A new study shows two-dose Moderna vaccine is highly effective against COVID-19 hospitalization, but protection against delta infection decreases over time

Two doses of Moderna COVID-19 vaccine were highly effective against all SARS-CoV-2 variants, according to Kaiser Permanente research published today in The British Medical Journal; however, vaccine effectiveness against the delta variant moderately dropped with increasing time after vaccination.

“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,” says Katia Bruxvoort, lead author of the study.

“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.”

This Moderna-funded study included 8,153 individuals who tested positive for SARS-CoV-2, with samples taken between March 1 and July 27, 2021. 91.3 percent of them were unvaccinated, 1.4 percent had received one dose of Moderna COVID-19 vaccine, and 7.3 percent had received two doses of Moderna COVID-19 vaccine.

For 5,186 of the samples, 39.4 percent were delta, 27.7 percent alpha, 11.4 percent epsilon, 6.9 percent gamma, 2.2 percent iota, 1.4 percent mu, and 11.1 percent other variations were discovered. The omicron type had not yet been discovered in the Southern California region at the time of this study.

People who tested positive were compared to those who tested negative. Researchers discovered:

• The 2-dose Moderna COVID-19 vaccine effectiveness against infection by variant:
  • Delta: 86.7%
  • Mu: 90.4%
  • Alpha: 98.4%
  • Other identified variants: 96 to 98%
• People who received the 2-dose Moderna COVID-19 vaccine were strongly protected against COVID-19 hospitalization associated with the delta variant:
  • Vaccine effectiveness against delta hospitalization was high at 97.5%
  • Vaccine effectiveness against hospitalization for non-delta variants was not estimated because there were no hospitalizations with non-delta variants identified among vaccinated people
• Among people who had received the 2-dose Moderna COVID-19 vaccine, no hospitalized deaths occurred
• Vaccine effectiveness of 2 doses of Moderna COVID-19 vaccine against delta infection was 87.9% among people ages 18 to 64, and 75.2% among people ages 65 and older

“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,” adds Hung Fu Tseng, senior author.

“The findings of moderately reduced vaccine effectiveness of Moderna COVID-19 vaccine over time against delta infection supports current booster dose recommendations.”

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