Mayo Clinic Staff
The delta (B.1.617.2) variant is nearly twice as contagious as earlier variants and might cause more severe illness. The greatest risk of transmission is among unvaccinated people.
People who are fully vaccinated can get vaccine breakthrough infections and spread the virus to others. However, it appears that vaccinated people spread COVID-19 for a shorter period than do unvaccinated people.
While research suggests that COVID-19 vaccines are slightly less effective against the delta variant, the Pfizer-BioNTech, Moderna and Janssen/Johnson & Johnson COVID-19 vaccines still appear to provide protection against severe COVID-19.
The omicron (B.1.1.529) variant spreads more easily than the original virus that causes COVID-19 and the delta variant. However, omicron appears to cause less severe disease.
People who are fully vaccinated can get breakthrough infections and spread the virus to others. But the COVID-19 vaccines are effective at preventing severe illness.
This variant also reduces the effectiveness of some monoclonal antibody treatments. Omicron has a few major offshoots (sublineages), including BA.1, BA.2, BA.3, BA.1.1 and BA.2.12.1.
BA.2 made up about 62% of COVID-19 infections that had genetic sequencing in the U.S. during a week in June, according to the CDC.