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Early Signs of the Severity of Delta

Here is a glimpse of the surge in incidence rates of the variant as it began to take hold in Mesa County, Colorado.

The COVID-19 pandemic continues to stress and surprise us. Questions regarding boosters, future variants, and the trajectory of this forth wave, all plague us. Australia is under fire against Delta, the United States has now seen several days of consecutive mortality rates paralleling those of March. We are not doing ok.

A part of outbreak response is to monitor for shifts in variants and their prevalence within a community. We rely on genomic sequencing to understand these dynamics and keep an eye on how diseases are spreading within a population. One of the first big indicators we saw within the United States was a rapid increase in the Delta (B.1.617.2) variant. In a recent Morbidity and Mortality Weekly Report from the Centers for Disease Control and Prevention (CDC), a review of an increase in Mesa County, Colorado from April to June of this year was discussed.

Starting in early May when the first cases of SARS-CoV-2 Delta was reported in Mesa County, they found that all cases were associated with a school setting. Not surprisingly, efforts were increased to address this worrisome situation. “Through early June, Mesa County experienced a marked increase in the proportion of Delta variant cases identified through sequencing: the 7-day proportion of sequenced specimens identified as B.1.617.2 in Mesa County more than doubled, from 43% for the week ending May 1 to 88% for the week ending June 5. As of June 6, more than one half (51%) of sequenced B.1.617.2 specimens in Colorado were from Mesa County.”

The research team found several interesting aspects of this sudden surge in cases. First, 27.5% of the population aged 65 and older were fully vaccinated, which was actually higher than other counties in Colorado. Moreover, of the 18,000+ sequenced specimens reported in Colorado through June 6, 783 were of the Delta variant, and 51.1% were from Mesa County residents despite the population only accounting for less than 3% of the population within the state.

The authors noted that, “during April 22–June 26, a total of 37 COVID-19 outbreaks were reported in Mesa County; 13 (35%) in residential care facilities, 11 (30%) in schools, two (5%) in correctional facilities, and 11 (30%) in other settings. Twelve outbreaks, including seven in residential care facilities, had at least one Delta variant case.

Average vaccination coverage in these seven residential facilities was 87% among residents (range = 50%–97%) and 50% among staff members (range = 6%–69%); attack rates among residents ranged from 0% to 54.6% (median = 1.2%) and among staff members from 2.2% to 25.5% (median = 10.0%). Five of these seven outbreaks involved at least one case in a fully vaccinated resident or staff member.”

While this study desperately reiterates the need for more genomic surveillance and sequencing across the United States, it reinforces the speed at which Delta can spread and the importance of both vaccines and non-pharmaceutical interventions like masks. Now more than ever, we are living the realities that one single intervention isn’t enough, but rather a complementary approach to COVID-19 infection prevention.