Vaccines appear to be starting to reduce new Covid-19 infections in the United States, a breakthrough that could allow people to resume more normal activities as infection fears fade, according to public-health officials.

The Centers for Disease Control and Prevention in the United States said on Tuesday that it was safe for fully vaccinated people to walk, exercise, and gather outdoors in small groups without masks.

According to federal data, more than 40% of adults in at least 17 states, ranging from Hawaii to the Dakotas and several states in the Northeast, are currently immunized. Following increases earlier this spring, newly reported cases in several of these states, including New Jersey, Vermont, New York, and Connecticut, have begun to fall again.

Nirav Shah, who directs Maine’s Center for Disease Control and Prevention, said a combination of high vaccination rates and protection from prior infections likely means that more than half the state’s population is well shielded. After beginning to rise two months ago, new case reports have begun to fall again. The CDC recently reported that 44.5 percent of Maine adults and 36.3 percent of the overall population were fully vaccinated.

The drop in cases is a hopeful sign after the United States saw cases drop from a wintertime peak, then plateau and begin edging higher again last month. One likely cause was the spread of a more contagious variant known as B.1.1.7 in places such as Michigan, where there was a significant increase that is now subsiding. Some epidemiologists have also stated that people and states were becoming too relaxed too quickly.

In the United States, the first clear indication of the vaccines’ impact was seen in nursing homes, where cases and deaths among residents have decreased, according to federal data. Soon after, federal data revealed a drop in hospitalizations and deaths among the elderly, who have accounted for a sizable proportion of Covid-19 fatalities since the pandemic began.

Eliminating cases in the general population can be more difficult, especially when younger people, who are less likely to be vaccinated and often spread the virus without symptoms, become infected. According to experts, the virus’s spread eventually slows because it can’t reach as many vulnerable people.

Some parts of the United States have already reached this point. Between late March and Monday, the number of fully vaccinated adults in rural Clinton County, New York, which has a population of about 80,000 people near the Canadian border, more than doubled to 42.7 percent, making it one of the fastest counties in the country to get vaccinated. According to CDC data, the seven-day average for new cases fell from 24 to 16 per 100,000 people during the same time period.

County health officials said an aggressive vaccine rollout, which targeted 5,000 students at a state university campus in the county seat of Plattsburgh, had aided. When they trace infections in the county to known contacts, they find that half of the people they contact have been vaccinated, according to Molly Flynn, a Clinton County Health Department official in charge of contact tracing.

Still, there are reasons to be cautious, according to public-health experts. While the national trend is toward fewer cases, not every state and county has been quick to vaccinate its residents. Better vaccination rates among states and counties do not always equate to fewer cases when compared to less-vaccinated areas, according to a Wall Street Journal data analysis. Population density, for example, can make comparisons difficult.

Covid-19 can cause outbreaks even in areas with high vaccination rates. This occurred in Clallam County, Washington, where nearly half of adults are fully vaccinated, after a child’s birthday party triggered hundreds of cases of the B.1.1.7 variant, according to authorities.

Dr. Fauci and other public-health experts are increasingly emphasizing the short-term goal of slowing virus transmission through widespread vaccination rather than the more elusive long-term goal of eradicating the virus through herd immunity.

As a result, the number of new infections has dropped dramatically. In early February, the county’s new infection rate peaked at 21 per 100,000 people; by late April, it had dropped to 3 new cases per 100,000 people.