Moderna COVID-19 vaccine effective against viral variants, delta protection diminishes over time

Two doses of the Moderna COVID-19 vaccine were highly effective against all SARS-CoV-2 variants, according to a study published in The British Medical Journal. However, vaccine efficacy against the Delta variant declined moderately over time after vaccination.

“While this study provides reassuring evidence of the effectiveness of 2 doses of Moderna COVID-19 vaccine in preventing COVID-19 infection and hospitalization due to variants, including delta, it also has implications for booster shots,” Hung says. Fu Tseng, PhD, researcher with the Kaiser Permanente Southern California Department of Research & Evaluation, in a press release. “The findings of moderately reduced vaccine effectiveness of the Moderna COVID-19 vaccine over time against Delta infection support current booster dose recommendations.”

To conduct the study, researchers collected positive SARS-CoV-2 samples from symptomatic and asymptomatic patients for whole-genome sequencing. Samples from a total of 8153 individuals were collected from March 1 to July 27, 2021.

Of these individuals, 91.3% had not been vaccinated, 1.4 had received a single dose of Moderna’s COVID-19 vaccine, and 7.3% had received 2 doses of the vaccine. Variants were identified in 5186 samples: 39.4% were Delta, 27.7% Alpha, 11.4% Epsilon, 6.9% Gamma, 2.2% Iota, 1.4% Mu and 11.1% other variants . The Omicron variant had not been detected in the Southern California region at the time of the study.

The researchers compared samples from individuals who tested positive with those from individuals who tested negative. They found that the 2-dose vaccination had an efficacy of 86.7% against the Delta variant, 90.4% against Mu, 98.4% against Alpha, and between 96% and 98% for other identified variants.

Furthermore, subjects receiving the 2-dose Moderna vaccine were highly protected from hospitalization, with 97.5% efficacy against Delta-related hospitalization and no reported hospitalizations in subjects with other variants. There were no in-hospital deaths among patients who received the 2-dose vaccine.

“We conducted a previous study demonstrating the high effectiveness of the 2 dose Moderna COVID-19 vaccine, but when the Delta variant gained the upper hand in early summer 2021, questions arose about its effectiveness against variants,” says Katia Bruxvoort, PhD, adjunct researcher with the Kaiser Permanente Southern California Department of Research & Evaluation, in the release. This study confirmed efficacy against all variants during the study period, although we found a decrease in efficacy against Delta infection over time, from 94% efficacy in the first 2 months after vaccination to 80% efficacy at 6 months. The protection against hospitalization by the Delta variant remained high at 98% effectiveness.”


Moderna COVID-19 vaccine effective against virus variants [news release]. Kaiser Permanente; December 15, 2021. Accessed December 15, 2021.