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Delta Variant: What We Know About the Science

August 26, 2021

Some data suggests, that The Delta Variant, might cause more severe illnesses, than the previous strains, in unvaccinated persons

On July 27th, 2021, The CDC, released updated guidance, on the need for urgently increasing, COVID-19 Vaccination coverage and a recommendation, for everyone in areas of substantial or high transmission, to wear a mask in public, indoor places, even if, they are fully vaccinated. The CDC, issued this new guidance, due to several concerning developments and newly emerging data signals. First, is a reversal in the downward trajectory of cases. In the days leading up to our guidance update, The CDC, saw a rapid and alarming rise in the COVID-19 Cases and hospitalization rates, around the country.

In late June, our 7-day moving average of reported cases, was around 12,000. On July 27, the 7-day moving average of cases, reached over 60,000. This case rate looked more like, the rate of cases that we had seen, before the Vaccines, were widely available.

Second, new data began to emerge, that The Delta Variant, was more infectious and was leading to increased transmissibility, when compared to other variants, even, in vaccinated individuals. This includes recently published data from The CDC and from our public health partners, have unpublished surveillance data, that will be publicly available, in the coming weeks, information included in The CDC’s updated Science Brief on COVID-19 Vaccines, Vaccinations and ongoing, outbreak investigations, that are linked, to The Delta Variant.

The Delta Variant, is currently the predominant strain of the virus, in the United States. Below, is a high-level summary of what, The CDC's Scientists, have recently learned about The Delta Variant. More information, will be made available, when more data is published or released, in other formats.

Infections And Spreading:
The Delta Variant, is highly contagious, nearly twice as contagious, as previous variants. Some data suggests, that The Delta Variant, might cause more severe illness than previous strains, in unvaccinated persons. In two different studies from Canada and Scotland, patients infected with The Delta Variant, were more likely to be hospitalized, than patients infected with The Alpha Strain or The Original Virus Strains.

Unvaccinated people, remain the greatest concern. Although breakthrough infections, happen much less often, than infections in unvaccinated people, individuals infected with The Delta Variant, including fully vaccinated people with symptomatic breakthrough infections, can transmit it to others. The CDC is, continuing to assess data on whether fully vaccinated people with asymptomatic breakthrough infections, can transmit. However, the greatest risk of transmission, is among unvaccinated people, who are much more likely to contract and therefore, transmit the virus.

Fully vaccinated people with The Delta Variant breakthrough infections, can spread the virus to others. However, vaccinated people, appear to be infectious for a shorter period: Previous variants, typically produced, less virus in the body of infected and fully vaccinated people, with breakthrough infections, rather, than in, unvaccinated people. In contrast, The Delta Variant, seems to produce the same high amount of virus in both unvaccinated and fully vaccinated people. However, like other variants, the amount of virus produced by The Delta Variant's breakthrough infections, in fully vaccinated people, also goes down faster than infections, in unvaccinated people. This means fully vaccinated people, are more likely, to be infectious for less time, than, with unvaccinated people.

The COVID-19 Vaccines authorized in the United States, are highly effective at preventing severe disease and death, including, against, The Delta Variant. But they are not 100% effective and some fully vaccinated people will become infected by a breakthrough infection and experience illness. For such people, the vaccine still provides them strong protection, against serious illness and death.

Given what we know about The Delta Variant, vaccine effectiveness, current vaccine coverage and layered prevention strategies, such as wearing masks, are needed to reduce the transmission, of this variant.

At this time, as we build the level of vaccinations nationwide, we must also use all of the prevention strategies available, including masking indoors, in public places, to stop the transmission and the epidemic.

Vaccines are playing a crucial role, in limiting the spread of the virus and minimizing, severe disease. Although vaccines are highly effective, they are not perfect and there will be vaccine breakthrough infections. Millions of Americans are vaccinated and that number is growing. This means that even though, the risk of breakthrough infections is low, there will be thousands of fully vaccinated people, who become infected and able to infect others, especially, with the surging spread of The Delta Variant. Low vaccination coverages in many communities, are driving the current rapid and large surges, in cases associated with The Delta Variant, which also increases the chances, that even more concerning variants, could emerge.

Understanding Variants:
Viruses constantly change through mutation, and new variants of a virus, are expected to occur. Sometimes, new variants emerge and disappear. Other times, new variants persist. Multiple variants of COVID-19 have emerged, in the United States. At this point, the original variant that caused the initial COVID-19 cases in January 2020, are no longer circulating, as newer variants, have increased.

How Variants Work:
If you think about a virus, like a tree growing and branching out; each branch on the tree, is slightly different than the others. By comparing the branches, scientists, can label them according to the differences. These small differences, or variants, have been studied and identified, since the beginning, of the Pandemic.

Some variations, allow the virus to spread more easily or make it resistant, to treatments or vaccines. Those variants, must be monitored, more carefully.

How Variants Change:
As the virus spreads, it has new opportunities to change and may become, more difficult to stop. These changes can be monitored, by comparing differences, in physical traits (such as, resistance to treatment) or changes in, the mutation's genetic code, from one variant, to another.

For more information visit the CDC's web-site at:

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