Two new studies from the U.S. Centers for Disease Control and Prevention show fully vaccinated Americans’ immunity to COVID-19 is waning as the more-transmissible Delta variant continues to spread across the country.
One study, which focused on frontline health care workers, found that vaccine effectiveness declined by nearly 30 percentage points since the Delta variant became the dominant strain in the U.S. The analysis also concluded that the COVID-19 vaccines were 80 percent effective in preventing infection among the frontline health care workers.
The second study examined 43,000 Los Angeles residents age 16 and older. It found that 25 percent of new infections from May to July were in fully vaccinated individuals, while 71 percent occurred in unvaccinated people. The study also showed that the hospitalization rate was significantly lower for fully vaccinated people than for unvaccinated people.
The publication of the studies comes a week after the agency released its first three reports on vaccine efficacy, the Delta variant and breakthrough infections. One of those analyses showed that vaccine effectiveness among adults in New York declined from 91.7 percent in early May to 79.8 percent by late July.
Both recent sets of CDC studies show that breakthrough infections in vaccinated people are still rare. But they also add to growing evidence that protection from COVID-19 shots lessens over time, which agency officials say supports their recommendation that Americans receive booster shots eight months after their initial vaccination.
The CDC has for months monitored breakthrough infections and vaccine efficacy in different populations across the U.S. The agency is working with a small group of state health departments to study all infections in fully vaccinated people, including those who do not end up in the hospital. It also continues to study vaccine efficacy among groups like frontline health care workers and nursing home residents and staff.