This tweet is not factually correct. While it is true that the vaccines are effective at prevention of serious illness, many studies show that they are also effective at preventing asymptomatic infection. That is, vaccines are effective at preventing you from catching COVID-19, and therefore reduce the risk that you transmit to others. On the other hand, unvaccinated individuals are more likely to both catch and spread COVID-19. Therefore, unvaccinated individuals are a greater threat to both vaccinated and unvaccinated individuals.