Why is the COVID Delta Variant So Infectious?

The “delta variant” of COVID-19 is far more virulent than the original strains, and is largely responsible for the recent surges in COVID cases in the U.S. and worldwide. Centers for Disease Control and Prevention Director Rochelle Walensky told the Senate on July 15 that the delta variant now makes up 83% of U.S. cases, up from 50% at the beginning of this month. It was first detected in India, then spread to the U.K. and the U.S., and around the world.

What is it that makes the delta variant so infectious? From a molecular point of view, here are the known functional mutations in the DNA that produces the “spike” proteins in the virus which bind to human cells:

Source: Stanford

Four of these mutations in particular are believed to contribute to the virulence of this strain, as discussed here. Among other things, they can cause the spike protein to bind more strongly to our cells, and inhibit our immune response. See here for 3-D model of the virus spike binding to human receptor, showing the locations of those mutated proteins.

As a result of those mutations the delta variant grows faster inside people’s respiratory tracts and reaches much higher levels. Per NPR,

On average, people infected with the delta variant had about 1,000 times more copies of the virus in their respiratory tracts than those infected with the original strain of the coronavirus, the study reported.

In addition, after someone catches the delta variant, the person likely becomes infectious sooner. On average, it took about four days for the delta variant to reach detectable levels inside a person, compared with six days for the original coronavirus variant.

… People who have contracted the delta variant are likely spreading the virus earlier in the course of their infection.

How can we stop it? It is pretty simple: get vaccinated (or never be in a closed space with other unvaccinated humans). Vaccines don’t totally prevent you from getting COVID initially, so you might still have early symptoms and also be able to spread the virus to others for a few days. However, vaccines are highly effective in helping your immune system to quickly shut down any infection you do get before the symptoms get severe. This is true for all for essentially all strains of COVID, including delta.

Again per NPR,

Preliminary data shows that in some U.S. states, 99.5% of COVID-19 deaths in the past few months were among people who weren’t vaccinated, said CDC director Dr. Rochelle Walensky at a White House press conference in early July.

And 97% of those currently hospitalized with COVID-19 are unvaccinated, according to Walensky.

Just compare the two maps below of which American states have high/low vaccinations and high/low COVID incidence, and draw your own conclusions: