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Fans reach for a home run ball during batting practice before the 2021 MLB All-Star Game Tuesday, July 13, 2021, at Coors Field in Denver. (The Gazette, Christian Murdock)

Denver health officials stressed Thursday that COVID-19 cases associated with the All-Star Game were in a fraction of the event's attendance and there had been "an expectation" that disease would spread enough at the event to qualify as an outbreak.

Thus far, 20 cases have been tied to All-Star Game events, but officials have not said if a specific event drove the cases.

Bob McDonald, executive director of the city's Department of Public Health and Environment, said in an email all COVID-19 cases associated with the All-Star Game at Coors Field would fall into one cluster, regardless of immediate relations. That's similar to how outbreaks at the University of Colorado and other institutions of higher learning have been tracked: They don't necessarily need to all be traced together to be counted as an outbreak, and all cases from a larger institution or event will be counted cumulatively.

Denver has experienced a steady uptick in cases in recent weeks, but the city "is not seeing an alarming number of cases as a result of the MLB All Star week, and we will continue to monitor, investigate, and collaborate with the state in the upcoming weeks," McDonald said.

A spokeswoman for the state Department of Public Health and Environment deferred to city health officials for comments. To qualify as part of an outbreak, "an individual has to have either attended the event while infectious or developed COVID-19 in the 14 days following the event," she said.

The All-Star Game festivities were just over two weeks ago.

"Anyone - regardless of vaccination status - who attended the All-Star Game and developed symptoms should get tested immediately," she added when asked about the need for testing of those at the game or its events.

McDonald wrote that anyone who knows they've been exposed or has developed symptoms should get tested. But he pointed to the large attendance relative to the small number of cases.

"The number of cases relating to the All Star-Game is incredibly low," he said. "Right now, that is 20 cases. The game had an attendance of 45,000 people."

The All-Star Game is not the only summer event to have led to an outbreak in Colorado. Three other events all have led to clusters, according to state data published each Wednesday. The Greeley Stampede led to 25 cases, the Country Jam in Grand Junction contributed 23 more, and the Meeker Range Call added 17 more, the data show. All of those outbreaks were identified in July.

Other events and gatherings have led to outbreaks elsewhere. Summer camps in Illinois and Texas led to more than 200 infections combined, according to NBC News. One of the most notorious cases was a motorcycle rally in Sturgis, South Dakota, a multi-day, outdoor event with minimal masking that led to scores of cases. But that event took place in 2020, months before the vaccine was available.

More evidence exists showing indoor events are drivers of transmission more than their outdoor counterparts. A review of several studies, published in the Journal of Infectious Disease in February, found that fewer than 10% of infections could be traced to outdoor settings; infections were more than 18 times more likely to occur indoors than outdoors. 

But there were limiting factors in the researchers' review of the studies, the authors wrote. For one, it's unclear how much indoor components of outdoor events - such as restaurants and bars around Coors Field - play a part in transmission associated with the bigger event. No active outbreaks have been associated with any Denver restaurants, bars or any other downtown location.

"These findings ... suggest that while outdoor transmission is less common than indoor, it is not impossible," the researchers wrote. "Case reports identified after our review had been completed provide further evidence that high-density outdoor gatherings, particularly with low mask use, may lead to higher transmission rates. "

Large gatherings - both indoor and outdoor - have been cleared in Colorado for weeks, after the pandemic slowed in the spring and the fourth wave in April and May was mitigated faster than previous highs. Coors Field's approved capacity was increased significantly before the game.

Guidance from the Centers for Disease Control and Prevention advises attendees of large gatherings to wear masks and keep apart from non-household members. Outdoor events are safer than indoor festivities, the agency wrote on its website in early May, but masks and distancing should still be encouraged.

Denver's vaccination rate is high: Just over 75% of the city's residents have been inoculated. But for the unvaccinated, any crowd can be dangerous, particularly now that the delta variant is dominant statewide, authorities have cautioned. 

"It's so contagious that if you're in the middle of a crowd and you've not been vaccinated, it will find you," Seth Foldy, of Denver Health, said last week of the COVID-19 delta variant.

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Health reporter

Seth Klamann is the health reporter for the Gazette, focused on COVID-19, public health and substance use. He's a Kansas City native and a University of Missouri alum, with stops in Wyoming, Omaha and Milwaukee before moving to Denver.