Last week CDC changed their guidance for people living in areas of high transmission (like Arizona) urging everyone to wear a mask whether or not they’re vaccinated. There was a lot more to the guidance – but that was the top line change.

I initially had a beef with the CDC on that change because they relied on unpublished data to make the policy change:  “… based on emerging evidence suggests that fully vaccinated persons who do become infected with the Delta variant are at risk for transmitting it to others”  (CDC COVID-19 Response Team, unpublished data).

I have a problem with agencies making policy changes without disclosing the data sources that they’re using. In this case, CDC made a major policy change that impacts the behavior of hundreds of millions of people based on data that was unpublished.

Decision-making like that makes it very difficult for public health people in the field to defend the new policy. The agency is essentially saying ‘trust us, we’re right but we won’t tell you what the data is that drove the decision‘. Furthermore, it impairs adherence to the new policy because it’s not backed up by reviewable data.

Later in the week, CDC finally published the data that they used to develop the policy change earlier in the week.

Some of those data were published Friday in CDC’s Morbidity and Mortality Weekly Report.  It presented data suggesting that Delta infection results in similarly high SARS-CoV-2 viral loads in vaccinated and unvaccinated people… suggesting an increased risk of transmission in vaccinated persons with breakthrough infections (this has not been known to be the case in previous strains).

I’m OK with the guidance now, but I sure wish they had published their sources before making the policy change.

Remember, that the vast majority of persons infected with the virus at any given time are unvaccinated, so I’d still characterize the CDC’s decision to recommend public masking of vaccinated persons something that is being done ‘out of an abundance of caution’.