The CDC’s Slender Reed

Based on thin evidence, the agency’s new mask guidance may undermine the vaccination effort.

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The Centers for Disease Control and Prevention (CDC) has reversed its guidance from two months ago and now recommends that fully vaccinated individuals should wear a mask in public indoor settings in areas of the country with high or substantial transmission—23 and 20 states, respectively, according to the CDC. There is little in the data to support this reversal. The agency is probably being overcautious or responding to political pressure to “do something.”

The CDC says that “preliminary evidence” suggests that breakthrough infections of vaccinated people from the Delta variant of Covid-19 can spread the virus to others. Masking vaccinated people will thus protect them from infection and from passing the virus to unvaccinated people. But the agency never quantifies either the risks of breakthrough infections and secondary transmission or the benefits of masking.

Delta poses little risk to the vaccinated. The vaccines protect against infection and are highly effective in preventing serious Covid-19 illness and death with all variants, including Delta. In addition, the symptoms and duration of illness in those vaccinated people suffering breakthrough infections have been attenuated compared with cases among unvaccinated people.

So what has changed? The CDC’s assessment of the risk that breakthrough cases might pose to others. Viral loads in breakthrough infections have been shown to be lower than those in infected unvaccinated people, suggesting that breakthrough cases would be less likely to transmit the infection. But now the CDC, citing a non-peer-reviewed, preprint study, along with unpublished data, claims that breakthrough Delta infections are more transmissible than breakthrough infections from other variants. The study examined breakthrough infections in 100 Indian health-care workers vaccinated with an Indian version of the Astra-Zeneca vaccine (not used in the U.S.). Infections with the Delta variant showed higher respiratory viral loads compared with non-Delta infections, leading to increased transmission between health-care workers.

The CDC also cited another preprint study showing that Delta-variant infections make up a higher percentage of breakthrough cases than other variants in Houston. But the study also showed that breakthrough cases were only 6.5 percent of all cases and were far less likely to require hospitalization. Significantly, the study showed that breakthrough cases had lower viral loads, indicating less transmission potential than in unvaccinated patients (though this portion of the study did not distinguish between Delta and non-delta variants).

These small and somewhat contradictory studies seem like a slim reed on which to base a nationwide change in policy. The CDC has not released any other data showing that breakthrough Delta infections present as high a transmission risk as infections of the unvaccinated. Despite the current Delta surge, vaccinated people likely pose a minimal risk to the uninfected.

The CDC acknowledges that even with Delta, only a small percentage of fully vaccinated people will be infected. For a vaccinated person to become infected, they have to encounter an infected person who is actively shedding the virus. A large percentage of the population is immune and unlikely to be infected or shedding; half the U.S. population is vaccinated, and at least another 20 percent to 25 percent more have natural immunity after recovering from Covid-19. Even in areas with high or substantial transmission, the prevalence is low; most of the unvaccinated we meet do not have an active Covid-19 infection. Moreover, even if they are infected, relatively prolonged close contact is needed to create a transmission risk. Finally, the vaccines appear to be about 90 percent effective in limiting transmission.

Read the full article at City Journal.