OLYMPIA — Masks will soon be required at large, outdoor events across Washington, Gov. Jay Inslee announced Thursday.

Effective Monday, all attendees, regardless of vaccination status, at gatherings of 500 or more people must mask up to blunt the spread of COVID-19’s fifth wave, which is being driven by the unvaccinated and pushing hospitals statewide closer to crisis standards of care.

“I have to tell you there’s a lot of frustration by those who are vaccinated, that they have to bear the burden of this irresponsible behavior,” Inslee said at a news conference. “I don’t think it’s a great thing that you have to wear a mask to go to a Husky game. I wish that were not so. But the reason is because so many people have refused to become vaccinated.”

The governor added he’s considering a statewide vaccine verification system for certain indoor businesses, similar to one being planned in King County.

His updated masking order mirrors those imposed in King and Pierce counties last week and had been under consideration in Snohomish County. It comes three weeks after the governor announced he was reinstating Washington’s requirement for everyone older than 5 to wear masks indoors.

Back then, the number of people hospitalized with COVID in Snohomish County hovered around 80.

As of this week, local hospitals were treating 106 virus patients, and 26 required ventilators to breathe.

And on Tuesday, the Snohomish Health District announced the county recorded its highest two-week case rate of the pandemic — 464 new infections per 100,000 people.

The previous high, which occurred last winter, was 462 per 100,000.

The fifth wave, experts say, is the direct result of not enough people getting vaccinated paired with the more contagious delta variant.

“We’ve got to understand how severe this is,” Inslee said. “More people die of COVID (nationwide) every three days than died on September 11th (2001) in the terrorist attacks. This is not a time for little tiny baby steps.”

But Inslee said he’s not looking to re-impose capacity restrictions on restaurants and shops, or shut down any sectors of the economy. Those dramatic moves were necessary at the outset of the pandemic to stop the spread of the virus. With masking and a vaccine, such actions are not needed.

“We’re doing the things we know that work and are readily available,” he said. “And if we do this, we will beat this pandemic.”

While “breakthrough” cases and hospitalizations among those who have been vaccinated are becoming more common, vaccines still provide strong protection, experts say, especially in preventing admission to an ICU.

Thursday, Inslee and Secretary of Health Dr. Umair Shah again pleaded with unvaccinated Washingtonians to get their shots.

“I’ve heard some people say it’s an individual decision — that is just so far from the truth,” Inslee said. “The fact is when you make a decision to not be vaccinated, it is not just about your health, it is about the health of everyone around you. We need people to stop thinking so much about ‘me,’ and start thinking more about ‘we.’”

Those skeptical about vaccines should consult their doctor, not conspiracy theories or social media misinformation, he added.

Across Snohomish County, about 200,000 eligible adults have yet to receive a dose.

County Executive Dave Somers welcomed the governor’s mask mandate expansion.

“I think any reasonable measure that will help lower the rate is advisable,” he said. “I would much rather have the governor do something that is uniform for the entire state. Different places having different rules can be confusing to folks.”

The requirement arrives after completion of the Evergreen State Fair in Monroe, one of the largest summertime events in the county. It will be in force for upcoming high school football games as well as fall activities like pumpkin patches and corn mazes.

It comes one month after the governor mandated that state employees and contractors, private health care workers and employees of schools and colleges be vaccinated against COVID-19 by mid-October or face potential job loss.

Mask and vaccine mandates are among the steps Inslee and local leaders are pursuing in response to a continuing surge in coronavirus infections, hospitalizations and deaths, the majority occurring among unvaccinated Washingtonians. It’s pushed the health care system to its limits.

“We’re concerned about it,” Somers said. “We are aware some people who need care and can’t get in to the ICU.”

Locally and nationwide, some elected leaders are looking to other tools, as well, to blunt the fifth wave.

On Thursday, President Joe Biden announced a sweeping mandate that requires upwards of 100 million Americans to get vaccinated.

Under a forthcoming rule from the federal Department of Labor’s Occupational Safety and Health Administration, all employers with 100 or more workers must require their staff to get vaccinated or be tested weekly for the coronavirus.

The department will also require employers to give staff paid time off for vaccine appointments and sick leave if they experience symptoms post-vaccination.

Companies that don’t comply could be fined $14,000 per violation.

Additionally, all federal employees and contractors must get their shots. So must workers at hospitals and other health care settings that receive Medicare and Medicaid funding.

It’s unclear when the new rules will take effect, though the White House said workers would have sufficient time to arrange for appointments.

In King County, officials announced this week a vaccine verification system for businesses, which will be required to check that customers have received their shots. The plan follows policies in New York City and San Francisco, as well as Clallam and Jefferson counties in Washington.

The verification system could be ready by October and would likely apply to restaurants, bars and other indoor settings.

Meanwhile, the Seahawks, Mariners, Kraken, Sounders and University of Washington all announced they would adopt plans to require attending sports fans to show proof of vaccination.

The state Department of Health is assisting with King County’s vaccine verification system. Inslee said he’s considering taking the program statewide if they can develop a means of making it easy for businesses to use.

Jerry Cornfield: jcornfield@heraldnet.com; 360-352-8623 @dospueblos

Joey Thompson: 425-339-3449; jthompson@heraldnet.com. Twitter: @byjoeythompson.