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 Los Angeles cohort study tells a very compelling story of the impact of vaccination in protecting very well against severe disease and in reducing infection,” said Leana Wen, an emergency physician and public health professor at George Washington University. “It also highlights why it matters to the vaccinated if others around them are unvaccinated — the infections among the unvaccinated are spilling over and increasing the likelihood of breakthrough infections among the vaccinated.”
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.
CDC officials pointed to recent state data on breakthrough infection states, and its New York study, as clear proof that Americans need booster shots. But critics inside the Biden administration and outside health experts have argued the CDC has not presented enough evidence to support doling a second round of shots domestically when dozens of countries are still struggling to get a hold of first doses.
Two senior administration officials said the CDC has kept its vaccine efficacy and breakthrough infection cohort data from other senior Biden officials. That has raised questions within the administration about why the agency decided to recommend booster shots.
The CDC’s secrecy as it scrubbed and analyzed the data has frustrated some in the White House who have insisted for weeks that breakthrough infections are rare, based on initial findings from the spring. But that was before the Delta variant became the dominant form of the coronavirus in the United States.
In speeches, Biden has repeatedly emphasized that Covid-19 is a “pandemic of the unvaccinated,” going as far in late July to assert that “over 99 percent of Covid-19 deaths had been among the unvaccinated.”
Yet as reports of breakthrough mounted and scrutiny intensified, health officials have clamored for the more recent data on the proportion of serious infections found in vaccinated Americans.