Gov. Baker launched a new colored-coded system to label cities and towns based off the severity of their COVID-19 infection rates on Tuesday, initiating a targeted approach to virus containment that he said should help inform and guide the state, communities, and their residents in making decisions about how to contain the coronavirus’s spread.

The state has identified 33 communities where it believes worrying trends in COVID-19 infection rates warrant intervention efforts, including four “red” cities - Chelsea, Everett, Lynn, and Revere - where residents are at the highest risk of becoming infected. A list of the other 29 communities flagged by the administration Tuesday was not available hours after the governor’s press conference.

The number of active COVID-19 cases in Massachusetts rose nearly 25 percent between July 29 and Aug. 5, and has been steady or climbing for nearly a month as new COVID-19 infections are once again on the rise.

Last week, Baker said the second step of Phase 3 of the state’s reopening plan will be put on hold and the gathering limit for any outdoor event will be reduced from 100 to 50 as of Tuesday. The limit for indoor gatherings will remain at 25, but Baker said the limits apply to all types of locations on public or private property. The revised order also mandates that everyone wear a mask or face covering anywhere that more than 10 people from different households could mix.

Movie theaters, gyms, casinos, museums, and more were allowed to reopen in early July as part of Phase 3. The Baker administration referred to it as “Step One of Phase III” but did not fully detail what would be included in the second step of Phase 3. On the state’s reopening website, it lists indoor theater or concert hall performances, and laser tag, roller skating, trampolines and obstacle courses as the activities that would be allowed to reopen in step two of Phase 3.

The state plans to offer all municipalities assigned a “red” or a “yellow” label assistance with testing, contact tracing, gathering-size enforcement, and public awareness campaigns. Parks, playgrounds and some businesses could be restricted or shut down in moderate- or high-risk communities if they have been shown to be contributors to a municipality’s higher infection rates, the governor said.

“People need to step up and be aware of the level of spread in each community, and especially in your own area, and to be vigilant,” Baker said at the press conference. “The virus doesn’t care about boundaries and it certainly takes every opening any of us give it.”

Communities that have demonstrated control over the virus, however, should be confident in moving ahead with their reopening strategies, including a return to school in the fall.

“If you’re in a green or a white community, I can’t imagine a good reason not to go back, whether it’s full-time or some sort of a hybrid, because for all intents and purposes you meet all the benchmarks that are being used across the country and across New England to make decisions about whether it’s safe to go back to school,” Baker said.

The new maps, which will be updated weekly on Wednesdays, are part of an effort to revamp how the state reports public health data on the COVID-19 pandemic to keep residents better informed about the status of the communities where they live, work, and shop.

“We also need to ensure that the businesses and individuals in those communities are aware of the level of COVID that exists in their communities, and what would be required of them to help control it,” Baker said.

Health and Human Services Secretary Marylou Sudders said the number of positive and negative antigen and serology tests reported to the Department of Public Health also would be added to the daily report, and the definition and reporting of “probable” COVID-19 cases would be updated to reflect the latest Centers for Disease Control guidelines.

The governor and his team reached out to leaders in all 33 “red” and “yellow” municipalities before Tuesday to offer assistance, and Baker said the common theme among officials he spoke to is that there have been too many informal gatherings with too many people, and people not wearing masks.

“We’re making progress and have made progress in our fight, but we’re seeing the effects of too many people letting their guard down and simply relaxing get in the way of some of our continued move forward,” Baker said.

The worst-off communities will be assigned a “red” designation signaling a daily infection rate of more than 8 cases per 100,000 people. Chelsea, Everett, Lynn, and Revere are the only four communities in that highest risk category, currently.

The moderate risk “yellow” designation means that an average of four to eight daily cases per 100,000 people have been diagnosed over the previous two weeks, while “green” communities have fewer than 4 cases per 100,000 people and “white” communities will have had less than 5 cases total in the past 14 days.

The administration released a map with cities and towns shaded based on their infection rates, but could not provide a list of the 33 communities colored red or yellow.

The Baker administration’s stepped up vigilance comes after case totals and the state’s positive testing rate had been creeping up over the past couple of weeks. Both seem to have stabilized, with the positive testing rate back under 2 percent for several days, but the governor said that’s not the case everywhere in Massachusetts.

Police can also enforce COVID-19 restrictions, picking up a task that previously had been under the purview of local boards of health.

Baker said enforcement of the gathering size limits is one way the state could help high- and moderate-risk communities, but he said he anticipated most of the citations or warnings would come as a result of people calling in complaints about businesses or parties, not random patrols or business spot checks.

The state is also offering to help cities and towns through a multi-agency COVID Enforcement and Intervention Team with road signs, public service announcements, reverse 911 calls or other strategies.