Delta Variant: What We Know About the Science

On July 27, 2021, the CDC released updated guidance on the need for an urgent increase in COVID-19 vaccination rates and a recommendation for everyone in areas with significant or high transmission to wear a mask in public indoor spaces, even if they are fully vaccinated. CDC has issued these new guidance in response to several worrying developments and emerging data signals. The first is a reversal in the downward movement of the falls. In the days leading up to our guidelines update, CDC saw a rapid and alarming surge in COVID cases and hospitalization rates across the country.

  • At the end of June, our 7-day moving average of reported cases was around 12,000. On July 27, the 7-day moving average of cases hit over 60,000. This fall rate was more similar to the fall rate we had seen before the vaccine was widely available.

Second, new data emerged that the delta variant was more contagious and resulted in increased communicability compared to other variants, even in vaccinated individuals. This includes recently released data from CDC and our public health partners, unpublished surveillance data that will be publicly available in the coming weeks, information from CDC's updated Science Brief on COVID-19 vaccines and vaccinations, and ongoing outbreak investigations in the Connection with the delta variant.

Delta is currently the predominant strain of the virus in the United States. Below is a high-level summary of what CDC scientists recently learned about the Delta variant. Further information will be provided as further data is published or published in other formats.

Infections and spread

The delta variant causes more infections and spreads faster than early forms of SARS-CoV-2

Shut down

  • The delta variant is more contagious: The Delta variant is highly contagious, almost twice as contagious as previous variants.
  • Some data suggest that the Delta variant could cause more severe illness than previous strains in unvaccinated individuals. In two different studies from Canada and Scotland, patients infected with the Delta variant were hospitalized more often than patients infected with Alpha or the original strains of the virus.
  • Unvaccinated people remain the greatest concern: Although breakthrough infections are much less common than infections in unvaccinated people, people infected with the Delta variant, including fully vaccinated people with symptomatic breakthrough infections, can spread it to others. CDC continues to evaluate data on whether fully vaccinated individuals with breakthrough asymptomatic infections can be transmitted. However, the greatest risk of transmission is from unvaccinated people, who are much more likely to be infected and therefore transmit the virus.
  • Fully vaccinated people with breakthrough Delta variant infections can spread the virus to others. However, vaccinated individuals appear to be contagious for a shorter period of time: Earlier variants typically produced fewer viruses in the body of infected fully vaccinated individuals (breakthrough infections) than unvaccinated individuals. In contrast, the delta variant appears to produce the same high levels of virus in both unvaccinated and fully vaccinated individuals. However, as with other variants, the amount of virus produced by breakthrough delta infections in fully vaccinated individuals declines faster than infections in unvaccinated individuals. This means that fully vaccinated people are likely to be less contagious than those who are not.


Vaccines in the US are highly effective, including against the Delta variant

  • The US-approved COVID-19 vaccines are highly effective at preventing serious illness and death, including against the Delta variant. However, they are not 100% effective and some fully vaccinated people become infected (called breakthrough infection) and become ill. For these people, the vaccine still offers strong protection against serious illness and death.


Given what we know about the Delta variant, vaccine effectiveness, and current vaccine protection, multi-layered prevention strategies such as wearing masks are needed to reduce the transmission of this variant

  • As we are currently increasing vaccination levels across the country, we must also use all available prevention strategies, including indoor masking in public places to stop transmission and stop the epidemic.
  • Vaccines play a vital role in limiting the spread of the virus and minimizing serious illness. While vaccines are highly effective, they are not perfect and breakthrough vaccine infections will occur. Millions of Americans are vaccinated, and that number is growing. This means that despite the low risk of breakthrough infections, thousands of fully vaccinated people can become infected and infect others, especially with the rapid spread of the Delta variant. The low immunization coverage in many communities is driving the current rapid and large increase in cases associated with the Delta variant, which also increases the likelihood that even more worrying variants could emerge.


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