Vaccines show declining effectiveness against infection overall but strong protection against hospitalization amid delta variant


Three studies published Wednesday by the Centers for Disease Control and Prevention show that protection against the coronavirus from vaccines declined in the midsummer months when the more contagious delta variant rose to dominance in the United States.

At the same time, protection against hospitalization was strong for weeks after vaccination, indicating the shots will generate immune fighters that stave off the worst effects of the virus and its current variations.

Data from these studies persuaded the Biden administration to develop a plan for additional doses to bolster the immune systems of people vaccinated months earlier. The Biden administration will begin offering coronavirus booster shots to fully vaccinated adults who received the Pfizer-BioNTech and Moderna shots Sept. 20, top health officials announced Wednesday, after concluding that a third shot is needed to fight off waning immunity.

“Examining numerous cohorts through the end of July and early August, three points are now very clear,” CDC director Rochelle Walensky said at a White House covid-19 news briefing Wednesday. “First, vaccine-induced protection against SARS-CoV-2 infection begins to decrease over time. Second, vaccine effectiveness against severe disease, hospitalization and death remains relatively high. And third, vaccine effectiveness is generally decreased against the delta variant.”

The trio of reports, published Wednesday in the Morbidity and Mortality Weekly Report, the CDC’s scientific digest, also reinforce the idea that vaccines alone will be unable to lift the nation out of the pandemic.

Masks and other precautions should be part of “a layered approach centered on vaccination,” wrote researchers from the New York State Department of Health and the University at Albany School of Public Health in their study of vaccine effectiveness across New York state.

All three reports measure vaccine effectiveness, which compares the rates of infection or hospitalization among vaccinated people with the rates among people who had not been vaccinated.

Until now, evaluations of vaccine effectiveness amid delta largely relied on observations from outside the United States. A recent New England Journal of Medicine study concluded the Pfizer vaccine was 88 percent effective against infections that caused symptoms in England.

Others, such as a study in Israel, found larger declines in protection against infection. One U.S. report that has not yet gone through peer review, collecting data from Mayo Clinic Health System facilities in five states, found a drop in the Pfizer-BioNTech vaccine’s effectiveness against delta infections to 42 percent. The other mRNA vaccine, made by Moderna, was 76 percent effective.

The new study from New York is the first to assess vaccine protection against coronavirus infection across the entirety of a U.S. state amid delta. The study authors found a modest drop in effectiveness: It descended from 92 percent in May to 80 percent in late July. Twenty percent of new infections and 15 percent of hospitalizations from covid-19, the disease caused by the coronavirus, were among vaccinated people.

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