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Why is the delta variant so much more contagious than other strains?

delta variant more contagious
Photo via Nik Anderson/Flickr (CC BY 2.0)

Almost a year and a half into the pandemic, researchers studying COVID-19 have reached a consensus that the delta variant is the most infectious variant of the virus to date. It reportedly has a transmission rate more than double previous variants, and as of early August was responsible for 98.3% of COVID-19 infections in the U.S. What makes the delta variant so much more contagious than other variants of COVID-19?

The delta variant is one of the most infectious respiratory diseases ever discovered, according to the director of the Centers for Disease Control and Prevention (CDC). 

“The delta variant is more aggressive and much more transmissible than previously circulating strains,” CDC Director Dr. Rochelle Walensky reportedly said in a recent briefing. “It is one of the most infectious respiratory viruses we know of, and that I have seen in my 20-year career.”

According to a recently published study, the viral load in a delta variant infection is over a thousand times higher than infections from earlier COVID strains. Researchers from the Guangdong Provincial Center for Disease Control and Prevention monitored 62 COVID-19 patients in mainland China that became infected with the delta variant during its first local outbreak between May 21 and June 18, 2021 and compared them to 63 COVID-19 patients infected the previous year with earlier versions of the virus. 

Based on study results, those infected in 2020 took an average of six days between first exposure to the virus and a positive PCR test, while those who contracted the delta variant in 2021 only took about four days before the infection was detected. These findings suggest that people who have contracted the delta variant are more likely to be transmitting the virus to other individuals earlier in their infection period.

The delta variant also has a mutation that makes it more efficient at destroying cells in the body. According to National Geographic, this mutation is what makes it easier for delta to “invade the host cell by fusing infected cells into structures called syncytium, which is a way of accelerating infection.” The delta variant’s spike protein has also reportedly gone through various mutations that have fortified the virus’ ability to bind to the ACE2 receptor and elude the body’s immune response. This makes the delta variant more contagious than other known COVID strains.

How infectious is the delta variant?

Epidemiologists use a metric called the basic reproductive number, or “Ro,” — the average number of people that an infected person is expected to infect — to track how quickly an infectious disease can spread. During the 1918 influenza pandemic, the Ro was estimated between 2.0 and 3.0. The first SARS coronavirus epidemic of 2002 reportedly had a Ro of 3.0, while the Middle East Respiratory Syndrome (MERS) coronavirus pandemic first detected in 2012 had a Ro between 0.69 to 1.3. 

By comparison, COVID-19 can reportedly be as transmissible as chicken pox, an infectious disease that has a Ro between 9.0 and 10, according to the CDC.

Compared to earlier strains of the virus, the delta variant also causes more severe infections. In a study conducted in Singapore, delta variant infection was linked to higher oxygen requirements, ICU admission rates, and death occurrences. Similar results were detected in a Canadian study, in which researchers noted that, in the absence of another variant of concern, the current delta strain’s “progressive increase in transmissibility and virulence” will “result in a significantly larger, and more deadly, pandemic than would have occurred.” A Scotland study found that the delta variant causes approximately double the amount of COVID-related hospital admissions and increases the risk of admission for patients with five or more relevant comorbidities.

Are the vaccines effective against the delta variant?

Being vaccinated is still your best defense against COVID-19. A full vaccination will also provide protection against the more contagious delta variant. According to the CDC, there is an eight-fold reduction in incidence of disease and a 25-fold reduction in incidences of hospitalization and death when you get vaccinated against COVID-19. Additionally, low vaccination coverage in many communities is what’s driving the delta variant surge, and is likely to heighten the chance of newer variants cropping up.

The effectiveness of the vaccines are expected to fluctuate over time, however, leading to concerns about the long-term protection provided by vaccines.

Delta has already been found to be more likely to evade antibodies. A Paris-based study published in July found that antibodies provided by the Pfizer and AstraZeneca vaccines are effective against the delta variant, but are around three to five times less effective than they are against the original virus strain. Similar results have been found for the Moderna vaccine.

The single-dose Johnson & Johnson vaccine, according to studies, has also been found to be effective against the delta variant, but the decrease in its efficacy is significantly greater compared to the mRNA vaccines. Not only is the delta variant more contagious than other variants, but it is also more likely to elude the vaccines.

This has led Pfizer to seek authorization of a booster shot of its vaccine, while Moderna is reportedly testing an updated mRNA vaccine booster shot. Those who have received a dose of the J&J vaccine in San Francisco can reportedly request a supplemental dose of any of the mRNA vaccines. Last month, the Biden administration announced plans to offer COVID-19 vaccine booster shots to Americans starting on Sept. 20, which is awaiting approval from the U.S. Food and Drug Administration.

To address the ever-shifting information regarding COVID-19 and the delta variant, the CDC has updated its guidance for fully vaccinated people. Guidance now includes wearing a mask in public indoor settings in areas of “substantial or high transmission,” and wearing a mask regardless of transmission rates if they or someone they live with are immunocompromised or have a heightened risk for severe disease from COVID-19. 

Sources: CDC [1, 2, 3, 4], NPR, CNBC [1, 2], Virological.org, National Geographic [1, 2], ScienceDirect, SSRN, medRxiv, The Lancet, Nature, The New England Journal of Medicine, bioRxiv, ClinicalTrials.gov, Kaiser Family Foundation


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