With temperatures plummeting to 20 degrees early Wednesday morning, the threat of EEE, a mosquito-borne virus, has ended for this year.

The virus, which is transmitted by mosquitos, is rare but serious, and can lead to the swelling of the brain and death. Those under the age of 15 and over the age of 50 are most at risk.

“In general, when large geographic areas have had hard frosts, the risk from mosquito-borne disease can be considered eliminated except in areas adjacent to places where a hard frost has not occurred,” said Omar Cabrera of the Massachusetts Department of Public Health on Tuesday.

The Department has reported a hard frost for all of Plymouth County. Temperatures between 27 and 29 degrees Fahrenheit occurring over wide geographic areas are considered sufficient to end the risk for mosquito-borne disease in that area.

The low temperatures mean that communities no longer need to take EEE into account when scheduling outdoor activities at night.

In a quirk of policy, however, the department does not remove the risk levels established during warmer weather until the entire state has had a hard frost.