At the protest outside City Hall in support of George Floyd on Saturday, people were spaced more than 6 feet apart — the recommended minimum for reducing the spread of the coronavirus. Most wore masks.

But later in the day, such as when crowds gathered outside the Philadelphia Museum of Art, many were not taking those precautions.

Could the protests here and elsewhere lead to a spike in COVID-19 cases? How about other recent outdoor events with large clusters of people, such as a Lake of the Ozarks pool party in Missouri on Memorial Day weekend?

The answer in either case will not be known for days, as symptoms from any new infections might not show up until two weeks after exposure.

There is one reason for cautious optimism: The events took place outside. Research to date suggests the coronavirus is far more likely to spread indoors, especially in poorly ventilated spaces.

Yet Philadelphia’s own history suggests the open air is no guarantee. In 1918, a spike in deaths from influenza has been attributed to a parade that drew more than 200,000 attendees along Broad Street.

Joel Hersh, former director of the bureau of epidemiology at the Pennsylvania Department of Health, is among those worried that history may repeat itself.

“The odds are not good that you would escape all these demonstrations without having somebody be positive,” he said. “I think it’s another disaster waiting to happen.”

Should someone at any mass event later test positive, public health officials will have difficulty notifying individuals who may have been exposed, he said.

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The best course may simply be a public announcement, such as what took place after one person at the Ozarks event was confirmed to have COVID-19. Health officials in Missouri have publicly shared that person’s schedule so other revelers can be on the lookout for symptoms.

The protests would pose a similar challenge, with an added complication. Many attendees were African American — a group in which the coronavirus has caused more severe health problems.

“It’s a pandemic, and people of color are getting hit harder,” Atlanta Mayor Keisha Lance Bottoms said Sunday on CNN. “We’re going to see the other side of this in a couple of weeks.”

On CBS News’ Face the Nation, former Food and Drug Administration Commissioner Scott Gottlieb cautioned that large demonstrations could lead to new waves of coronavirus infections. Minnesota, he said, has seen an increase in cases and hospitalizations in recent days.

“There’s going to be a lot of issues coming out of what’s happened in the last week, but one of them is going to be that chains of transmission will have become lit from these gatherings,” Gottlieb said.

As a rule, a person’s chance of becoming infected with a virus depends on the degree of exposure. For an airborne microbe such as the coronavirus, the more time you spend near an infected person, the greater the chance of transmission.

That is partly why being outdoors is thought to be less risky than indoors, as the concentration of any viral particles should dissipate more quickly. Another outdoors benefit: viruses can be inactivated by ultraviolet light from the sun.

A Japanese analysis of 110 coronavirus infections suggested that transmission was far more likely to occur indoors than outdoors.

The type of activity also matters. Singing and shouting, for example, can increase the risk of transmitting an airborne virus, as an infected person is more likely to expel virus-laden droplets of saliva. Dozens of people tested positive after attending a March choir practice in Skagit County, Wash.

No matter the location or activity, physicians warn that continued vigilance is needed. Most people still have not been exposed to the coronavirus, so their immune systems are less equipped to fend it off. And the virus can be spread by people who have no symptoms.

This article contains information from the Washington Post.