In the study, fully vaccinated adults 65 and older were 94 percent less likely to be hospitalized with covid-19 than than unvaccinated people of the same age, according to the CDC. People who were partially vaccinated were 64 percent less likely to be hospitalized with the disease than the unvaccinated.
The risk for severe illness increases with age, and because older adults are at highest risk, the CDC prioritized them for vaccination. About 68 percent of adults 65 and older in the United States — more than 37 million people — have been fully inoculated, the data shows.
The analysis is one of many by the CDC and other groups to assess the effectiveness of the coronavirus vaccines in real-life conditions. In the United Kingdom, another study released Wednesday found that a single dose of the Pfizer-BioNTech or Oxford-AstraZeneca vaccine may reduce transmission of the coronavirus within households by almost 50 percent. Researchers from Public Health England said that protection was seen around two weeks after vaccination — regardless of a person’s age or contacts.
British Health Secretary Matt Hancock called the findings “terrific news,” adding that the results further reinforced the need for people to get vaccinated to end the pandemic.
The Public Health England study found that those infected with the coronavirus three weeks after receiving one dose of vaccine were between 38 and 49 percent less likely to pass on the infection to close contacts, compared with those who were unvaccinated. The study was based on 57,000 people from 24,000 households who were considered contacts of a vaccinated person.
In the United States, CDC Director Rochelle Walensky welcomed that agency’s findings about protection for fully vaccinated older adults.
“The results are promising for our communities and hospitals,” Walensky said in a statement. “As our vaccination efforts continue to expand, covid-19 patients will not overwhelm health care systems — leaving hospital staff, beds, and services available for people who need them for other medical conditions.”
Until now, there had not really been “system-level evidence for both mRNA vaccines at the community level,” said Jeanne Marrazzo, director of infectious diseases at the University of Alabama at Birmingham.
Death rates for hospitalized older adults can be six to eight times that of people younger than 65, Marrazzo said in an email. Nearly half the individuals in the analysis were over 75.
The data gives clinicians even greater confidence in telling patients, “This vaccine will keep you out of the hospital if you get covid — and that is not a place you want to be!” she said.
Carlos del Rio, a professor of medicine and global health at Emory University, said: “When you see that being fully vaccinated essentially eliminates the risk of being hospitalized with covid even if you get infected, it makes me very happy.”
Unlike during the clinical trials, the CDC analysis of data took place as more-transmissible and potentially more-deadly variants of the virus were circulating, primarily B.1.1.7, which was first identified in the United Kingdom and is now the dominant strain in the United States.
The CDC study looked at hospitalizations among 417 participants during the first three months of this year at 24 hospitals in 14 states. Researchers compared prior covid-19 vaccination in a group of 187 patients who tested positive for the coronavirus infection with a very similar control group of 230 patients who tested negative. To determine the real-world effectiveness of the vaccines in preventing hospitalization, they compared the odds of prior vaccination between these groups.
Among the 187 patients who tested positive, for example, only one person (less than 1 percent) had been fully vaccinated. But in the other group, 18 (8 percent) had been fully vaccinated. Even though the numbers are relatively small, the difference was still significant because it shows that patients with covid-19 were significantly less likely to have completed full vaccination with the Pfizer-BioNTech or Moderna vaccine, the data shows.
As expected, the analysis confirmed that vaccination provided no protection to people who had received their first dose less than two weeks earlier. It takes two weeks for the body to mount an immune response after vaccination.
Jennifer Hassan in London contributed to this report.
A previous version of this article incorrectly said that over half of patients in the CDC analysis were over 75. After publication, the CDC said it had erred in providing some information and that nearly half the patients were over 75, correcting its earlier press release. The article has been corrected.