All viruses—including SARS-CoV-2, the virus that causes COVID-19—change over time. When a virus copies itself inside human cells, or replicates, small changes can occur. A virus that continues to replicate with one or more new changes—called mutations—is referred to as a “variant.”
Since we published a previous blog post on variants of SARS-CoV-2, a new variant, called Delta, has emerged and spread rapidly throughout the world.
Delta Variant
The Delta variant (also known as B.1.617.2) is considered a “variant of concern” (VOC) because it is possibly 20-60% more transmissible (more easily spread between humans) than previous strains, may be associated with a higher risk of severe disease requiring hospitalization, and could be less susceptible to an immune response, even in people who are vaccinated or who previously had COVID-19.
Originally identified in India in late 2020, Delta is thought to be partially responsible for that country’s devastating second wave of infections beginning in February 2021. Since then, the VOC has been detected in at least 85 countries and all 50 U.S. states. A European Centre for Disease Prevention and Control (ECDC) Threat Assessment for Delta projects that the variant will be responsible for 90% of cases on the continent by the end of August.
Delta continues to spread rapidly throughout the world, particularly among unvaccinated populations and in regions where public health measures are being loosened. More COVID-19 cases likely will lead to more people needing hospitalization, further stressing health systems, and healthcare workers. Though new variants are expected and will continue to develop, the world can prevent the evolution of new—and possibly more harmful—variants by preventing new COVID-19 cases.
Below we explore the Delta variant in the U.S., and how continuing vaccination and public health measures can help prevent transmission.
Delta in the U.S.
Delta currently accounts for an estimated 25% of new COVID-19 cases in the U.S. and likely will become the dominant (most common) strain in a matter of weeks. Many health experts warn the VOC is the United States’ greatest threat in dealing with COVID-19, urging states and territories to keep vaccinating their residents, especially in areas with low vaccination rates.
Less than half of the total U.S. population is fully vaccinated for COVID-19, and that percentage is much lower in some areas of the country. After weeks of declining new COVID-19 cases in the U.S., the number is beginning to rise again, likely driven by cases caused by Delta. The most significant increases are in communities and states with lower vaccination rates, mostly in the South and Mountain West.
Vaccines & Delta
Several studies have shown that while the current COVID-19 vaccines are effective at preventing infection with Delta, they are slightly less so than against most other variants. However, full vaccination remains highly effective at preventing serious disease, hospitalization, and death from all known SARS-CoV-2 strains.
One study from Public Health England (PHE) showed two doses of the Pfizer-BioNTech vaccine is 88% effective at preventing symptomatic disease from Delta, and another PHE analysis showed two doses were 96% effective at preventing hospitalization among people infected with the VOC. According to a another study (preprint), the Moderna vaccine is likely highly effective against Delta, although about two times weaker than against the original strain of the virus. For individuals who have received only one dose of a two-dose regimen, protection against the variant is significantly reduced, the first PHE study showed.
Scientists estimate the Johnson & Johnson (J&J) vaccine could offer significantly less protection from symptomatic COVID-19 than either of the mRNA vaccines from Pfizer-BioNTech and Moderna, around 60%. J&J is researching whether a two-dose regimen of its one-dose viral vector vaccine might be more efficacious. Other research shows that a mix of shots from an mRNA vaccine and a one-dose viral vector vaccine may provide better protection than the viral vector vaccine alone. However, those studies are ongoing.
Masking & Delta
Even in countries with relatively high vaccination rates, including the United Kingdom and Israel, the Delta variant is causing an uptick in cases. Therefore, the World Health Organization (WHO) recently urged fully vaccinated people worldwide to continue to wear masks in public, physically distance, and practice other COVID-19 control strategies.
The U.S. CDC in May told fully vaccinated Americans they no longer need to wear masks indoors or physically distance themselves from other individuals. However, that was before the spread of Delta. Some U.S. localities, including Los Angeles County in California, are following the WHO’s lead and recommending that everyone continue to wear masks in public settings, regardless of vaccination status.
On June 30, CDC Director Dr. Rochelle Walensky reiterated the agency’s guidance that people who are vaccinated are “safe” from current SARS-CoV-2 variants and do not need to wear masks. Dr. Walensky also said U.S. communities should continue to tailor their masking guidance based on local vaccination and infection rates.
How You Can Help
The best ways to prevent COVID-19 include getting vaccinated and avoiding being exposed to the virus. Even if you are vaccinated, mask wearing and physically distancing continue to be effective tools for reducing the risk of virus transmission, especially in indoor areas, in communities with low vaccination rates, and for individuals who are ineligible for or do not have access to vaccines.
If you are looking for a place to receive a COVID-19 vaccine, you can visit Vaccines.gov, where you can enter your ZIP code to search for available vaccination sites near you.
Post-vaccination studies such as HERO-TOGETHER are an important way to help researchers learn more about COVID-19 vaccines. If you have recently received a COVID-19 vaccine, please join us in HERO-TOGETHER. It takes less than 10 minutes to sign up and is an easy way to continue to fight COVID-19. You will help our understanding of the long-term outcomes of the authorized COVID-19 vaccines and receive up to $200 compensation and information about what we are learning.