New CDC data shows COVID-19 vaccines still work well against Delta

New data from the US Centers for Disease Control and Prevention (CDC) is offering up-to-date insights into the effect of the Delta variant on vaccine effectiveness. The reports indicate unvaccinated people are still 10 times more likely to suffer hospitalization or death from COVID-19 compared to vaccinated people.

Over the past two to three months the Delta variant has swiftly become the predominant type of SARS-CoV-2 in the United States. In June the variant accounted for around 10 percent of COVID-19 cases. Now, about 99 percent of genomically isolated cases are found to be the Delta variant. Alongside Delta’s rapid dominance a new wave of cases have spiked and these new reports from the CDC offer some of the first insights into how effectively vaccines are holding up against this new variant.

The biggest of the three new CDC reports looked at more than 600,000 COVID-19 cases across 13 US states between early April and mid-July. In order to track any changes relating to Delta’s influence, the study compared data before and after June 20 – the date from which Delta accounted for more than 50 percent of all infections in the country.

The report found vaccine effectiveness held strong against Delta, with protection from hospitalization only dropping from 92 percent to 90 percent, and protection from death dropping from 94 percent to 91 percent. Against basic symptomatic infection the report found vaccine effectiveness dropped from 91 percent to 78 percent as Delta became predominant.

The report notes that while the number of symptomatic cases in vaccinated people did increase as Delta spread, rates of hospitalizations and death did not rise. This indicates vaccines are still offering robust protection from this variant despite the small rise in vaccine breakthrough infections.

“Looking at cases over the past two months, when the Delta variant was the predominant variant circulating in this country, those who were unvaccinated were about four and a half times more likely to get COVID-19, over 10 times more likely to be hospitalized, and 11 times more likely to die from the disease,” says CDC Director Rochelle Walensky.

Although Delta accounts for more infections in vaccinated people, the rise in hospitalizations and deaths are primarily in those unvaccinated
Although Delta accounts for more infections in vaccinated people, the rise in hospitalizations and deaths are primarily in those unvaccinated


Another new CDC report looked at 32,867 cases of COVID-19 needing some kind of medical encounter between June and August, the period when Delta had become predominant. This data showed unvaccinated individuals were between five and seven times more likely to need emergency department care due to COVID-19 compared to those vaccinated.

Perhaps more interesting is this particular study’s breakdown of different COVID-19 vaccine effectiveness. This data suggests differences in effectiveness between the two commonly used mRNA vaccines. The study indicates the Moderna vaccine is 95 percent effective at preventing COVID-19 hospitalization, while the Pfizer vaccine is only 80 percent effective at preventing hospitalizations.

The researchers note this is not the first study to find small differences in effectiveness between the Moderna and Pfizer mRNA vaccines. A recent study is cited showing Moderna may be twice as effective at preventing symptomatic COVID-19, however, that study is still yet to be peer-reviewed and published in a journal.

It is unclear exactly why there could be efficacy differences between Moderna and Pfizer’s relatively similar mRNA vaccines. One hypothesis suggests it could be because Moderna’s vaccine doses are significantly higher than the Pfizer vaccine, but there are plenty of other factors that could be influencing these mRNA vaccine differences. For example, this particular study did not track time since vaccination, so it is unknown whether waning efficacy accounts for these differences.

Shaun Grannis, from the Indiana University School of Medicine, says the biggest takeaway from this new data is that vaccines are still extraordinarily potent against severe COVID-19 caused by the Delta variant.

“Despite the differences in effectiveness, vaccines continue to offer much more protection than not getting one at all,” says Grannis. “While breakthrough cases do happen, data shows the symptoms are less severe. The vast majority of COVID hospitalizations and deaths continue to be among unvaccinated individuals. COVID-19 vaccines are powerful tools for combating the pandemic.”

Source: CDC, Regenstrief Institute

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