Three new cases of whooping cough in Anne Arundel County that made headlines in Maryland this week are part of an ongoing uptick of the highly contagious disease in the United States.

But as seriously as Marylanders should take the illness — particularly those who have contact with children under a year old — the spike is not an unexpected development, said Amesh Adalja, a senior scholar with the Johns Hopkins Center for Health Security, a department of the university’s Bloomberg School of Public Health.

Like many infectious diseases, whooping cough — cases of which are up tenfold in Maryland since last year — follows natural cycles, proliferating and decreasing at regular intervals. The nation was due for the jump it typically experiences every three to five years, Adalja said.

Whooping cough, formally known as pertussis, is a bacterial infection that affects the respiratory tract. Physicians say its early symptoms can resemble those of the common cold, including a cough, mild fever and a runny nose. A week or two later, sufferers can develop long coughing fits, including the high-pitched “whooping” sound upon inhaling that gives the infection its name. The effects can last for 10 weeks or more, though antibiotics can help shorten the length of infection and prevent its spread.

Vaccines are generally effective both at preventing pertussis and minimizing its spread — the DTaP vaccine is used for children under 7, the Tdap for older individuals and people who are early in their third trimester of pregnancy. But public health experts say that when infection does occur, it can lead to serious complications, particularly for infants, even proving fatal in a small percentage of cases.

According to the U.S. Centers for Disease Control and Prevention, more than 17,500 pertussis cases have been reported nationwide so far this year. That’s more than five times the number from the equivalent span in 2023, according to the agency. A comparable rise is happening in Maryland. The state had reported 121 confirmed pertussis cases to the CDC by Oct. 22 this year, a more than tenfold increase over the 11 it reported over the same span in 2023, according to the Maryland Department of Health, which tracks pertussis annually.

The department has also reported 24 probable, but unconfirmed, cases in 2024, up from only 10 reported by Oct. 22 of last year, health department spokesman Chase Cook reported in an email to The Baltimore Sun.

Anne Arundel County Public Schools reported two cases this month, both at Crofton High School in Gambrills, and one in September at Rolling Knolls Elementary School in Annapolis. The infections were widely reported in local media.

The Anne Arundel County Department of Health does not believe the two cases at Crofton High School are related, according to a communication from the school district to Crofton High School families. The three cases came in addition to 15 already reported in the county this year.

Five cases of whooping cough have been reported in Baltimore City, according to Blair Adams, a spokesperson for the city’s health department.

Cases of whooping cough have also been reported in Baltimore County, Elyn Garrett, a spokesperson for the county health department, said in an email to The Sun, and the department’s communicable disease program is monitoring those.

The department is also working with Baltimore County Public Schools “to provide information to families/staff around what symptoms to be aware of and encouraging steps to take to protect against this issue,” added Garrett, who said the county is advising residents to get vaccinated.

Conditions were ripe for a climb in numbers this year, Adalja said. The type of pertussis vaccines in use today are less potent than the “whole-cell” vaccines most patients received before the early 1990s, and the number of vaccinations given dropped sharply during the coronavirus pandemic in 2020.

Public health officials in Maryland and beyond, in fact, have said that the current numbers are similar to the ones reported on a regular basis in the years leading up to 2019. The COVID-19 virus first appeared in the U.S. late that year, prompting the closure of many doctors’ offices and disrupting childhood immunizations.

At the current rate of infection across the country, the United States would see about 22,000 cases of pertussis this year, according to the CDC. The national average was about 18,000 per year between 2015 and 2019. Maryland would see about 145 reported cases; it saw 121 in 2019.

“In one sense, we’ve had these outbreaks before,” Adalja said. “I don’t think there’s any reason to panic. But there are certainly people at more risk for developing severe infection, especially neonates and infants, so it’s really important to try and protect children in their first year of life.”

Cases of pertussis, once viewed as a common childhood illness in the U.S., dropped significantly as vaccines were created or improved. A whole-cell inactivated vaccine developed in 1939 — and improved via combination with typhoid and diphtheria toxoids about a decade later — caused a drop in annual numbers from figures that regularly hovered between 150,000 and 200,000 to less than half that number through 1959.

From the early 1960s through the early 1990s, the number of cases fell to well below 10,000 per year — a drop likely affected by the fact that by the mid-1960s, many states had made pertussis vaccines mandatory for public school students.

In the early 1980s, however, concerns about side effects associated with the whole-cell vaccine — some of which proved valid while others did not — led to a change in medical protocol. A new, acellular vaccine that produced fewer side effects but was less potent against pertussis and remained effective for shorter periods of time became the norm in the U.S. by the mid-1990s.

“The current vaccine is not as effective, and it wanes faster,” Adalja said. But it is still the most reliable protection against pertussis.

In 2003, the number of whooping cough cases topped 10,000 in the U.S. for the first time in 39 years, according to CDC figures. The figures never fell below that number between 2003 and the onset of the pandemic, topping 20,000 seven times.

In addition to the change in vaccines and the cyclical nature of pertussis, Adalja said the number of reported cases might be on the rise because more information is available to the general public. He added that the true number of whooping cough cases in a community is typically considerably higher than the number that gets reported.

Bridget Byrne contributed to this article. Have a news tip? Contact Jonathan M. Pitts at jpitts@baltsun.com, 410-332-6990 and x.com/@jonpitts77.