More than 300 people in B.C. are now estimated to have contracted measles in the current outbreak, and officials south of the border in Washington state say the number of B.C.-linked measles cases has also grown.
Dr. Greg Stern, a health officer with Whatcom County, says four members of the same family are confirmed to have contracted measles.
Stern said one member of the family contracted measles while in B.C. before the virus spread to three other family members.
The family is now in isolation, he said.
The family belongs to the same church congregation as many of those who came down with measles in B.C.'s Fraser Valley.
Fraser Health, the health authority in B.C., says there are now an estimated 320 cases in total spread throughout Abbotsford, Mission, Chilliwack, Agassiz, Harrison Hot Springs and Hope.
The B.C. outbreak is now entering its fourth week, and officials say it is expected to continue for another two weeks as second and third waves move through the affected communities.
The outbreak was first declared March 8 in parts of the region where immunization rates were low within some religious communities and schools.
The health authority then warned the virus was spreading and confirmed over two weeks ago that a student from BCIT in Burnaby, B.C., was infected.
Fraser Health says the spread of the virus into communities outside the Fraser Valley has been limited.
The health authority has also been working with schools, community groups, churches and medical officials to control the virus, and it has set up dedicated immunization clinics.
Stern said there is nothing to suggest measles will spread into the wider community in Whatcom County, but he's urging residents who haven't been vaccinated to make an appointment.