Viruses constantly change through mutation. When a virus has one or more new mutations it�s called a variant of the original virus. Currently, one variant of the virus (SARS-CoV-2) that causes coronavirus disease 2019 (COVID-19) is creating concern in the U.S.

• Delta (B.1.617.2). This variant is now the most common COVID-19 variant in the U.S. It�s nearly twice as contagious as earlier variants and might cause more severe illness. The greatest risk of transmission is among unvaccinated people. People with vaccine breakthrough infections also may spread COVID-19 to others. However, it appears that vaccinated people spread COVID-19 for a shorter period than do unvaccinated people. This variant also might reduce the effectiveness of some monoclonal antibody treatments and the antibodies generated by a COVID-19 vaccine.

The alpha, gamma and beta variants continue to be monitored but are spreading at much lower levels in the U.S. The mu variant is also being monitored.

While research suggests that COVID-19 vaccines are slightly less effective against the variants, the vaccines still appear to provide protection against severe COVID-19. For example:

• Early research from the U.K. suggests that, after full vaccination, the Pfizer-BioNTech COVID-19 vaccine is 88% effective at preventing symptomatic COVID-19 virus caused by the delta variant. The vaccine is 96% effective at preventing severe disease with the COVID-19 virus caused by the delta variant. The research also showed that the vaccine is 93% effective at preventing symptomatic COVID-19 virus caused by the alpha variant.
• Early research from Canada suggests that, after one dose, the Moderna COVID-19 vaccine is 72% effective at preventing symptomatic COVID-19 virus caused by the delta variant. One dose of the vaccine is also 96% effective at preventing severe disease with the COVID-19 virus caused by the delta variant.
• The Janssen/Johnson & Johnson COVID-19 vaccine is 85% effective at preventing severe disease with the COVID-19 virus caused by the delta variant, according to data released by Johnson & Johnson.

To strengthen protection against COVID-19 and the delta variant, the CDC recommends additional doses and booster doses of COVID-19 vaccines in specific instances:

• Additional dose. The CDC recommends a third dose of an mRNA COVID-19 vaccine for some people with weakened immune systems, such as those who have had an organ transplant. People with weakened immune systems might not develop enough immunity after vaccination with two doses of an mRNA COVID-19 vaccine. An additional dose might improve their protection against COVID-19.

  The third dose should be given at least 28 days after a second dose of an mRNA COVID-19 vaccine. The additional dose should be the same brand as the other two mRNA COVID-19 vaccine doses you were given. If the brand given isn�t known, either brand of mRNA COVID-19 vaccine can be given as a third dose.

• Booster dose. The CDC recommends a booster dose of the Pfizer-BioNTech COVID-19 vaccine for people age 65 and older, residents in long-term care settings and people ages 50 to 64 who have underlying medical conditions, such as cancer, diabetes and heart disease. A booster dose of the Pfizer-BioNTech COVID-19 vaccine is also available to people ages 18 to 49 with underlying medical conditions and people ages 18 to 64 who have a job that puts them at increased risk of being exposed to COVID-19. A booster dose provides additional protection for people whose immune response weakened over time.

  The booster dose should be given at least 6 months after the second dose of the Pfizer-BioNTech COVID-19 vaccine.

There isn�t enough research to determine if people with weakened immune systems who got a Janssen/Johnson & Johnson COVID-19 vaccine also have an improved response after getting an additional dose of the same vaccine. The CDC also hasn�t made recommendations about booster doses for people who got the Moderna or Janssen/Johnson & Johnson COVID-19 vaccine.