NEW HAVEN \u2014 The Pfizer and Moderna COVID-19 vaccines have been shown to protect against multiple variants of the coronavirus, including delta, according to a Yale University study. The research, published this week in the journal Nature, also found that people who were vaccinated after having COVID-19 had stronger immunity to variants than those who were vaccinated but had not had the disease. The study was conducted during an increase in so-called breakthrough infections, mostly caused by the delta variant, in vaccinated people, according to a release. \u201cVaccines induce high levels of antibodies against delta and most variants,\u201d said Akiko Iwasaki, professor of immunobiology in the Yale School of Medicine and an author of the paper, in the release. \u201cAnd two shots are better than one.\u201d The research also found that booster shots of Pfizer-BioNTech and Moderna, both mRNA vaccines, are effective against the coronavirus. In the study, led by Iwasaki, Nathan Grubaugh, associate professor of epidemiology at the Yale School of Public Health, and Saad Omer, director of the Yale Institute for Global Health, blood samples were collected from 40 unvaccinated health care workers in the Yale New Haven Health System between November 2020 and January 2021. More samples were taken after the health care workers received their first and second doses of Moderna or Pfizer vaccines. The blood samples were then exposed to 16 coronavirus variants, and antibodies and T cells were measured. There was an increased immune response in all blood samples, and the response to the delta variant was robust, increasing after the second shot of vaccine, according to the release. Iwasaki said breakthrough cases are more likely the result of how contagious the delta variant is, not because the vaccines did not work. \u201cThe delta variant is more infectious than earlier variants,\u201d Grubaugh said in the release. \u201cThe high transmissibility of the variant, not its escape from our vaccine-induced immune response, best explains infections among the vaccinated.\u201d Those who had had a case of COVID before being vaccinated showed a better immune response. \u201cRecovering from an initial infection is like getting a first vaccine shot,\u201d Iwasaki said. A booster shot could have a similar effect, she said.