Minnesota's biggest measles outbreak since 1990 is winding down, with no new cases reported for the past two weeks. But state health officials aren't ready to call an official end to an outbreak that has sickened 78 people over the past two months.

The two rural counties affected by the outbreak, with a total of six cases, will be considered all-clear as of Tuesday, and Ramsey County, which had three cases, will get that designation Thursday if no new cases develop.

Hennepin County, the epicenter of the outbreak with 69 cases, is still under watch for new infections. It can take three weeks for measles symptoms to develop.

"It has actually been quiet," said Kris Ehresmann, infectious disease director for the Minnesota Health Department. "We are really seeing a drop off in measles activity."

The Health Department's laboratory, the only facility in the state capable of testing for the measles virus, has seen a dramatic drop-off in samples sent by hospitals and clinics. And state epidemiologists are not aware of any people with symptoms that need to be monitored.

As a result, the state's measles control team is meeting just twice a week, instead of daily, and a number of Health Department employees who had been assigned to measles detection and prevention are now back at their regular jobs.

Although 78 people came down with measles, thousands were exposed, including hundreds who were not vaccinated or did not have natural immunity from a previous measles infection. The Health Department identified 8,880 people who were potentially exposed to known cases in day care centers, health care settings, schools and other community settings. Public health officials contacted many of them or checked their vaccination status using the state's immunization registry.

That work identified 596 at-risk people, and they were asked to voluntarily limit their activities to avoid exposing others. "We really worked hard to reduce the opportunity for transmission, and that has had an impact," Ehresmann said.

Health officials took that approach when an unvaccinated child in Crow Wing County contracted measles after a family visit to Hennepin County. Unvaccinated family members, as well as other unvaccinated children from Le Sueur County exposed to the virus at a large family gathering, were counseled on preventive measures.

"We isolated those folks from the get go," Ehresmann said. "That made a huge difference."

Because measles no longer occurs naturally in the United States, officials believe this outbreak started with someone traveling from abroad. Measles is prevalent in many African countries, China and Europe, which has seen a resurgence in infections. However, the exact source has not been identified.