Whooping cough cases are soaring. Can infecting people help test a better vaccine?

By Brenda Goodman and Deidre McPhillips, CNN

(CNN) — Whooping cough is surging in the United States, with cases now five times higher than they were at the same point last year, according to preliminary data from the US Centers for Disease Control and Prevention that was reported Friday.

It’s the highest number of whooping cough infections since 2014, “with no indication of slowing down,” said Dr. Susan Hariri of the CDC’s National Center for Immunization and Respiratory Diseases, who presented the data Friday at a meeting of experts who advise the US Food and Drug Administration on its vaccine decisions.

This year’s spike in cases marks a return to pre-pandemic levels of this infection, which is also called pertussis.

Even before the Covid-19 pandemic, whooping cough cases were trending upward in the US, because the vaccines that are currently used offer strong initial protection that wears off after two or three years.

“We know that we do not have very good pertussis vaccines,” said Dr. Archana Chatterjee, a pediatric disease specialist and dean of the Chicago Medical School, at the meeting.

“These vaccines have done a yeoman’s job for us for many decades, but they are not doing the job as well as they used to do,” said Chatterjee, who is a member of the FDA’s Vaccines and Related Biological Products Advisory Committee.

On Friday, the committee met to discuss whether it might be time to allow pharmaceutical companies to purposely give people whooping cough as the primary test of their vaccines, a type of study called a human challenge trial.

Whooping cough cases climbing for decades

As with many infectious diseases, cases of whooping cough dropped to unusually low levels during the pandemic as people limited social interactions and took other precautions to defend against Covid-19.

In 2020, the US had about 6,000 cases of pertussis, and that number dropped even further in 2021 to roughly 2,000 cases. There were about 3,000 cases in 2022 and about 5,000 last year.

The US has seen 14,569 cases this year, compared with 2,844 at the same time last year, according to the CDC’s Nationally Notifiable Diseases Surveillance System.

Whooping cough starts with symptoms that look a lot like a regular cold: a runny nose, sneezing, a low-grade fever and a tickly cough. A week or two later, the infection moves into a new stage, leading to a violent full-body cough.

These coughing fits can be so severe that they cause patients to vomit or break ribs, and they’re often accompanied by a whooping sound as the person tries to catch their breath. Antibiotics can treat the infection, but only if it’s caught within the first few weeks, before the arrival of the exhausting, painful cough. Then, the only treatment is comfort care with plenty of rest and fluids while the infection runs its course.

Unvaccinated babies and adults are those at highest risk of severe infection and, rarely, death. But doctors have increasingly been seeing vaccinated people with infections, too.

Young children typically receive five doses of a vaccine against pertussis – called DTaP or TdaP, which also protects against tetanus and diphtheria – by the time they’re 6 years old. Boosters are recommended for children who are 11 or 12. Pregnant women are also recommended to be vaccinated against the infection, and adults are supposed to get boosters every 10 years.

Pertussis vaccination rates are high in the United States – around 90% – and did not dip during the pandemic, Hariri said, as they did in some other countries.

Despite that, the upward trend in infections has continued.

Protection from vaccines is waning

The increase in cases is not fully understood. There’s evidence that it may be due to changes in the bacteria that causes the infection, said Dr. Tod Merkel, associate director of research in the FDA’s Office of Vaccines Research and Review, in a presentation to the committee Friday.

In the 1990s, the US changed the type of vaccines used to prevent pertussis, moving from a whole-cell vaccine that caused children to spike fevers and experience other worrisome symptoms after their shots to a newer vaccine that didn’t cause as many reactions but also offered less protection that didn’t last as long.

Other countries that changed to the newer vaccines have seen cases increase too, Merkel said.

The incomplete protection from the newer vaccines, experts believe, has allowed the infection to continue to spread. It’s also caused the bacteria to mutate to escape the immune response generated by the shots.

Merkel said Friday that 90% of bacterial strains cultured in human infections in the United States are now those associated with mutations to skirt the protections of the vaccine.

For young children who are vaccinated against pertussis, protection now wanes quickly, sometimes after two or three years – well before age 11, when a booster is recommended.

Regulators consider human challenge studies

Although babies are most at risk from severe disease, infections are rising in older children and teens as protection wears off.

That’s led to a hunt for better, more durable immunizations against whooping cough that also don’t cause as many side effects.

Tests of new vaccines have been difficult, however, because whooping cough cycles, with major outbreaks every three to five years, and researchers can’t predict where or when it will surge. Manufacturers need large numbers of infected people to test their inoculations.

Enter human challenge studies.

In some ways, whooping cough is an ideal infection to test in human challenge models. It’s curable with antibiotics in its early stages, which protects study volunteers from harm.

On the other hand, most people have few symptoms in those early stages. Investigators can measure the vaccine’s ability to prevent mild early symptoms and whether it can prevent bacteria from setting up shop in nasal passages, but the pertussis challenge models don’t allow them to directly measure how well a vaccine may prevent severe disease, which is the goal of most vaccines.

Researchers also don’t know what immune factors may indicate that a person is protected from whooping cough, so they can’t measure whether people are getting protection that’s at least as good as that provided by the approved vaccines, a strategy called bridging.

On Friday, labs in Canada and the UK presented their models for how to conduct challenge trials for pertussis.

The FDA advisers were then asked whether they thought these kinds of trials could measure the right things and, if so, if they would provide enough evidence to be the basis of a vaccine approval.

The labs’ models were met with mixed reactions from the experts. On the one hand, they said, this approach was exciting and would surely speed the testing of new vaccines. On the other hand, they weren’t sure these studies had been fine-tuned enough.

“I’m very supportive of this; whether or not we are quite there yet, I’m not totally sure,” said Dr. Melinda Wharton, associate director for vaccine policy and clinical partnerships at the CDC’s National Center for Immunization and Respiratory Diseases.

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