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Since Americans first began rolling up their sleeves for coronavirus vaccines, health officials have said that those who are immunized are very unlikely to become infected, or to suffer serious illness or death. But preliminary data from seven states hint that the arrival of the Delta variant in July may have altered the calculus. Breakthrough infections in vaccinated people accounted for at least one in five newly diagnosed cases in six of those states and higher percentages of total hospitalizations and deaths than had been previously observed in all of them, according to figures gathered by The New York Times. The absolute numbers remain very low, however, and there is little doubt that the vaccines remain powerfully protective. This continues to be “a pandemic of the unvaccinated,” as federal health officials have often said. Still, the trend marks a change in how vaccinated Americans might regard their risks. “Remember when the early vaccine studies came out, it was like nobody gets hospitalized, nobody dies,” said Dr. Robert Wachter, chairman of the department of medicine at the University of California, San Francisco. “That clearly is not true.” The figures lend support to the view, widely held by officials in the Biden administration, that some Americans may benefit from booster shots in the coming months. Federal officials plan to authorize additional shots as early as mid-September, although it is not clear who will receive them. “If the chances of a breakthrough infection have gone up considerably, and I think the evidence is clear that they have, and the level of protection against severe illness is no longer as robust as it was, I think the case for boosters goes up pretty quickly,” Dr. Wachter said. The seven states — California, Colorado, Massachusetts, Oregon, Utah, Vermont and Virginia — were examined because they are keeping the most detailed data. It is not certain that the trends in those states hold throughout the United States. In any event, scientists have always expected that as the population of vaccinated people grows, they will be represented more frequently in tallies of the severely ill and dead. “We don’t want to dilute the message that the vaccine is tremendously successful and protective, more so than we ever hoped initially,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston. “The fact that we’re seeing breakthrough cases and breakthrough hospitalizations and deaths doesn’t diminish that it still saves many people’s lives.” The C.D.C. declined to comment on the states’ numbers. The agency is expected to discuss breakthrough infections, hospitalizations and vaccine efficacy at a news briefing on Wednesday. Most analyses of breakthrough infections have included figures collected through the end of June. Based on the cumulative figures, the C.D.C. and public health experts had concluded that breakthrough infections were extremely rare, and that vaccinated people were highly unlikely to become severely ill. The states’ data do affirm that vaccinated people are far less likely to become severely ill or to die from Covid-19.