More than five times as many pertussis cases had been reported as of mid-October compared with the same time last year, according to provisional numbers from the Centers for Disease Control and Prevention.

Pertussis, or whooping cough, is characterized by persistent, chronic fits of coughing followed by a “whoop” sound, and is sometimes called the “100-day cough.”

Though rates of the illness had been low since the start of the coronavirus pandemic, the number of reported cases this year — almost 20,000 — is similar to pre-pandemic levels. The CDC also says infection numbers are probably underestimated. There are diagnostic tests, but they are more reliable early in the illness.

Antibiotics can be effective in treating the disease but should be given early on. By the time pertussis presents with a persistent cough, it can be too late to test for or treat. A pandemic-related decrease in pertussis vaccination rates and waning effectiveness of the vaccine may be factors in the rise in cases, the CDC said.

Two versions of the pertussis vaccine are licensed in the United States — pediatric DTaP and adolescent/adult Tdap; both are combined with components that protect against diphtheria and tetanus. A series of five boosters, the DTaP vaccine is given to babies and young children up to age 6. According to the CDC, DTaP protects against illness in 98 out of 100 children within a year of their last shot and 7 in 10 children five years later. The Tdap vaccine, given to those 7 and older, protects about 7 in 10 people in the first year and about 3 or 4 in 10 people four years after vaccination.

At a recent Food and Drug Administration meeting, experts stressed the need for “new safe and effective pertussis-containing vaccines that increase the duration of disease protection.”