Kaiser Permanente research in Southern California published Nov. 25, 2021, in The Lancet Regional Health – Americas confirmed high Moderna COVID-19 vaccine effectiveness up to 5 months after the second dose. Effectiveness was 87% against COVID-19 infection, 96% against COVID-19 hospitalization, and 98% against COVID-19 death.

"This research provides reassuring evidence of the high effectiveness of the Moderna COVID-19 vaccine in preventing infection, hospitalization, and death from COVID-19," said Katia Bruxvoort, PhD, an adjunct investigator with the Kaiser Permanente Southern California Department of Research & Evaluation. "One of the important aspects of this study was that it included more than 700,000 adults who were racially and ethnically diverse and had a broad range of underlying conditions including chronic diseases, immunocompromising conditions, and autoimmune conditions."

This study evaluated the 5-month effectiveness of the Moderna COVID-19 mRNA vaccine as part of a 5-year observational study within Kaiser Permanente, an integrated health care organization with 4.7 million members in Southern California.

The study included 352,878 recipients of 2 doses of Moderna COVID-19 vaccine matched to 352,878 unvaccinated individuals by age, sex, race, and ethnicity. Vaccinated individuals received 2 doses of Moderna COVID-19 vaccine from December 18, 2020, to March 31, 2021. Individuals in both groups were followed until June 30, 2021.

During follow-up, COVID-19 infections occurred among 289 vaccinated patients and 1,144 unvaccinated patients. Vaccine effectiveness against COVID-19 infection was 87%.\

COVID-19 hospitalization occurred among 13 vaccinated and 182 unvaccinated patients, and COVID-19 in-hospital deaths occurred among 1 vaccinated and 25 unvaccinated patients. Vaccine effectiveness against COVID-19 hospitalization was 96%, and vaccine effectiveness against COVID-19 in-hospital deaths was 98%.

Vaccine effectiveness against COVID-19 infection remained high across age, sex, racial, and ethnic subgroups, with results ranging from 83% to 92%.

Vaccine effectiveness was higher against symptomatic COVID-19 (88%) than asymptomatic COVID-19 (73%).

From March to June 2021, there were 5,619 SARS-CoV-2 positive specimens that were successfully sequenced. The most prevalent variants were alpha (42%), epsilon (18%), delta (12%), and gamma (9%), with delta increasing to 54% of variants by June 2021.

"This study adds evidence of real-world Moderna COVID-19 vaccine effectiveness, particularly among the general population," said Hung Fu Tseng, PhD, a researcher with the Kaiser Permanente Southern California Department of Research & Evaluation and the senior author of the study. "Additionally, our follow-up on these fully vaccinated patients occurred through June 2021, a period that overlapped with the emergence of the delta variant in the United States. Long-term follow-up is ongoing to further evaluate the durability of protection."