Department of Research & Evaluation News

Researchers from the Centers for Disease Control and Prevention and Kaiser Permanente Southern California found that COVID-19 cases caused by the delta variant increased rapidly from April 15 to July 21, particularly among people ages 18 to 44.

The study was published October 7, 2021, in the CDC’s Morbidity and Mortality Weekly Report, known as MMWR. The paper describes the distribution of SARS-CoV-2 variants, the virus that causes COVID-19, across Kaiser Permanente Southern California from March 4 to July 21, 2021. The distribution of the most common variants identified during this period were compared by various patient characteristics.

“We witnessed COVID-19 infections caused by the delta variant increase rapidly from 0% to 95% of all infections in this population between mid-April and late-July 2021, highlighting the ability for new variants to quickly dominate transmission,” said the lead author, Debbie Malden, DPhil, MSc, a CDC Epidemic Intelligence Service officer stationed with the Kaiser Permanente Southern California Department of Research & Evaluation. “Continued monitoring of SARS-CoV-2 variants, in conjunction with effective preventive measures will be key to controlling the COVID-19 pandemic.”

For this study, researchers linked whole genome sequencing data from 6,798 SARS-CoV-2-positive samples to patient data among all positive tests collected from Kaiser Permanente Southern California members from March 4 to July 21, 2021 (Kaiser Permanente Southern California performs SARS-CoV-2 testing for all members upon request, regardless of symptoms, and prior to hospital admission or medical procedures).

During that time, COVID-19 cases caused by the delta variant increased rapidly, quickly becoming the predominant variant. Relative to other variants, infections attributed to the delta variant were more common among patients between ages 18 and 44 and were associated with an apparent increased risk of hospitalization within 14 days of sample collection among unvaccinated patients.

Among 6,798 sequenced positive specimens:

• 88% (5,994) were in unvaccinated persons,
• 10% (648) were in fully vaccinated persons, and
• 2% (156) were in partially vaccinated persons.
• The weekly proportion of infections caused by the alpha variant increased from 20% to 67% during March 4 through May 19.
• The weekly proportion of delta variant infections increased from 0% to 95% during April 15 through July 21, 2021.
• The weekly proportion of variants was similar among fully vaccinated and unvaccinated people. Infection with the delta variant was more common among younger people, particularly those ages 18 to 44 years.
• Among unvaccinated persons, infections with the delta variant were associated with an increased risk of hospitalization compared with all other variants.

“This study shows the importance of monitoring COVID-19 variants for new mutations, as well as the importance of vaccinations, physical distancing, and other preventive measures to control the spread of COVID-19,” said Sara Tartof, PhD, an infectious disease epidemiologist with the Department of Research & Evaluation and the senior author of the research.

In addition to Dr. Malden and Dr. Tartof, researchers from Kaiser Permanente Southern California Department of Research & Evaluation included Katia J. Bruxvoort, PhD; Hung Fu Tseng, PhD; Bradley Ackerson, MD; Soon Kyu Choi, MPP; Ana Florea, PhD; Julia Tubert, MPH; Harpreet Takhar, MPH; Michael Aragones, MD; and Vennis Hong, MPH.