Rapid increase in circulation of the SARS-CoV-2 B.1.617.2 (Delta) variant — Mesa County, Colorado, April–June 2021
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Rapid increase in circulation of the SARS-CoV-2 B.1.617.2 (Delta) variant — Mesa County, Colorado, April–June 2021

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      What is already known about this topic? The highly transmissible B.1.617.2 (Delta) variant of SARS-CoV-2 has become the predominant circulating U.S. strain. What is added by this report? During April–June 2021, COVID-19 cases caused by the Delta variant increased rapidly in Mesa County, Colorado. Compared with that in other Colorado counties, incidence, intensive care unit admissions, COVID-19 case fatality ratios, and the proportion of cases in fully vaccinated persons were significantly higher in Mesa County. Crude vaccine effectiveness against symptomatic infection was estimated to be 78% for Mesa County and 89% for other Colorado counties. What are the implications for public health practice? Vaccination is critical for preventing infection, serious illness, and death associated with SARS-CoV-2 infection (including the Delta variant). Multicomponent prevention strategies, such as masking in indoor settings irrespective of vaccination status as well as optimal surveillance testing and infection prevention and control, should be considered in areas of high incidence. On May 5, 2021, the Colorado Department of Public Health and Environment (CDPHE) identified the first five COVID-19 cases caused by the SARS-CoV-2 B.1.617.2 (Delta) variant in Mesa County in western Colorado (population 154,933, <3% of the state population). All five initial cases were associated with school settings. 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. CDPHE assessed data from surveillance, vaccination, laboratory, and hospital sources to describe the preliminary epidemiology of the Delta variant and calculate crude vaccine effectiveness (VE). Vaccination coverage in early May in Mesa County was lower (36% of eligible residents fully vaccinated) than that in the rest of the state (44%). Compared with that in all other Colorado counties, incidence, intensive care unit (ICU) admissions, and COVID-19 case fatality ratios were significantly higher in Mesa County during the analysis period, April 27–June 6, 2021. In addition, during the same time period, the proportion of COVID-19 cases in persons who were fully vaccinated (vaccine breakthrough cases) was significantly higher in Mesa County compared with that in all other Colorado counties. Estimated crude VE against reported symptomatic infection for a 2-week period ending June 5 was 78% (95% confidence interval [CI] = 71%–84%) for Mesa County and 89% (95% CI = 88%–91%) for other Colorado counties. Vaccination is a critical strategy for preventing infection, serious illness, and death from COVID-19. Enhanced mitigation strategies, including masking in indoor settings irrespective of vaccination status, should be considered in areas with substantial or high case rates. Whole genome sequencing is performed in the CDPHE laboratory on specimens submitted as part of sentinel surveillance (38 sites across Colorado, including one acute care hospital in Mesa County), as well as for cluster and outbreak response and on suspected variants (reverse transcription–polymerase chain reaction [RT-PCR]–positive specimens with S-gene target failure associated with the B.1.1.7 lineage) (1). The Colorado Electronic Disease Reporting System (CEDRS), a surveillance system managed by CDPHE, was used to identify reported confirmed or probable cases of COVID-19 occurring from April 27, the date of illness onset for the first Delta variant case in Mesa County, to June 6, when sequencing identified B.1.617.2 as the dominant variant in Colorado (2). The Colorado Immunization Information System (CIIS) was used to verify COVID-19 vaccination status; vaccine breakthrough infections were identified using personally identifying information to match cases in CEDRS to CIIS entries* (3). Crude VE against reported symptomatic infection was estimated and compared among Mesa County and all other Colorado counties using a screening method outlined by the World Health Organization† as a rapid tool to assess whether a vaccine is performing as expected (4). To better determine settings where the Delta variant was spreading, outbreak data during April 22–June 26 were obtained from the CDPHE outbreak database, which contains information on all reported COVID-19 outbreaks in Colorado and outbreak line lists.§ Residential care facility vaccination data were obtained from EMResource, a capacity planning tool used by CDPHE for facility-level reporting of aggregate COVID-19 vaccinations. Incidence of SARS-CoV-2 infection and proportions of outcomes and vaccination rates among patients living in Mesa County and all other Colorado counties were compared and p-values were calculated using chi-square or Fisher’s exact tests. This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy. Suggested citation for this article: Herlihy R, Bamberg W, Burakoff A, et al. Rapid Increase in Circulation of the SARS-CoV-2 B.1.617.2 (Delta) Variant — Mesa County, Colorado, April–June 2021. MMWR Morb Mortal Wkly Rep. ePub: 6 August 2021. mm7032e2.htm?s_cid=mm7032e2_w mm7032e2-H.pdf
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