A wave of whooping cough infections is continuing to worsen nationwide, data from the Centers for Disease Control and Prevention suggests, with weekly reported cases now accelerating to the highest levels seen in the U.S. for years. 

At least 259 pertussis cases were reported during the first week into August, the CDC now tallies, up from 215 weekly cases towards the end of June. 

Pennsylvania has reported 1,489 cases so far this year, the most of any state. New York has reported the next largest total of cases, at 1,266 infections.

The increase comes as CDC officials have been warning for months of a return to the pre-pandemic trends of the illness. Whooping cough is caused by the bacteria Bordetella pertussis, which had plummeted to record lows during the COVID-19 pandemic, likely due to more people at the time wearing masks and staying home from schools and offices. 

So far this year, a total of more than 10,000 pertussis cases have been reported by health departments. By mid-June, total cases this year reached more than triple what they were compared to the same time last year. 

Reported cases this year are also now higher than they were right before the pandemic, which had reached around 8,000 cases by early August of 2019.

Why are whooping cough cases accelerating?

Many diseases that spread from person-to-person through the air like pertussis saw rates of infections drop during the COVID-19 pandemic.

“It’s likely mitigation measures used during the pandemic (e.g., masking, remote learning) lowered transmission of pertussis,” the CDC said in July. 

That interrupted what had been signs that pertussis rates were starting to trend up again, after dropping from a previous peak in 2012. 

“The increase in pertussis that we are observing continued right up through the start of the COVID-19 pandemic in 2020, after which we saw the dramatic decline in reported pertussis, which was similar to trends in other nationally notifiable diseases,” Tami Skoff, a CDC epidemiologist, said at an April event hosted by the National Foundation for Infectious Diseases.

A number of factors were behind the gradual increase in pertussis cases before the pandemic, Skoff said, like better rates of diagnosing and reporting infections. 

Another reason may trace back to the switch in the 1990s to a safer type of vaccine, known as “acellular” shots, whose protection lessens over time.

“We believe one of the driving factors and one of the largest contributors is waning protection from acellular pertussis vaccines,” Skoff said.

Skoff said the CDC’s vaccine experts do not currently plan to revisit the topic of pertussis vaccines, after deciding against booster recommendations for the 2012 surges. But new vaccines could result in changes to the agency’s guidance, she said.

“There are vaccines moving forward in the pipeline. Hopefully in the next few years, we’ll have vaccines with longer duration of protection available,” she said.

What are the symptoms of whooping cough?

Doctors typically divide whooping cough into at least three stages, which start with an initial onset of symptoms similar to the common cold, like cough and runny nose. Symptoms typically set in around a week after the patient was first exposed to another person who was contagious. 

The cough worsens for up to six more weeks, often into the characteristic “whooping” sound, which occurs as patients struggle to breathe after bursts of rapid coughs to try to expel mucus building up in the body’s airways. 

That can result in the skin turning blue or purple, because of the lack of oxygen in their body, and vomiting afterwards. 

The symptoms can look different depending on the age of the patient. Infants are at the greatest risk of severe pertussis, with the highest rates of hospitalizations.

While vaccines and better treatments have prevented the thousands of childhood deaths pertussis used to cause each year early during the 20th century, severe cases can result in complications like pneumonia and neurologic issues, especially in infants.

How is whooping cough treated?

Antibiotics can be effective at reducing whooping cough’s severity, especially if started early during an infection, the CDC says. They can also be given to at-risk close contacts of pertussis cases to prevent them from getting sick. 

Doctors can test patients with symptoms using a swab pushed deep into the nose. Those tests are most accurate when done within the first three weeks of the cough, the CDC says.

“In the setting of waning immunity and we are seeing more and older individuals and oftentimes I’ll hear from clinicians, we don’t think about pertussis in older individuals or they come to healthcare much later when some of the diagnostic tests aren’t accurate,” Skoff said.

Testing is not always needed to start patients on antibiotics, especially if they are at risk of severe disease. People diagnosed with pertussis can be given antibiotics like azithromycin for up to five days to treat infections, though it does not guarantee that people will see faster relief from their symptoms.

“Antibiotics are intended to prevent transmission of pertussis to others and do not shorten the disease course or improve symptoms,” doctors said in an article published by the American Academy of Family Physicians, citing a review of 13 trials.