Several recent studies have reached similar conclusions around transmission of COVID-19 among vaccinated individuals:

1. Individuals who are fully vaccinated transmit the virus less than those who are partially vaccinated (received one dose only) and unvaccinated individuals have the highest risk of transmission.
2. Vaccines are less effective against preventing transmission of the Delta variant, when compared to other variants of COVID-19. 

Vaccines can reduce transmission by preventing infections and reducing the amount of infectious virus in someone’s body if they do get sick. However, the exact rate of transmission among vaccinated individuals (though it is rare) is still uncertain. A lot of cases of infection in vaccinated people do not progress to symptomatic disease, making it harder to know who is sick and collect data on how contagious they may be. 

A study published by researchers at the University of Oxford in September examined the contacts of confirmed COVID-19 cases. The cases included individuals vaccinated with Pfizer-BioNTech or the AstraZeneca vaccines. The study found that transmission was reduced as a result of both vaccines. The Pfizer-BioNTech vaccine was more protective against transmission than the AstraZeneca among people who received both doses of either vaccine. Additionally, people who received two doses of either one contributed less transmission to their contacts than individuals who had only received one dose. 

In addition to different types of vaccines, the study also looked at transmission for both the Alpha and Delta variants, and found that the vaccines were more protective against transmission of the Alpha variant. Furthermore, this study enforced previous published data about the Delta variant in a study from Singapore. 

At the beginning of an infection with the Delta variant, both vaccinated and unvaccinated people tend to have similar amounts of the virus in their system. However, vaccinated people are able to fight the virus and clear it from their system in a much shorter duration, which may reduce transmission among the vaccinated people. 

In a preprint study from China examining the records of transmission among an outbreak that happened in Guangdong, China in May-June 2021, researchers found people who received two doses of a vaccine transmitted the virus less than those who received only one dose or none at all. 

As with waning immunity after vaccination, experts also predict a decline in the vaccine’s ability to prevent transmission. The University of Oxford study found that after three months of vaccination, the Delta variant was transmitted similarly by vaccinated and unvaccinated individuals. 

Another study out of the UK found that vaccinated individuals with a breakthrough infection of the Delta variant are twice as likely to pass on the infection than those who had a breakthrough infection with the Alpha variant. This waning protection suggests that booster campaigns can be a good strategy for preventing further transmission by the Delta variant in the future.

The United States Center for Disease Control and Prevention published a report in August about an outbreak in Massachusetts where 90% of the cases among the vaccinated was due to an infection by the Delta variant, and continued to say that the data from this report is not enough to draw conclusions about the effectiveness of COVID-19 vaccines against the Delta variant. 

More research is needed about the relationship between transmission of COVID-19 and the current vaccines in order to understand how vaccination affects transmission in the long run. 

Despite the recent evidence that antibodies produced by both vaccines and previous infections decrease over time, experts continue to remind us of the bigger picture:

1. Breakthrough cases among the vaccinated are rare
2. The majority of new cases are among the unvaccinated 
3. Vaccines are highly effective for preventing severe consequences of COVID-19

Experts continue to support vaccination as a key strategy to control the pandemic, and recommend combining it with social distancing and mask wearing. With enough immunity in the population COVID-19 can be a disease the society can live with, if enough people are vaccinated.