Highly contagious respiratory illness whooping cough is making a comeback in Colorado after three years of a significant decline during the COVID-19 pandemic.

Rates of pertussis — colloquially referred to a “whooping cough” — in Colorado decreased roughly 80% from 2020 through 2023, but now appear to be returning to pre-pandemic levels.

“We’ve seen an increase compared to what we saw in the pandemic,” said Grace Nelson, a disease intervention unit supervisor with the Denver Department of Public Health & Environment. “It’s kind of a return to baseline.”

In the five years before the 2020 pandemic,  the state saw an average of 675 pertussis cases each year, according to the Colorado Department of Public Health & Environment.

During pandemic from 2020 to 2023, Colorado averaged 146 cases.

Year-to-date, 535 pertussis cases have been reported in Colorado.

“There has also been a noted increase in clusters and outbreaks in the past year, mostly in school settings — which is a common setting for these outbreaks to occur,” Kayla Glad, a state health department spokesperson, said in an email to The Denver Gazette.

Initial reports show that the highest rates are among Coloradans younger than 18-years-old, particularly those aged 15-to-18 years old.

“These numbers are similar to numbers we had in Colorado prior to the pandemic, and we are monitoring case reports for other notable trends,” Glad said.

Outbreaks are common, particularly in schools and childcare centers, hospitals and large geographical areas.

Denver Public Schools, Colorado’s largest school district, had five confirmed and two suspected whooping cough cases as of Monday, said Stephanie Eastland, a district spokesperson.

Children’s Hospital Colorado is “not seeing anything that’s cause for concern right now,” Courtney Mally, a hospital spokesperson, said in an email.

Typically, roughly 10,000 whooping cough cases are reported annually in the United States, according to the Centers for Disease Control and Prevention (CDC).

Early symptoms mimic a common cold. Symptoms — which can take five to 10 days to materialize after exposure — include a runny or stuffed-up noes, low grade fever of less than 100.4°F or a mild and occasional cough.

One-to-two weeks after the first symptoms, patients may develop coughing fits that can last up to 10 weeks. Generally, the cough worsens as the illness progresses.

Coughing fits can cause vomiting, fatigue and a high-pitched “whoop” sound when inhaling and can result in the lips, tongue and finger nailbeds turning blue.

A cough that persists longer than two weeks is a concern, said Eileen Tran, a disease intervention specialist with the county health department.

Whooping cough can cause serious and occasionally deadly complications in babies and young children. One-in-three children younger than 1 year’s old are hospitalized each year, according to the CDC.

Among this year’s reported cases statewide, about two-thirds of patients were up to date on their vaccines.

Health officials encourage everyone be up to date on their vaccinations, to stay home when sick and finish taking all their antibiotics, if perscribed.

Since the pandemic, health officials in the United States and Denver have been concerned about vaccine hesitancy, a growing phenomenon that hampered the national COVID-19 vaccination campaign.

State data shows that vaccination rates for Tdap (the vaccine for older children and adults) and DTaP (for infants and young children) have cumulatively decreased about 2.5% since the 2019-2020 school year.