If we’ve learned one thing about the Centers for Disease Control and Prevention over the past almost year and a half, it’s that much of what it considers to be hard scientific evidence is … not that.

The agency updated its mask guidance last week because of new data suggesting that fully vaccinated adults carry higher viral loads than unvaccinated adults when they catch the coronavirus. This would mean that vaccinated adults are just as likely to transmit the virus as unvaccinated adults, which would lead to an increase in breakthrough infections among the vaccinated and regular infections among the unvaccinated. (Even though this study suggests vaccinated and unvaccinated adults have a similar chance of transmitting the delta variant, an individual would have to contract the virus first. Vaccination against COVID-19 makes infection much less likely.)