studies suggest that important amino acid mutations may be behind the ferocious infectivity of delta mutants.

A team of researchers led by Pei-YongShi, a virologist at the University of Texas School of Medicine, focused on key mutations that alter a single amino acid in the SARS-CoV-2 peplomer. This change, called P681R, converts proline residues to arginine, according to a study published in the Nature journal. The change occurs at the furin cleavage site of the peplomer.

To penetrate the cell, the SARS-CoV-2 spike protein must be cleaved twice by the host protein. The furin cleavage site of Covid-19 is important because it means that the host enzyme containing furin can make the first cleavage. After this, newly formed virus particles emerge from the infected cells and can be more efficiently infected by the host cells.

Alpha variants also have mutations in the same place, but contain different amino acid changes. Studies show that in the case of delta, mutations that alter furin cleavage had a significant effect.

In a preprint of a recently available study, researchers found that spike proteins were cleaved much more efficiently in delta variant particles than alpha, and the P681R mutation was the main reason for spikes to be clipped efficiently. I said there is.

The study states that the P681R mutation holds the key to the high infectivity and rapid infection of delta mutants. Researchers have found that delta mutants spread more rapidly than alpha in infected cultured human airway epithelial cells. However, when researchers removed the P681R mutation, the difference in transmissibility was lost.

The study reflects the results of a study led by a team led by Kei Sato, a virologist at the University of Tokyo, where the P681R-bearing peplomer is nearly three times faster than the mutation in the protoplasm of uninfected cells. It turned out that it can be fused with the membrane. being insufficient.

Source: CoVariants.org and GISAID via Our World in Data

Are other important mutations also responsible for the rapid transmission of the delta?

Scientists say that there are several important mutations in the delta mutant, and while changes in P681R are important, they are likely not the only reason to promote their rapid transmission.

Ugandan researchers said the P681R modification was present in a widespread variant in the country in early 2021, but has not proven to be as infectious as Delta. NS Kappa variantDelta’s siblings identified in India had the same mutation, but researchers found that the peplomer was cleaved less frequently and did not fuse very efficiently to the cell membrane.

Previous studies have shown that other important mutations in Delta variants, such as L452R and D6146, allow the virus to attach more tightly to receptor cells and allow immunity to escape more easily.

In fact, the delta mutant has multiple mutations in the S1 subunit of the peplomer, which contains three of the receptor-binding domains, improving its ability to bind to the ACE2 receptor and evade the body’s immune system. increase.

“I think the virus has been successful in quantity and speed. It has become a much more efficient virus. Gary Whittaker, a virologist at Cornell University, told Nature.

Whittaker further said that this study coronavirus.. He added that P681R would not be the last mutation at the furin cleavage site that caused concern.

Source: CoVariants.org and GISAID via Our World in Data

Why is it so difficult to stop the spread of delta variants?

One of the other reasons for the rapid spread of delta variants, as discovered during a study in Guangzhou, China, is that patients are very infectious even in the presymptomatic stage. This means that there is a risk of spreading the virus, even before people suspect that they are infected.

According to a recent study by the University of Hong Kong epidemiologist Benjamin Cowling and his colleagues, people were 5.8 days after being infected with Delta, that is, 1.8 days after the first positive viral RNA. Symptoms began to appear. Therefore, this leaves a dangerous window where virus infections occur.

In other words, the onset of symptoms occurs later, so more people infected with the delta variant can pass on the infection to others, even before they even suspect they may be ill. ..

The study found that 74% of delta infections occurred in the presymptomatic stage. Researchers also found that in the case of Delta, the basic reproduction number is 6.4, which represents the number of people who can be expected to infect the disease with R-naught, or on average one infected person. To look at things, according to some previous studies, the R-naught of the Wuhan lineage was between 2 and 4.

Is reduced vaccine efficacy against Delta a concern?

Several studies have shown that vaccines are less effective against Delta variants and that fully vaccinated individuals are vulnerable to breakthrough infections.

Early public health studies England Oxford-AstraZeneca Vaccine Effectiveness 64% for delta variants..A study previously published in The Lancet found that people who were completely vaccinated with the Pfizer vaccine 5 times or more Compared to the original strain, the level of neutralizing antibody against the delta mutant is low.

Recently, according to data from the Israeli Ministry of Health, two shots of Pfizer Provides 64% protection For Covid, an era when more than 90% of the cases reported in Japan were caused by delta mutations.

Another concern is the effectiveness of the vaccine, which diminishes over time. In a recent study, researchers at the University of Oxford in the United Kingdom and the Office for National Statistics in the United Kingdom found that the effectiveness of the Pfizer vaccine dropped to 90% a month after the second vaccination and 85% two months later. 78 percent after 3. For AstraZeneca, the equivalent protection provided was 67, 65, and 61 percent.

The study also found that infections caused by delta variants produce peak viral load similar to that of unvaccinated individuals, even in fully vaccinated individuals. Similar results have been obtained in studies conducted in the United States and Singapore.

This means that the biggest concern is that even fully vaccinated people can spread the delta mutant. This makes it very difficult to break the chain of infection.

According to a report released this month by the CDC, 469 Covid-19 cases were reported in the state after a large rally in Provincetown, Massachusetts, of which nearly three-quarters were vaccinated. They were people.

The analysis found that as many people took jabs as they did unvaccinated. Ct value, Indicates that the amount of virus is high. After genome sequencing of 133 samples, it was found that 90% of cases were caused by delta mutants. Following the findings, the July 27 CDC renewed its guidelines, recommending and even masking even vaccinated people in areas with “substantial” and “high” infections of Covid-19. Must be worn.