Their release comes as the Biden administration is grappling with how to blunt the surge of cases driven by the highly contagious Delta variant as schools and offices reopen, travel increases and cooler weather sends people inside.
In a speech on Thursday, President Joe Biden made his strongest push yet to get Americans vaccinated against the virus — announcing that all federal workers and contractors will now be required to get the shot. The nation’s top health agencies, meanwhile, are ironing out a plan for the distribution of booster shots later this month.
The exact details of the booster plan are still hazy, in part because the administration was waiting for more data from the CDC on vaccines’ effectiveness over time, the rate of breakthrough infections and how any changes in those variables were affecting hospitalization rates, according to two senior administration officials familiar with the matter.
CDC began last month to release the results of targeted vaccine effectiveness studies, showing protection against infection beginning to wane in residents of New York and Los Angeles and among frontline health care workers. But the agency has taken weeks to complete one of its largest and most comprehensive analyses of breakthrough infections — based on data from 13 jurisdictions with the ability to match immunization records with Covid-19 lab reports.
That study, one of the three released Friday, compared the relative risks of infection, hospitalization and death between people who are fully vaccinated and those who are not across different age groups. The CDC looked at 600,000 people infected with Covid-19 from April through mid-July. It found that overall, vaccine effectiveness against severe Covid-19 disease remains high. Incidence rate ratios for hospitalization and death changed relatively little after the Delta variant became the most dominant strain of the virus in the U.S., the study showed.
But the vaccines’ ability to prevent any infection — including mild disease — decreased from 91 percent to 78 percent after the Delta variant took over this summer.
Between April 4 and June 19, before Delta’s rise, fully vaccinated people accounted for 5 percent of cases, 7 percent of hospitalizations and 8 percent of deaths. Those figures roughly doubled between June 20 to July 17 as the variant spread. Fully vaccinated individuals accounted for 18 percent of cases, 14 percent of hospitalizations and 16 percent of deaths.
The CDC also released two reports that looked more closely at hospitalizations associated with the Delta variant and vaccine effectiveness.
One of those reports pulled on data from 1,175 patients 18 and older who were hospitalized at five Department of Veterans Affairs facilities between February and August. VA hospitals generally treat people who are older, with a higher prevalence of underlying medical conditions, than the general population.
The vaccines were 87 percent effective overall at preventing hospitalization, protection that remained relatively consistent before and after Delta became the country’s dominant variant, the study shows. But that figure masks notable differences between age groups. The shots were only 80 percent effective at keeping adults over 65 from being hospitalized with Covid, while they were 95 percent effective for people between ages 18 and 64.
A third CDC study looked at more than 32,000 adults who sought care, received Covid-19 molecular testing and were discharged with a Covid-like illness from 187 hospitals and 221 emergency departments and urgent care clinics across nine states from June to August.
The CDC identified infections among 19 percent of the unvaccinated and 3 percent among the fully vaccinated. Vaccine efficacy was “significantly lower” among adults aged 75 and older at 76 precent, the study said. Efficacy was higher among Moderna vaccine recipients at 95 percent than among Pfizer-BioNTech at 80 percent or Johnson & Johnson at 60 percent.