Top health expert says vaccinated people are spreading delta variant: Not testing vaccinated people may be overlooking some transmission Posted on Christopher Murray said that vaccinated individuals may be contributing to the spread of COVID-19. As the delta variant becomes the most dominant strain, Murray posits that not testing vaccinated individuals may overlook possible transmission points. New data suggests that the delta variant may also be more resilient to the Pfizer vaccine. As some states report marked upticks in COVID-19 infections while the delta variant becomes the most common strain in the U.S., public health experts are encouraging further vaccinations to help curb transmission.