Vaccine Breakthrough Cases in South Dakota: Impact of SARS-CoV-2 Variants in a Rural State.

Background: Emergence of the SARS-CoV-2 Delta variant raised concern for greater transmissibility and severity of illness compared to the Alpha variant. Our objective was to compare SARS-CoV-2 vaccine breakthrough cases in South Dakota during the time periods where the Alpha and Delta variants of SARS-CoV-2 predominated.

Methods: Data were obtained from the South Dakota Department of Health's electronic disease surveillance system and South Dakota's Health Information Exchange. SARS-CoV-2 cases were matched with the immunization system data to verify vaccination status of vaccine breakthrough cases (VBC). The Alpha variant time-period (ATP) was defined as April 15-May 10, 2021 and the Delta variant time-period (DTP) as July 18-31, 2021. Case rates, demographics, risk factors, symptomology, and outcomes were compared for VBC during these periods.

Results: A total of 155 VBC were reported during the ATP and 153 during the DTP. The rate of SARS-CoV-2 VBC was 1.88 times higher for the DTP than the ATP. VBC during the ATP were more likely to present with no symptoms and during the DTP were more likely to present with subjective fever, cough, headache, loss or altered smell/taste, congestion, or postnasal drip. The average hospital length of stay was 6 days for the ATP and 4 days for the DTP. A total of 5 deaths were reported during the ATP compared to 1 death during the DTP. The non-statistically significant relation of the ATP and the DTP for hospital length of stay and number of deaths indicated a similar severity of disease.

Conclusions: In fully vaccinated South Dakotans, the SARS-CoV-2 Delta variant was shown to cause 1.88 times higher breakthrough cases but resulted in similar severity of illness compared to the Alpha variant.