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Why do COVID cases come in waves?

Cases in the U.S. began a downward trend in mid-September.
Credit: AP
FILE - In this Aug. 31, 2021 file photo medical professionals pronate a 39 year old unvaccinated COVID-19 patient in the Medical Intensive care unit (MICU) at St. Luke's Boise Medical Center in Boise, Idaho. Idaho's public health leaders have expanded health care rationing statewide amid a massive increase in the number of coronavirus patients requiring hospitalization. The Idaho Department of Health and Welfare made the announcement Thursday, Sept. 16, 2021. St. Luke's Health System, Idaho's largest hospital network, asked state health leaders to allow "crisis standards of care" on Wednesday because the increase in COVID-19 patients has exhausted the state's medical resources. (AP Photo/Kyle Green, File)

ATLANTA — COVID-19 cases in the United States continue on a downward trend, but medical experts warn that we could be hit by another wave if Americans let their guard down.

Like a boat bobbing across the ocean, the COVID-19 pandemic has taken us to frightening highs and promising lows. U.S. cases have surged and dropped about every two or three months.

Most recently, cases started rising in July only to start falling again in mid-September.

“Anyone who says they fully understand this isn’t telling the truth,” Georgia State University’s Dr. Harry Heiman said.

The most recent drop in cases comes with just over half of all Americans vaccinated and many school children going back to meeting in-person. Dr. Heiman said the summer surge that brought illness to so many also brought natural protection.

“A surge in people having natural immunity,” Dr. Heiman said. “For every case we measure, there’s probably five or ten more we’re not capturing.”

Viruses typically surge in the winter months. That was certainly the case last winter with COVID.

The more contagious Delta variant triggered the surge that began this past summer.

Medical experts say people naturally become more cautious when cases begin to rise, turning to masks and social distancing, which can help cases drop.

Once that happens, it’s easy to let your guard down, encouraging cases to rise again.

“I think it’s a combination of those things that have led to the situation we’re seeing now,” Dr. Heiman added.

He points out that even as cases began dropping in September, Georgia was averaging over 100 COVID deaths a day.

“While optimistic that may be trending the right way, this is no time for anyone to drop their guard,” he said.

There’s always the possibility of another variant like Delta which could cause a dramatic rise in cases once again.