Decreases in vaccine efficacy are driven more by the delta variant and behavioral changes than the vaccine itself, according to a study of 8.8 million New Yorkers.
The study, which was published Oct. 9 in the preprint server MedRxiv, compared vaccine cohorts defined by age, vaccine manufacturer and duration of full vaccination to age-specific unvaccinated cohorts. The study linked statewide hospital, testing and vaccine registry databases.
Below are six key findings:
- Declines in vaccine efficacy against COVID-19 infection occurred across all groups during the weeks when the delta variant was rapidly spreading in New York. After the delta variant reached 85 percent prevalence, changes in vaccine efficacy plateaued, with more recently vaccinated people having higher protection levels in some groups.
- Between May 1 and Aug. 28, decreases in efficacy against COVID-19 infection were greatest for Pfizer's vaccine. The efficacy decreased by 24.6 percent among people ages 18-49, 19.1 percent for people ages 50-64 and 14.1 percent for people ages 65 and older.
- The efficacy for Moderna's vaccine decreased by 18 percent among people ages 18-49, 11.6 percent among people ages 50-64 and 9 percent for people ages 65 and older.
- The efficacy for Johnson & Johnson's vaccine decreased by 19.2 percent among people ages 18-49, 10.8 percent among people ages 50-64 and 10.9 percent for people ages 65 and older.
- Since the study was conducted during the summer months, when mitigation measures were lifted, New Yorkers' changes in COVID-19 prevention behaviors likely contributed to the declines in vaccine efficacy against infection.
- Across all groups, vaccine effectiveness against hospitalization among people ages 18-64 was more than 86 percent. Among people ages 65 and older, vaccine efficacy against hospitalization declined from 95 percent to 89.2 percent for Pfizer recipients, from 97.2 percent to 94.1 for Moderna recipients and from 85.5 to 82.8 for Johnson & Johnson recipients.