Health officials have raised concern about the spread of coronavirus, as thousands of people brave the pandemic to participate in national protests sparked by the death of George Floyd.

The US Centers for Disease Control and Prevention (CDC) said Sunday it was closely monitoring demonstrations taking place across the US, and warned that such gatherings could spur coronavirus transmission. Some states are already seeing upward trends of new cases.

For three months, the country passed one grim milestone after the other, hitting 100,000 coronavirus deaths in late May. Public health officials have said that without the lockdowns most states put in place, the death toll could have been significantly higher.

As those lockdowns were lifted and other measures were loosened, the CDC and other top health officials urged Americans to use face coverings when they go out, and always maintain a distance.

But large protests make it hard to keep to the recommended social distancing guidelines and “may put others at risk,” CDC spokesperson Kristen Nordlund said in a statement.

“It is too early to know what, if any, effect these events will have on the federal Covid-19 response. Every local situation is different. State and local officials will make decisions to protect public health and safety based on circumstances on the ground,” she said.

Earlier this month, CDC Director Dr. Robert Redfield said protesters should be evaluated and tested for the virus.

“I do think there is a potential, unfortunately, for this to be a seeding event,” he said — especially in metropolitan areas where there has been significant transmission.