Delta Variant: What You Need to Know

The Delta variant is an example of how the virus that causes COVID-19 can change as it spreads. Many people are wondering about what it means for fully vaccinated people. Here are your questions answered and some important things you should know about the Delta variant to protect yourself and others.

What you Should Know about the Delta Variant

  • Increase in Cases: Toward the end of the summer, Delta was the cause of more than 99% of new U.S. COVID-19 cases, according to CDC estimates.
  • Extremely Contagious: The CDC says this variant is more than two times as contagious as previous variants. It is as contagious as chicken pox (which is more contagious than a cold or flu).
  • Breakthrough Infections: Vaccinated people can get breakthrough infections of the Delta variant and may be contagious. In the U.S. as a whole, 0.004% of COVID breakthrough infections to date have resulted in hospitalization and/or death.
  • Fully vaccinated people with 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 fully vaccinated people (breakthrough infections) 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 Delta breakthrough infections in fully vaccinated people also goes down faster than infections in unvaccinated people. This means fully vaccinated people are likely infectious for less time than unvaccinated people.
  • Delta Variant and Vaccines: The COVID-19 vaccines approved or 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 (called a breakthrough infection) and experience illness. For all people, the vaccine provides the best protection against serious illness and death.
  • 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. 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.
  • Mask Update from the CDC: Given what we know about the Delta variant, vaccine effectiveness, and current vaccine coverage, layered prevention strategies, such as wearing masks, are needed to reduce the transmission of this variant. On July 27, 2021, 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.
  • Scientists are Monitoring: Viruses constantly change through mutation, and new variants of a virus are expected to occur. Scientists monitor these changes, including changes to the spikes on the surface of the virus. By carefully studying viruses, scientists can learn how changes to the virus might affect how it spreads and how sick people will get from it.

FAQs about the Delta Variant

1. Should I be concerned about the Delta Variant?

According to the CDC, new data shows that the Delta variant is different than past versions of the virus and is much more contagious.
  • Vaccinated people can get breakthrough infections of the Delta variant and may be contagious.
  • Vaccinated individuals represent a very small amount of total transmissions.
However, if you are vaccinated, your risk of infection is lower, and your risk of getting ill, being hospitalized and dying from COVID-19 is much lower.

2. Should I wear a mask?

According to the CDC, in all areas of substantial or high transmission, everyone should wear a mask in public indoor settings to help prevent the spread of the Delta variant and to protect others. Most spread is among unvaccinated people in areas of low vaccination rates. Vaccines help prevent Delta from spreading further.

3. Are the vaccines effective?

Yes! The 163+ million fully vaccinated people in the United States have protection against all known variants, including Delta. Vaccination helps them avoid severe illness, hospitalization, and death. Unvaccinated individuals account for nearly all the COVID-19 hospitalizations and deaths in the country.
Despite a surge in infections that are similar to what we saw last summer, deaths are down more than 70% thanks, in part, to vaccination. According to the CDC, getting fully vaccinated is the best thing you can do to protect yourself and those around you.

The Bottom Line: 

More than half of the entire American population are not yet fully vaccinated, CDC data shows. Nationwide, the number of cases has been increasing since around late June. The best way to stop the spread of Delta is to get vaccinated against COVID-19. Unvaccinated individuals should still get vaccinated and wear a mask until you do. Everyone in areas of substantial or high transmission should wear a mask, even if they are vaccinated.


CDC – National Center for Health Statistics – Homepage. August 26, 2021.