Infectiousness of breakthrough infections can be measured by viral densities. Higher SARS-CoV-2 viral density in the upper airways of people infected with the virus are thought to increase transmission to household members.5, 6 If vaccines reduce viral density in those who do become infected despite vaccination, it would probably lead to lower infectiousness and less onward transmission. Hence, the authors compared the viral kinetics in breakthrough delta variant infections in vaccinated people with delta variant infections in unvaccinated people. They report that peak viral loads showed a faster decline in vaccinated compared with unvaccinated people, although peak viral loads were similar for unvaccinated and vaccinated people.