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See how Massachusetts’ COVID-19 risk map has evolved week-by-week

A sign outside the mass vaccination site at the Hynes Convention Center.
A sign outside the mass vaccination site at the Hynes Convention Center.Suzanne Kreiter/Globe staff

Massachusetts’ weekly public health reports show how town-by-town risk for coronavirus spread has fluctuated over a period stretching from mid-October to mid-March.

The state releases risk data on a weekly basis, and each map represents data from a two-week period. These maps lay out the data since the Nov. 6 public health report, when the state began evaluating risk using a combination of percent positivity and daily incidence rate per 100,000 people. While these maps represent 22 weekly public health reports, they represent 23 weeks of data.

On Nov. 6, when the state changed its metrics for determining the risk level of COVID-19 transmission, 16 communities were considered high risk,

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The number steadily rose from there, to 30, to 62, to 81, 97, 158, 187, 188, 190, 219, before peaking at 229 and then falling to 222, 192, 153, 110, 66, 28, 19, 14, 20, 32, and 55 in last week’s report. This week’s total of 77 high-risk communities marks a fourth straight week-to-week increase.

Boston remained at moderate risk for COVID-19 in the state’s most recent report; at one point it spent nearly a month in the high-risk category. The city’s positivity rate slowly increased from 1.12 percent over the period of Oct. 18 to 31, to a peak of 5.94 percent percent over the period from Dec. 20 to Jan. 2 and its average rate of positive tests per 100,000 went from 18.4 for the report covering Oct. 18 to 31, to a peak of 72.4 for the report covering Dec. 27 to Jan. 9.

In the state’s most recent report, covering data from March 21 to April 3, Boston had a positivity rate of 1.69 percent and an average daily incidence rate of 34.5 cases per 100,000 people.

Explore the state’s recent risk maps here:

The map includes a color-coded ranking system, by town size. Here’s how to track the map:

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For towns with under 10,000 people:

Grey (lowest risk): Less than or equal to 10 total cases.

Green (lower risk): Less than or equal to 15 total cases.

Yellow (moderate risk): Less than or equal to 25 total cases.

Red (high risk): More than 25 total cases.

For towns with 10,000-50,000 people

Grey: Less than or equal to 10 total cases.

Green: Less than 10 cases per 100,000 people and more than 10 total cases.

Yellow: 10 or more cases per 100,000 people, or a positivity rate equal or greater to 5 percent.

Red: 10 or more cases per 100,000 people, and a positivity rate equal or greater to 5 percent.

For towns with more than 50,000 people

Grey: Less than or equal to 15 total cases.

Green: Less than 10 cases per 100,000 people and more than 15 total cases.

Yellow: 10 or more cases per 100,000 people, or a positivity rate equal or greater to 4 percent.

Red: 10 or more cases per 100,000 people, and a positivity rate equal or greater to 4 percent.

Before it changed its metrics on Nov. 6, the state had evaluated risk solely on the basis of the daily incidence rate, with an exception for communities with fewer than 5 total cases.


Peter Bailey-Wells can be reached at peter.bailey-wells@globe.com. Follow him on Twitter @pbaileywells. Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1.