Hasty about-face distorts Delta force
WE finally know what drove the CDC to revise its guidance days ago without explanation to recommend that fully vaccinated individuals wear masks in public indoor settings in areas of the country with high or substantial COVID-19 transmission. The Washington Post released a July 29 CDC slide presentation, and the CDC confirmed that one of the unpublished studies cited was pivotal in its decision.
The slides largely just confirm what was already known: The Delta variant is more contagious than earlier variants.
Two of the slides suggest people with breakthrough Delta infections can spread the virus to others, leading to the claim vaccinated people must wear masks. In fact, a nonpeer-reviewed, preprint study claimed that breakthrough Delta infections are more transmissible than breakthrough infections from other variants. It examined breakthrough infections in 100 Indian health-care workers and found higher viral loads in Delta infections compared with non-Delta infections, leading to increased transmission. But — some say crucially — this small study used an Indian version of the AstraZeneca vaccine which is not used in the US. It may have limited relevance to the mRNA vaccines used here.
The slides also reported unpublished data, now updated by the
CDC, that convinced the agency to take action: 74 percent of the 469 cases in an outbreak in Barnstable County, Mass., following the July 4 weekend were in vaccinated people and most showed symptoms. Among cases who had genomic sequencing and viral load determinations, there was no difference in Delta viral load between 127 vaccinated cases and 84 unvaccinated cases. This suggests Delta breakthrough cases pose as high a transmission risk as regular Delta cases, which is higher than other variants.
But does this one study really warrant a large change in policy?
It is difficult to assess unpublished data. Only time and published studies will tell if this report is accurate.
But it is odd that there are no similar findings in studies from areas like India and the UK, where the Delta variant has been prevalent for months. An outbreak where threequarters of cases were breakthrough cases among those who had been vaccinated and most were symptomatic is unprecedented. It strongly suggests detection bias — only cases that became ill were testing leading to an undercount of asymptomatic cases. In addition, the CDC acknowledged that the outbreak occurred after public events marketed to adult men and that the study lacked information about underlying health conditions, including im-munocompromising conditions, that might have made study participants particularly susceptible to infection and higher viral titers.
Unfortunately, the report did not describe the severity of symptoms or connect them to the amount of virus. Yet it is hugely important to note that few patients were hospitalized and none died. It may be that, as with earlier variants, most breakthrough cases are only mildly symptomatic and that asymptomatic and mildly symptomatic Delta breakthrough cases have low viral loads
and pose little threat to others. It would have been helpful to answer this question before acting.
The CDC acknowledges that even with Delta, only a small percentage of fully vaccinated people will be infected, so why do all vaccinated people need to wear a mask? And why change guidance for a variant surge that is likely to be over soon? In both India, where Delta originated and first surged and where few are vaccinated, and in the UK, where both Delta and vaccination are common, case counts peaked and rapidly fell over just a few weeks. In the UK, health services were not overwhelmed and a rise in deaths did not match the vertiginous spike in cases.
Half of the US population is vaccinated, which CDC data show is nearly as effective against Delta as against earlier variants. Many millions more — perhaps as many as 20 to 25 percent of the population — have natural immunity, which, as one of the studies on the CDC slides showed, provides good protection against Delta reinfection for at least six months. Most of the unvaccinated are young and healthy and unlikely to suffer severe COVID-19 cases, which explains why, even after a month of rising Delta cases, hospitalizations have only risen modestly and deaths are basically flat.
Rather than recommend universal masking, the CDC could have advised those most at risk — people who are immunocompromised and account for almost half of breakthrough infections and the elderly who comprise 75 percent of hospitalized or fatal breakthrough cases — to take mitigating measures like distancing until the Delta surge passes.
The CDC guidance may provide a pretext for more restrictive measures like lockdowns and school closings. The CDC announcement of the Barnstable results suggests that even areas without high or substantial transmission should consider masking and other restrictions. American Federation of Teachers President Randi Weingarten already appears to be hedging on her earlier commitment to reopen schools, saying that the new guidance is a “curveball” and only that “we’re going to try to open up schools.”
Moreover, the CDC action is sure to undermine confidence in vaccines needed to combat Delta and end the pandemic. An AP-NORC poll, published just before the CDC announcement, reported that 30 percent of Americans were not confident that vaccines are effective against new variants. Only 28 percent were very confident the vaccines work. Among the unvaccinated, 64 percent lacked confidence in the vaccines.
At such a crucial time, the CDC and the Biden administration must be sure it has cast-iron scientific proof to change its guidance or risk sowing unnecessary alarm and confusion across the country. It is far from convincing it has that proof.