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Washington state is canceling events with more than 250 attendees in three counties through the end of March, Gov. Jay Inslee announced Wednesday at a press conference. The moratorium is the state’s latest and toughest effort to stem the spread of COVID-19, the disease caused by the novel coronavirus that the World Health Organization declared a pandemic earlier today.

The vast majority of COVID-19-related deaths in the U.S. have occurred in Washington, where the first known American patient tested positive for the virus in January. Testing for COVID-19 was stalled in the weeks that followed, allowing the virus to spread undetected. As of Wednesday, Washington had at least 267 COVID-19 cases and 24 deaths, according to state health officials. Inslee said this week that the actual number of cases in the state could be more than 1,000.

“Starting today I’m ordering, pursuant to my emergency powers that certain events in King, Snohomish, and Pierce County with more than 250 people are prohibited by order of the governor,” Inslee said.

Coronavirus Live Updates: The latest COVID-19 developments in Seattle and the world of tech

Those counties were chosen because of “significant transmission, outbreaks, and they are large population centers,” according to Inslee.

The announcement applies to sporting events, concerts, conferences, and social and family gatherings in the Seattle region. Inslee said the prohibition will last until the end of the month but cautioned, “it is very highly likely that it will be extended beyond March.”

If it’s extended, the event ban could have major consequences for tech industry events in the coming months. Microsoft Build, the annual developer conference that draws about 5,000 people to Seattle, is still scheduled for May 19-21. Sakura-Con’s website says the Seattle event will still take place April 10-12, and a Northwest Convention League directory lists several events in the Pacific Northwest that haven’t been canceled at this time. The GeekWire Awards, originally scheduled for March 26, are now slated for May 19. The event draws more than 800 attendees.

The Seattle Mariners organization is working with Major League Baseball to make alternative plans for games scheduled at the end of March, according to a statement circulated Wednesday. The organization said ticket holders will be refunded or credited for games that aren’t played.

“Missing the home opener is a big, big deal to the state of Washington,” Inslee said, but he expects full compliance from the Mariners and others throughout the state.

To reiterate … the Mariners and MLB have been discussing contingency plans for this situation for over a week. They've discussed playing the games in Arizona. Playing the games in Texas and Minnesota, and playing an empty stadium. All of them have potential problems — Ryan Divish (@RyanDivish) March 11, 2020

Shortly after the press conference, the Seattle Sounders announced a match scheduled for March 21 at CenturyLink Field has been postponed. The organization said it will continue to heed recommendations from public health officials.

The Seattle Dragons announced it will play a home game at CenturyLink Field this Sunday, March 15 without fans.

The Tacoma Defiance announced that it will play Wednesday evening’s USL Championship match at Cheney Stadium in Tacoma with no fans. Its home match on March 22 will be rescheduled.

And the University of Washington, which moved classes online last week, is restricting attendance at on-campus sporting events.

Inslee said the order does not apply to airports or casinos, due to the state’s relationship with Native American Tribes. “We’re having further discussions with the tribes on it,” Inslee said at the press conference. Restaurants and bars are not subject to the order but officials plan to issue guidance for those businesses.

Officials said during the press conference that schools throughout the state should begin preparing for closures as the virus continues to spread.

“This is an extremely dangerous event that we are facing but we are not helpless,” Inslee said.” We have the ability to seize our own destiny and the safety of our loved ones.”

King County Executive Down Constantine said during the press conference that events with fewer than 250 attendees will also be canceled unless “they meet public health guidelines” for preventing the spread of the virus. Those guidelines include social distancing, hand washing, and regular sanitation.

Major employers in the state, including Amazon and Microsoft, have asked workers who can telecommute to do so at the recommendation of public health officials. Those companies and others have promised to continue paying their hourly workers regular wages, even if their hours are diminished due to the virus.

“For employers like Microsoft to ensure that hourly workers will be paid even if they are not currently needed … those are the kinds of forward-looking measures that we need from all leaders public and private in our state,” Constantine said.

Editor’s note: This story has been updated to clarify that the ban does not apply to casinos, due to the state’s relationship with Native American Tribes.