The Center for Disease Control and Prevention’s (CDC) revised recommendations that people who are fully vaccinated should mask have been based upon investigation into the Delta variant’s changed behavior with regard to vaccines. In particular, more of those who have been vaccinated can still get infected than with previous variants; and those that do, emit a viral load not much different than does a non-vaccinated infected person. This simply was not known back on May 13 when the CDC loosened their restrictions and allowed for the fully vaccinated to go without masks. Further, the virus has not promised to stay within its present behavior.
CDC director Rochelle Walensky privately briefed the Congress on July 29, based upon these new studies. The CDC plans to reveal their studies shortly; however a leaked version appeared in the Washington Post on July 30, later publicly confirmed in essentials by the CDC. That version shows that breakthrough cases will increase as a proportion of all cases, as the percentage of vaccinated Americans rises. That rise will especially show up in “congregate settings'’ and amongst the at-risk, “immune compromised, elderly, etc.” It is estimated that there are about 35,000 symptomatic infections per week amongst 162 million vaccinated. The vaccinated, compared to the unvaccinated, show an 8-fold reduction in infection; and a 25-fold reduction in hospitalizations and deaths. (The numbers would be even better, except for the fact that the present vaccinated population includes a higher proportion of at-risk people than the general population.) Up until April 10, when Delta variant was not a factor in the U.S., those vaccinated with even only one dose who did get infected, had a 40% lower mean RNA viral load and shorter duration of symptoms (10.3 days vs 16.7 days). The numbers are not all in, but they have gotten worse.
