Staff Writer

The Delta (B.1.617.2) coronavirus variant originally discovered in India last December has now become the most dominant — and worrisome — strain of the coronavirus circulating globally. Research indicates that it is the most transmissible variant yet — as much as 60 percent more contagious than the Alpha (U.K./B.1.1.7) variant, which itself fuelled numerous waves of the pandemic around the world.

Delta has already spread to at least 98 countries, and prompted explosive outbreaks in countries or areas of countries with low vaccination rates. In Zimbabwe, Delta has quickly become the dominant strain: On July 20, Cabinet estimated that Delta now accounts for 83 percent of all new sequenced cases in the country up from 50 percent at the beginning of the month.

On top of all that, Delta may be more likely to infect people who are only partially vaccinated than other strains, and may also come with a higher risk of hospitalization. Below is what we know about the Delta variant.

How is Delta different from other variants, and why may it be more dangerous?

The Delta variant “is faster, it is fitter, and it will pick off the more vulnerable more efficiently than previous variants,” warned Dr. Mike Ryan, the executive director of the World Health Organization’s Health Emergencies Program, on June 21.

Delta has multiple mutations that appear to give it an advantage over other strains. The most important apparent advantage is that the mutations may make the strain more transmissible than any other variant, which would also make it the most dangerous variant yet.

Professor Neil Ferguson, a leading epidemiologist at Imperial College London and one of the chief pandemic advisers to the U.K. government, said on June 4 that Delta is estimated to be 60 percent more transmissible than Alpha, which is itself more transmissible than the original strain of the coronavirus that emerged in China in late 2019 and that is why scientists believe it became a dominant variant globally. Other estimates from the U.K. have said that Delta may be 40 or 50 percent more transmissible than Alpha.

Numerous COVID experts and the WHO warn that Delta variant will soon become the most dominant COVID strain in the world and drive rapid outbreaks among unvaccinated populations.

There is limited research regarding whether or not the Delta variant causes more severe illness than other variants. According to Public Health England, early data suggests that Delta is more likely to lead to hospitalization than Alpha, but that could be due to increased transmissibility rather than it being more pathogenic. 

Vaccines are effective against Delta — but may be slightly less effective

Research by the U.K. government has found that full vaccination is effective against the Delta strain but may be slightly less effective than against other variants, particularly after only one dose.

One set of U.K. government research found that two doses of a COVID vaccine provided 81 percent protection against the B.1.617.2 variant (compared with 87 percent protection against the B.1.1.7 variant). One dose only provided 33 percent protection against symptomatic infection from B.1.617.2 (compared with 51 percent protection against B.1.1.7). That means, according to a Financial Times analysis, that a single dose is 35 percent less effective against B.1.617.2 than it is against B.1.1.7.

If that is accurate, it means that Delta may be the variant that currently poses the biggest threat to partially vaccinated populations worldwide.

Again, as with every known variant, full vaccination works against the Delta strain, preventing serious illness at the very least but there are still signs that the variant marks a worrisome evolution in the coronavirus, and it seems likely it could raise the stakes for countries that continue to struggle to vaccinate their populations.

In a study released in Nature on July 8, French researchers tested unvaccinated people who had survived a bout of COVID-19 in order to study how well antibodies produced by natural infection, as well as vaccines, can neutralize Delta. Antibodies from those who were previously infected did not neutralize Delta very well, but a single dose of vaccine dramatically boosted their antibody levels.

“This is an important study for confirming the immune evasiveness property of Delta, which is a feature that adds to its enhanced transmissibility, making it the most formidable version of the virus to date,” Eric Topol, director of the Scripps Research Translational Institute, told STAT News. 

“No surprises, but further characterization of the variant, which reinforces why it is so challenging.”

In order to get back to the world we want and not to lose loved ones, please let us all get vaccinated. #MaskUp and get #Vaccinated

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