HARTFORD, Conn. -- Covid-19 cases in vaccinated people have become increasingly common in recent months amid the spread of the delta variant and the waning of immunity from vaccination, according to new Yale research conducted in Connecticut.
Yale researchers found that from August to mid-October, about 22% of patients with covid-19 at Yale New Haven Hospital were fully vaccinated and experiencing severe symptoms -- an increase from 1.4% in the same health system from March to July.
These findings could owe in part to the spread of the delta variant during the second half of the year, but the most likely explanation was the decline of vaccine immunity over time, said Hyung Chun, associate professor of medicine at Yale and the new study's senior author. In that way, Chun said, the new research underscores the importance of booster shots for vaccinated people.
"The emerging data is pretty clear that the efficacy of the first vaccines are waning but that the boosters remain to be the best treatment available to prevent one from catching covid-19 or becoming sick with covid-19," Chun said.
Chun noted that the Yale study was conducted before the recent emergence of the omicron variant but that the new strain doesn't change the need for people to get vaccinated and to seek booster shots.
As of mid-November, all American adults are eligible for booster shots six months after their second dose of the Pfizer-BioNTech or Moderna vaccine and two months after their dose of the Johnson & Johnson vaccine. According to the Centers for Disease Control and Prevention, 27.2% of Connecticut's fully vaccinated adults had received boosters as of Friday.
The new Yale study, which was published Friday in The Lancet Microbe, is consistent with other research suggesting that immunity from covid-19 vaccines wanes over time, leaving people increasingly vulnerable as they get further and further out from their shots.
Despite finding an increase in breakthrough cases over the second half of the year, Chun emphasized that vaccination remains highly important in slowing covid-19.
According to the Yale study, vaccinated covid-19 patients tended to be older than unvaccinated patients and had more serious underlying conditions but still typically had shorter hospital stays and lower mortality rates.
"The first message is that the vaccines are still highly effective and that even those people who are coming in with breakthrough cases do have better outcomes than patients who are unvaccinated and needing hospitalization," Chun said.
According to state data, unvaccinated Connecticut residents have been about five times as likely as vaccinated residents to test positive for covid-19 in recent weeks, as well as more than 15 times as likely to die from the disease.