Moderna COVID-19 Vaccine Effective Against Viral Variants, Delta Protection Wanes Over Time


Study shows moderately reduced efficacy of the Moderna COVID-19 vaccine over time against Delta infection, which supports current booster dose recommendations.

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, the vaccine’s efficacy against the Delta variant moderately declined over time following 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,” said Hung 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 Moderna COVID-19 vaccine over time against Delta infection supports current booster dose recommendations.”

To conduct the study, investigators gathered positive SARS-CoV-2 specimens 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.

Among these individuals, 91.3% were unvaccinated, 1.4 had received a single dose of Moderna’s COVID-19 vaccine, and 7.3% 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 yet been detected in the Southern California region at the time of the study.

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

Further, individuals who received the 2-dose Moderna vaccine were strongly protected against hospitalization, with 97.5% efficacy against Delta-related hospitalization and no reported hospitalizations among individuals with other variants. There were no hospitalized deaths among patients who received the 2-dose vaccine.

“We conducted a previous study that showed the high effectiveness of the 2-dose Moderna COVID-19 vaccine, but as the Delta variant became predominant in early summer of 2021, questions arose about effectiveness against variants,” said Katia Bruxvoort, PhD, adjunct investigator with the Kaiser Permanente Southern California Department of Research & Evaluation, in the release. This study confirmed effectiveness against all variants during the study period, although we did find a drop in effectiveness over time against Delta infection, from 94% effectiveness in the first 2 months after vaccination to 80% effectiveness after 6 months. Protection against hospitalization due to 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.

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